Aqueous Solutions, Acids, and Bases

Transcription

1 Principles of Biology contents 7 Aqueous Solutions, Acids, and Bases The chemistry of acids and bases happens in aqueous solutions. The ocean. The ocean is seawater, a solution of salts and water. Photo courtesy of Lily Keyes. Some rights reserved. Topics Covered in this Module Aqueous Solutions Acids, Bases, and ph Buffers Stabilize ph Major Objectives of this Module Understand ph and how acids and bases interact with water. Describe how a buffer contributes to ph stability. Understand how ph affects organisms and ecosystems. Understand the mole, and describe various units of concentration. page 34 of pages left in this module

2 Principles of Biology contents 7 Aqueous Solutions, Acids, and Bases Earth is a watery planet. Oceans, lakes and rivers, glaciers, icebergs and rainfall are features of nearly every corner of the globe. And every living organism is made primarily of water. We have already learned about some of water's unique properties. In this module we explore the interaction of water with other substances, and, in particular, we introduce the chemistry of acids and bases in aqueous solution. Acids and bases are abundant and vitally important in living organisms. We close this module with a discussion of acid-base reactions in cells, organisms and ecosystems. Aqueous Solutions Recall that substances can be grouped into two broad categories based on their interactions with water. Substances that readily interact with water are hydrophilic, or "water loving." Polar molecules and ionic compounds are typically hydrophilic. Substances that repel water are hydrophobic, or "water fearing." Nonpolar molecules are hydrophobic. Hydrophilic substances readily dissolve in water to form solutions. For example, seawater is a solution of water and a number of salts. A solution is a completely homogenous mixture of two or more compounds. In a solution, the substance present in the greatest quantity is called a solvent. In aqueous solutions, such as seawater, water is the solvent. All other substances in the mixture are called solutes. Solutes can be gasses, liquids or solids. Gaseous carbon dioxide and oxygen are both nonpolar molecules, but because they are so small, they are able to form a solution with water. Polar liquids, such as alcohols, can also form solutions with water, as can many ionic salts, such as sodium chloride. Sodium chloride dissolves in water because it is composed of positively and negatively charged ions that interact with polar water molecules. Solution concentration. Being able to calculate the concentration of solute in solution matters. For this reason, scientists quantify concentration in a number of different ways. The simplest method involves measuring the mass of solute per volume of solvent. A liter of seawater contains about 35 grams of sodium chloride; therefore, the concentration of sodium chloride in seawater is 35 g/l. Square brackets are often used to signify concentration, so sodium chloride's concentration in the case of seawater can be written as "[NaCl] = 35 g/l." Blood has a sodium chloride concentration of 9 g/l, so seawater is about four times saltier than blood. Concentration can also be given as a mass percent (3.5% for seawater, 0.9% for blood). Often, scientists want to know how many particles of solute are present. Particles may be atoms, molecules, or units of ionic compound (a unit of sodium chloride is one NaCl). The number of particles can be calculated using a value called the mole (abbreviation: mol). By definition, one mole contains 6.02 x particles. The number 6.02 x 10 23, known as Avogadro's number, is very large because atoms are very small. The mass of one mole of an element equals its atomic mass in grams. For example, from the periodic table we know that sodium has an atomic mass of about 23 amu, so one mole, or 6.02 x atoms of sodium, has a mass of 23 grams. Molar mass is the mass, in grams, of one mole of particles. The molar mass of a compound equals the combined mass of its component atoms. Chlorine has an atomic mass of about 35, so the mass of one mole of NaCl is , or 58 grams. Molar mass has units of grams per mole, so

3 the molar mass of NaCl is 58 g/mol. The molar concentration of a solution is defined as the number of moles of solute in a given volume of water. Molar concentration has units of moles/liter, or molarity (M). Table 1 compares the NaCl concentration of seawater and blood in various units. The number of NaCl units per volume (far right) was calculated by multiplying the molar concentration with Avogadro's number. Since the number of particles is so large, scientists generally report concentration in molarity. Notice that, regardless of the units used, seawater is four times saltier than blood. g/l % mol/l (or M) NaCl units/liter seawater x blood x Table 1: Concentration of sodium chloride in seawater and blood. Adapted from: 1. Salinity of seawater: National Oceanic and Atmospheric Administration website: /whysalty.html; 2. Salinity of blood: Wurts, W. A. Why can some fish live in freshwater, some in salt water, and some in both? Vertebrate & Fish Evolution World Aquaculture 29, 65 (1998) /VertebrateFishEvolution.htm. BIOSKILL Concentration in the Laboratory A scientist has discovered a strain of yeast with particularly efficient respiration. She wants to compare how well the yeast grows in two different types of sugar: glucose (C 6 H 12 O 6 ) and sucrose (C 12 H 22 O 11 ). For her experiment, she must make one liter of a 0.5 M solution of each sugar. How many grams of each sugar will she need? Let's start with the glucose solution. From the periodic table, we know that the molar mass of carbon is 12, the molar mass of hydrogen is 1, and the molar mass of oxygen is 16. To determine the molar mass of glucose, we multiply the molar mass of each element by the number of atoms present, then add the masses together: C: 6(12) = 72 g/mol H: 12(1) = 12 g/mol O: 6(16) = 96 g/mol Molar mass = 180 g/mol One liter of a 0.5 M (or 0.5 mol/l) solution contains 0.5 moles of glucose. To calculate the mass of glucose required, multiply 0.5 moles times the molar mass: 0.5 moles x (180 g/mol) = 90 g

4 Test Yourself Calculate how many grams of sucrose the scientist will need to make one liter of 0.5 M solution. Explain why different masses of each sugar are needed to make a 0.5 M solution. Submit BIOSKILL IN THIS MODULE Aqueous Solutions Acids, Bases, and ph Buffers Stabilize ph Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? A Sea of Microbes Drives Global Change Do floating microbes in the ocean s surface waters play an outsize role in global climate? The Climate Connection How is life on Earth reacting to climate change? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View Download SCIENCE ON THE WEB Acid Rain and Pollution Politics. Watch archived news clips about acid rain, starting ph affects movement of drugs across cellular membranes. This interactive animation shows that the direction of flow of certain drugs across cell membranes depends on intracellular ph. page 35 of pages left in this module

5 Principles of Biology contents 7 Aqueous Solutions, Acids, and Bases Acids, Bases, and ph Sour receptors on the human tongue are able to detect lemon juice, which is acidic. Bitter receptors detect baking powder, which is basic, or alkaline. Most foods are slightly acidic, but very acidic substances, such as battery acid, are corrosive and harmful. Many alkaline substances, such as detergents, are unpalatable, and very alkaline substances, such as bleach, are caustic. To understand what makes a substance acidic or basic and how acidity and alkalinity affect cells, organisms, and ecosystems, we first need to understand the chemistry of pure water. Water forms hydroxide and hydronium ions. As we learned in the last chapter, water molecules can form hydrogen bonds. Sometimes, a hydrogen nucleus (also called a proton, or H + ) is transferred from one water molecule to the lone pair of electrons on the other (Figure 1). This transfer results in the formation of two ions: a positively charged hydronium ion (H 3 O + ) and a negatively charged hydroxide ion (OH - ). Figure 1: Hydroxide and hydronium ion formation. The transfer of an H + from one water molecule to another results in the formation of hydronium ions (H 3 O + ) and hydroxide ions (OH - ) Nature Education All rights reserved. Figure Detail The reaction that forms H 3 O + and OH - is reversible. The forward and reverse reactions occur continuously, but at any given time the total number of H 3 O + and OH - ions remains constant. One liter of pure water contains moles of each ion, so the concentration of each ion is 10-7 M. Certain substances, called acids and bases, alter the ratio of H 3 O + and OH -. In the next section, we will explore the properties of acids and bases. Test Yourself Two water molecules can react to form hydroxide and hydronium ions. Write the reverse reaction for this process. Submit Acids and bases. Pure water has an equal concentration of H 3 O + and OH -. An acid is a

6 substance that increases the concentration of H 3 O + in a solution. A base is a substance that decreases the concentration of H 3 O + in a solution (Figure 2). Figure 2: Acidic and basic solutions. a) Pure water has an equal number of H 3 O + and OH -. b) An acidic solution has more H 3 O + than OH -. c) A basic solution has more OH - than H 3 O Nature Education All rights reserved. Figure Detail Acids can be strong or weak (Table 1). A strong acid irreversibly dissociates when placed in solution, releasing H + that react with water to form H 3 O +. Hydrochloric acid (HCl) is an example of a strong acid. A weak acid reversibly dissociates in solution. Carbonic acid (H 2 CO 3 ) is an example of a weak acid. Notice that the released H + reacts with water to form a hydronium ion (H 3 O + ). Strong acid (hydrochloric) Weak acid (carbonic) Dissociations reaction HCl H + + Cl - H 2 CO 3 H + + HCO 3 - H + + CO 3 2- Reaction with water H + + OH - H 2 O H + + OH - H 2 O Result H 3 O +, OH - H 3 O +, OH - Table 2: Strong and weak acids. Adapted from: Masterton, W., Slowinski, E. & Stanitski, C. Chemical Principles, Alternate Edition. Saunders College Publishing, 1983, pp Bases can also be strong or weak. Sodium hydroxide (NaOH) is an example of a strong base that irreversibly dissociates in solution. The OH - reacts with H 3 O +, forming two water molecules. Ammonia (NH 3 ) is an example of a weak base. In aqueous solution, NH 3 reacts with H 3 O +, forming an ammonium ion (NH 4 + ) and water. Dissociation reaction Reaction with water Result Strong base (sodium hydroxide) NaOH Na + + OH - H 3 O + + OH - 2H 2 O H 3 O +, OH - Weak base (ammonia) none NH 3 + H 3 O + NH H2 O H 3 O +, OH -

7 Table 3: Strong and weak bases. Adapted from: Masterton, W., Slowinski, E. & Stanitski, C. Chemical Principles, Alternate Edition. Saunders College Publishing, 1983, pp Test Yourself What reaction do you expect to occur if CO 3 2- (also known as bicarbonate, the final dissociation product of carbonic acid) is added to water? Is CO 3 2- an acid or a base? Submit The ph scale. Recall that in pure water, the concentration of H 3 O + is 10-7 M and the concentration of OH - is 10-7 M. An acidic solution has a higher concentration of hydrogen ions than pure water (>10-7 M), and a basic solution has a lower concentration (<10-7 M). In aqueous solution, the product of the concentration of H 3 O + and OH - is constant and can be calculated as follows: [H 3 O + ] [OH - ] = 10-7 x 10-7 = M To understand why this product remains constant, take a look again at Tables 2 and 3. Acid (H + ) reacts with OH - to form water, hence removing OH - from solution. Bases (OH - or NH 3 ) react with H 3 O +, hence removing H 3 O + from solution. In other words, as H 3 O + increases, OH - decreases, and vice versa. Because the H 3 O + concentration is very small, scientists have developed a convenient logarithmic scale, called the ph scale, to quantify [H 3 O + ]. For simplicity, [H 3 O + ] is often referred to as the concentration of protons, or [H + ]. Thus, ph is defined as follows: ph = -log [H 3 O + ] = -log [H + ] The ph scale ranges from 0 to 14 (Figure 3). Water, which has an H 3 O + concentration of 10-7, has a ph of 7. Acidic solutions have a ph less than 7. The lower the ph, the more acidic the solution. Basic solutions have a ph greater than 7. The higher the ph, the more alkaline the solution. Because ph is a log scale, a one-unit change in ph corresponds to a ten-fold change in acidity. Thus, coffee, which has a ph of 5.0, is ten times more acidic than urine, which has a ph of 6.0. Figure 3: The ph scale. Pure water has a ph of 7. Acids have a ph less than 7, and bases have a ph greater the 7. The ph of most body fluids, including blood, cytosol, and extracellular fluid, is known as the physiological ph and is slightly basic, with a ph of The ph of several biological solutions is compared with that of several common household solutions Nature Education All rights reserved. Figure Detail

8 Test Yourself Ammonia has a ph of 11. (a) What is the concentration of H 3O + in ammonia? (b)what is the concentration of OH -? Submit IN THIS MODULE Aqueous Solutions Acids, Bases, and ph Buffers Stabilize ph Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? A Sea of Microbes Drives Global Change Do floating microbes in the ocean s surface waters play an outsize role in global climate? The Climate Connection How is life on Earth reacting to climate change? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View Download SCIENCE ON THE WEB Acid Rain and Pollution Politics. Watch archived news clips about acid rain, starting ph affects movement of drugs across cellular membranes. This interactive animation shows that the direction of flow of certain drugs across cell membranes depends on intracellular ph. page 36 of pages left in this module

9 Principles of Biology contents 7 Aqueous Solutions, Acids, and Bases Buffers Stabilize ph A reaction between CO 2 and H 2 O forms carbonic acid, a weak acid that dissociates into carbonate and bicarbonate ions: CO 2 + H 2 O = H 2 CO 3 = H + + HCO 3 - = 2H + + CO3 2- If H+ is added to a carbonic acid (H 2 CO 3 ) solution, it reacts with bicarbonate (CO 3 2- ) and carbonate (HCO3 - ), which drives the reaction to the left. If OH - is added, it removes H + from solution, which pulls the reaction to the right. Because carbonic acid is able to remove both excess H + and excess OH - from aqueous solution, it is a buffer, or a substance that minimizes changes in ph. Weak acids and weak bases act as buffers in aqueous solution. ph buffering in organisms. Many enzymes are exquisitely sensitive to ph, and ph gradients across intracellular compartments are necessary for various cellular functions. Thus, intracellular ph is carefully regulated. In eukaryotic cells, the cytoplasm and each organelle have a characteristic ph (Figure 4a). To ensure that perturbations in ph are rapidly buffered, an enzyme called carbonic anhydrase increases the rate of conversion between carbon dioxide and carbonic acid (Figure 4b). The direction of the reaction depends on the ph of the solution in which the enzyme is bathed.

10 Figure 4: Carbonate buffering in eukaryotic cells. a) The cytosol and each intracellular compartment in a eukaryotic cell have a characteristic ph. b) Carbonic anhydrase increases the speed of buffering by increasing the rate of conversion between carbon dioxide and carbonic acid. Two different forms of carbonic anhydrase (abbreviated CA) are shown: CA2 and CA Nature Publishing Group Casey, J., Grinstein, S., & Orlowski, J. Sensors and regulators of intracellular ph. Nature Reviews Molecular Cell Biology 11, (2010) doi: /nrm2820. Used with permission. Figure Detail

11 Test Yourself A carbonate buffering system is also used to regulate the ph of blood, which ranges between 7.35 and Liver failure, excess alcohol consumption, and low blood sugar can cause acidosis, a condition in which the blood is too acidic. Fever, high altitude, and liver disease can cause alkalosis, a condition in which the blood is too basic. Both conditions result in fatigue and confusion. Sodium bicarbonate (NaCO 2-3, or baking soda) alleviates one condition. Breathing into a paper bag, which increases CO 2 in the blood, alleviates the other condition. Which treatment is used for which condition? Explain your answer. Submit ph buffering in ecosystems. Pollutants from the burning of fossil fuels undergo chemical reactions that produce acids, including carbonic (H 2 CO 3 ), sulfuric (H 2 SO 4 ), and nitric (HNO 3 ) acids. These acids react with water in the atmosphere, resulting in acid fog and acid precipitation. In dry weather, acid dust may fall to the ground and acidify groundwater. Rainwater is normally somewhat acidic due to the presence of dissolved CO 2 and has a ph of about 5.6. The ph of acid precipitation falling in the eastern United States and Canada ranges from 4.2 to 5.0. Environments rich in limestone (CaCO 3 ) have a high buffering capacity and are relatively immune to acid rain, while limestone-poor environments, such as the Appalachian Mountains of the eastern United States, are particularly vulnerable. In vulnerable areas, acid precipitation leaches nutrients from the soil and solubilizes aluminum, which is toxic to plant roots. A combination of acid precipitation and other stressors, such as climate change, can kill trees. Acid precipitation also affects aquatic ecosystems. In the central and southern Appalachian Mountains, the brook trout (Salvelinus fontinalis) has experienced an extensive decline over the past one hundred years. This decline can be attributed to a variety of causes, including overfishing, sedimentation, climate change, and the introduction of exotic species. However, acid precipitation alone has a clear impact: in many parts of the central Appalachians, acid precipitation has decimated 60-80% of spawning habitat. In a process called acid remediation, ecologists have deposited CaCO 3 - rich limestone at the headwaters of streams native to brook trout. The dissolved CO 3 2- moves downstream and raises the water's ph. Acid remediation enhances and in some cases completely restores the brook trout biomass. However, this method treats the symptoms but not the cause of acid precipitation. As long as acid precipitation falls, the treatment must be continued indefinitely to maintain brook trout populations. Ocean acidification. As a result of burning fossil fuels, the amount of carbon dioxide in ocean water is increasing. The carbon dioxide reacts with ocean water to form carbonic acid, which acidifies ocean water. Figure 5 illustrates how ocean acidification may affect ocean life.

12 Figure 5: Ocean acidification. Rising atmospheric carbon dioxide (CO 2 ) levels are damaging the oceans' buffering system. As atmospheric carbon dioxide (CO 2 ) levels increase and dissolve in the ocean, levels of carbonic acid (H 2 CO 3 ) in the ocean rise. Through a series of chemical reactions, this ultimately leads to a decrease in available calcium carbonate (CaCO 3 ). This decrease harms many marine organisms, such as corals and snails, which use CaCO 3 to build shells and similar structures Nature Education All rights reserved. Future perspectives. The Whiskey Creek Shellfish Hatchery in Netarts Bay, Oregon, supplies Pacific oyster (Crassostrea gigas) larvae to regional shellfish growers. Until 2007, the hatchery never had a production problem. Then, for three months during the summer of 2007, the hatchery didn't grow any viable larvae. The hatchery's owners thought that bacterial contamination was to blame and installed a system to clean seawater pumped in from the bay. Although the system helped, productivity problems continued. Alan Barton, an employee at the hatchery, wondered if acidified seawater might be responsible for larval mortality. He sent water samples to researchers at Oregon State University, who confirmed that the ph was abnormally low. Calcium carbonate (CaCO 3 ) comes in two varieties: a more soluble aragonite and a less soluble calcite. Although adult bivalves, including oysters, mussels, and clams, have calcite shells, their larval forms have aragonite shells that are particularly susceptible to acidification. During summer months off the coast of Oregon, upwelling brings cold water to the surface. Calcium carbonate is generally more soluble in cold water, and because of ocean acidification, the cold, upwelled water is already acidic enough to dissolve aragonite. The hatchery now adds soda ash (Na 2 CO 3 ) to increase the ph of acidified water. The solution works well in the hatchery but, for logistical reasons, cannot be used in the open ocean. Many types of plankton, tiny floating

13 plants and animals that are an important food source for many marine fishes, have aragonite shells. Coral reefs, which provide fish habitat and protect human communities from storm surges, are also made from aragonite. The potential effects of ocean acidification on plankton and coral reefs, and on marine ecosystems in general, are poorly understood. Extensive networks are needed to monitor ocean ph, and new research is needed to understand how acidification will impact ocean life. Scientists also believe that new regulations are needed to limit the causes of ocean acidification. Recommendations in the United States include adopting a National Ocean Policy and declaring CO 2 as a pollutant in the Clean Water Act. Establishment of protected marine areas and fishing limits might help species recover from the impacts of ocean acidification. CAREERS Chemical Oceanography Chemical oceanographers study the chemistry of seawater. Marine organisms, ocean sediments, surface runoff, and atmospheric conditions all affect the chemistry of seawater. Nevertheless, seawater composition has remained reasonably constant for millions of years. Today, manmade pollutants are disrupting the ocean's chemistry. A better understanding of seawater chemistry is needed to mitigate effects of pollution and enhance productivity of fisheries. Chemical oceanography requires a strong background in math and science. Good communication skills are also beneficial because chemical oceanographers often work to promote changes in policy and to educate the public about the impact of pollutants on ocean chemistry. Those wishing to become chemical oceanographers typical obtain a basic science undergraduate degree, followed by a master's degree or PhD in chemical oceanography. CAREERS IN THIS MODULE Aqueous Solutions Acids, Bases, and ph Buffers Stabilize ph Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? A Sea of Microbes Drives Global Change Do floating microbes in the ocean s surface waters play an outsize role in global climate? The Climate Connection How is life on Earth reacting to climate change? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View Download SCIENCE ON THE WEB Acid Rain and Pollution Politics. Watch archived news clips about acid rain, starting ph affects movement of drugs across cellular membranes. This interactive animation shows that the

14 direction of flow of certain drugs across cell membranes depends on intracellular ph. page 37 of pages left in this module

15 Principles of Biology contents 7 Aqueous Solutions, Acids, and Bases Summary OBJECTIVE Understand ph and how acids and bases interact with water. Two water molecules can react to form hydronium ions (H 3 O + ) and hydroxide ions (OH - ). Acids are substances that increase the concentration of hydronium ions, and bases are substances that increase the concentration of hydroxide ions. ph, a measure of the acidity of a solution, is defined as the following: ph = -log [H 3 O + ] = -log [H + ] The ph scale ranges from 0 to 14. A neutral solution has a ph of 7. A basic solution has a ph greater than 7, and an acidic solution has a ph that is less than 7. Because the ph scale is logarithmic, a one-unit change in ph is equivalent to a 10-fold change in acidity. OBJECTIVE Describe how a buffer contributes to ph stability. A buffer is a substance that minimizes changes in ph. Weak acids and weak bases are buffers. A buffer reaction, which is reversible, can be written as follows: HA = H + + A - If acid (H + ) is added to the buffer solution, it reacts with A -, and the reaction shifts toward the left. If base (OH - ) is added to the solution, it strips an H + from HA, and the reaction shifts toward the right. OBJECTIVE Understand how ph affects organisms and ecosystems. Many enzymes are exquisitely sensitive to ph, and ph gradients across intracellular compartments are necessary for many cellular functions. Thus, the intracellular ph of both prokaryotic and eukaryotic cells is carefully regulated. Eukaryotic organisms employ an enzyme called carbonate anhydrase to increase the rate of the buffering. Ecosystems are also sensitive to changes in ph. Acid precipitation damages forests and aquatic ecosystems, particularly in regions where the soil is not well buffered. Carbon dioxide levels in ocean water are increasing due to the burning of fossil fuels. Carbon dioxide reacts with water to form carbonic acid, which acidifies ocean water in a process called ocean acidification. Calcium carbonate, which forms the shells and skeletons of many marine organisms, dissolves in acidic conditions. OBJECTIVE Understand the mole, and describe various units of concentration. The mass of a mole of an element equals that element's atomic mass. One mole of a substance contains 6.02 x particles. Concentration is the amount of solute dissolved in water. Units of concentration include grams per liter, percent, and moles per liter (molar, or M). Therefore, molar concentration indicates the number of particles in solution. Key Terms acid Substance that increases the [H 3 O + ] in an aqueous solution. acid precipitation Any form of precipitation in which the ph is lower than "normal" rain; that is, lower than approximately ph 5.6.

16 alkaline Basic; a solution with a ph greater than 7. Avogadro's number The number of particles in a mole; 6.02 x base Substance that decreases the [H 3 O + ] in an aqueous solution. buffer Substance that minimizes changes in ph. Weak acids and weak bases are buffers. hydronium ion H 3 O +, forms when two water molecules react. hydrophilic Any substance that is "water loving" and interacts with water. Polar and charged substances are typically hydrophilic. hydrophobic Any substance that is "water fearing" and repels water. Nonpolar substances are typically hydrophobic. hydroxide ion OH -, forms when two water molecules react. molar concentration The number of moles of solute in one liter of solvent. molar mass The mass of one mole of an element or compound. mole One mole contains Avogadro's number (6.02 x ) of particles; the mass of one mole of an element equals the element's atomic number in grams. ocean acidification As a result of the burning of fossil fuels, CO 2 levels in ocean water are increasing. CO 2 reacts with water to form carbonic acid, which acidifies ocean water. ph Logarithmic scale used to indicate the acidity of a solution, defined as: ph = -log [H 3 O + ] = [H + ] plasma membrane Phospholipid bilayer that forms a hydrophobic barrier around the cell. proton [H + ], the nucleus of a hydrogen atom; acids transfer protons to H 2 O to form [H 3 O + ]. solute The substance that dissolves in a liquid solvent, such as water, to form a solution. solution A homogenous mixture of two or more substances. solvent Liquid in which solutes can be dissolved. IN THIS MODULE Aqueous Solutions Acids, Bases, and ph Buffers Stabilize ph Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER?

17 A Sea of Microbes Drives Global Change Do floating microbes in the ocean s surface waters play an outsize role in global climate? The Climate Connection How is life on Earth reacting to climate change? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View Download SCIENCE ON THE WEB Acid Rain and Pollution Politics. Watch archived news clips about acid rain, starting ph affects movement of drugs across cellular membranes. This interactive animation shows that the direction of flow of certain drugs across cell membranes depends on intracellular ph. page 38 of pages left in this module

18 Principles of Biology contents 7 Aqueous Solutions, Acids, and Bases Test Your Knowledge 1. Sulfur (S) has a molar mass of 32 g/mol. How many moles of sulfur are in 64 grams? 0.5 moles 2 moles 1 mole The answer cannot be determined from the information x Carbonic acid has the formula H 2 CO 3. Given that hydrogen (H), carbon (C), and oxygen (O) have molar masses of 1, 12, and 16, respectively, what is the molar mass of carbonic acid? 30 g/mol 62 M 62 g/mol 6.02 x The answer cannot be determined from the information. 3. The [OH - ] of pickle juice is 10-9 M. What is the ph? The answer cannot be determined from the information given. 4. Given that milk has a ph of 6 and lemon juice has a ph of 2, which statement is correct? Milk is 10,000 times more acidic than lemon juice. Lemon juice is four times more acidic than milk. Lemon juice is 1,000 times more acidic than milk. Lemon juice is 10,000 times more acidic than milk. Lemon juice has a higher hydroxide ion concentration than milk. 5. Complete the following sentence: A buffer is typically a... weak acid or a weak base. strong acid. strong base. hydronium ion. None of the answers are correct. 6. In ocean acidification, the following reaction occurs: CO 2 + H 2 O = H 2 CO 3 = H + + HCO 3 - = 2H + + CO3 2- Which of the following statements about ocean acidification is NOT true? Ocean acidification is already affecting oyster fisheries. Ocean acidification may have a great impact on organisms with calcium carbonate shells and skeletons.

19 As a result of more CO 2 entering the ocean, the level of H 3 O + will increase, which will cause CaCO 3 to dissolve. As a result of more CO 2 entering the ocean, the level of CO 3 2- will increase, which will cause CaCO 3 to dissolve. All of the statements are true. Submit IN THIS MODULE Aqueous Solutions Acids, Bases, and ph Buffers Stabilize ph Summary Test Your Knowledge WHY DOES THIS TOPIC MATTER? A Sea of Microbes Drives Global Change Do floating microbes in the ocean s surface waters play an outsize role in global climate? The Climate Connection How is life on Earth reacting to climate change? PRIMARY LITERATURE How elevated carbon dioxide levels affect coral reefs Losers and winners in coral reefs acclimatized to elevated carbon dioxide concentrations. View Download SCIENCE ON THE WEB Acid Rain and Pollution Politics. Watch archived news clips about acid rain, starting ph affects movement of drugs across cellular membranes. This interactive animation shows that the direction of flow of certain drugs across cell membranes depends on intracellular ph. page 39 of 989