1 Bio 105 Lab 3: Chemistry: ph and solutions Part 1. Acid and Base Chemistry A. Introduction BIO 105 Summer 2013 Name One of the most important concepts in biology is acid/base chemistry. We are familiar with acids and bases in our daily life. We encounter acidic and basic solutions everyday. A few common acids include lemon juice and vinegar; common bases include ammonia and many household cleaners. Acids are defined as substances capable of donating a hydrogen ion, whereas bases are capable of accepting hydrogen ions. Acids increase the concentration of hydrogen ions by breaking into hydrogen (H + ) ions and another compound when placed in an aqueous solution. For example, hydrochloric acid (HCl) dissolves in water as follows: HCl H + + Cl - Bases decrease the concentration of hydrogen ions in a solution by accepting them. For example, a typical base is sodium hydroxide (NaOH), which in the presence of water ionizes into Na + and OH - (the H + acceptor): NaOH Na + + OH - If we were to add an acid and base together, the base neutralizes the acid by taking up the H + ions. The resulting products are water and a salt. ACID BASE HCl + NaOH NaCl + H 2 O Scientists use the ph scale to measure the strength of acids and bases. The concentration of free hydrogen ions (H + ) in the solution determines the ph. The ph scale ranges from 0 to 14.
2 Pure water (H 2 O) has a [H + ] = 1 x 10-7 mol/l of solution and a ph of 7. HCl has a [H + ] = 1 x 10-1 and a ph of 1. Does water or HCl have a higher [H + ]? Is HCl more or less acidic than water? Solutions with higher concentrations of H + are acidic and have a ph<7. Basic solutions have lower concentrations of H + and a ph>7. It is important to remember that there is an inverse relationship between ph and [H + ]. Each point increase or decrease in ph represents a ten-fold difference in H + concentration. A simple way to think about ph is that it equals the exponent on the H + concentration, ignoring the minus sign. NaOH has a [H + ] = 1 x 10-14 What is the ph of NaOH? Is NaOH considered an acid or base? [H + ] ph Example 1 X 10 0 0 HCl 1 x 10-1 1 Stomach acid 1 x 10-2 2 Lemon juice Acids 1 x 10-3 3 Vinegar 1 x 10-4 4 Soda 1 x 10-5 5 Rainwater 1 x 10-6 6 Milk Neutral 1 x 10-7 7 Pure water 1 x 10-8 8 Egg whites 1 x 10-9 9 Baking soda 1 x 10-10 10 Tums antacid Bases 1 x 10-11 11 Ammonia 1 x 10-12 12 Mineral lime - Ca(OH) 2 1 x 10-13 13 Drano 1 x 10-14 14 NaOH
3 B. Measuring ph The easiest way to measure the ph of a solution is to use ph paper. This paper has been treated with chemicals that change color depending on the H + concentration. Procedure 1: We will be measuring the ph of common solutions Step 1: Using a transfer pipet, apply one drop of each of the solutions on a strip of ph paper. Step 2: Consult the indicator chart to determine the ph of the solution. Step 3: Record your results in the table below. Vinegar Cola Milk Baking soda solution Tap water Orange juice Coffee Tea ph C. Antacids The stomach produces hydrochloric acid (HCl) to digest food. Acid reflux into the esophagus can cause heart burn. Antacids work by neutralizing H + in the stomach or by limiting the ability of the stomach to produce HCl. We will next test the effectiveness of various brands of antacids. Procedure 2: Testing antacids Step 1: Crush antacid into 100 ml of water, stir until dissolved. Step 2: Pipet 5 ml of antacid solution into test tube Step 3: Add 4 drops of indicator bromcresol purple to the tube, mix. Step 4: Add 0.1 M HCl dropwise to the tube, mixing after each drop is added. Continue adding drops of HCl until the solution turns yellow. This indicates that the solution is now acidic (below ph of 5.2). Step 5: Record how many drops of HCl were added to the tube. Step 6: Repeat these steps for each brand of antacid. Step 7: Repeat steps 3-6 with no antacid added Brand of antacid - Number of acid drops added
4 1. Which antacid absorbed the most acid? 2. Which absorbed the least? 3. Which antacid worked the best? Part 2: Solutions A. Introduction: A solution is a mixture of two or more substances: the solutes are the substances that are dissolved, the solvent is what the solute is dissolved in. For example, if you added sugar into your tea, the sugar is the solute and the tea is the solvent. In biological preparations, the solvent is usually water. Solubility is the maximum quantity of solute that can be dissolved in a specific amount of solvent. Concentration is the amount of a solute in a given amount of solution, for example grams of solute/liter of solution. Sometimes you will be asked to make a solution that is concentrated to a certain extent. i.e. How would you prepare an aqueous 15% solution of ethanol? This question is asking you to make a solution that is 15% ethanol. However, in order to answer this question we need more information. Suppose we re-phrase the question to be more accurate: Example 1: How would you prepare 100 ml of an aqueous 15% v/v solution of ethanol? What does v/v mean? The solute and solvent are both measured by volume. 100 * (0.15) = 15, so we will need 15 ml of ethanol. Since the total volume of the solution has to be 100 ml, 100-15 ml = 85, so 85 ml of the solution will be the solvent (in this case water). 15 + 85 = 100 ml. Example 2: A solution may also be made up in terms of relative weights. If we made up a solution with 5 grams of powder and 100 grams of liquid both measures are by weight so that the solution is weight by weight, i.e. w/w. 4. How would you prepare 500 ml of an aqueous 8% w/w solution of NaCl? 5. How would you prepare 150 ml of an aqueous 8% v/v solution of NaCl?
5 Molarity is a measure of concentration based on the mass of the solute. The concentration of the solution is expressed as molar (M). A one molar solution is a 1 mol/l solution. Remember that moles are based on the molecular weight, so a 1 molar solution of NaCl is the MW g/1 L or 58 g NaCl/1 L salt solution. Often you will need to make a dilution of a concentrated solution (the stock solution). For example if you need a final concentration of 1 x 10-6 M it would be hard to weigh out the small quantity of solute needed. Therefore it is often better to make a more concentrated solution and then dilute it to the needed concentration. You can use the following conversion formula to make dilutions: Dilution Equation: C 1 V 1 = C 2 V 2 (units need to be the same on both sides of the equation) Where: C 1 = Concentration of initial (stock) solution V 1 = Volume of initial solution to be used C 2 = Concentration of final dilute solution V 2 = Volume of final dilute solution Show your work as you solve the following problems: 6. You are asked to prepare 100 ml of a 0.650 M aqueous solution of CaCl 2. The stock solution that is available to you is a 2 M CaCl 2 solution. a. How much of the stock solution will you need? b. How much water will you need to add to the stock in order to get the correct final volume?
6 7. Suppose that you have a stock solution of a 200 mg/dl dye solution and you are asked to make 1.5 ml of a 100 mg/dl dye solution. a. How much stock will you need? b. How much water? 8. You are given a 100 mm NaCl stock solution and are asked to make 5 ml of a 20 mm NaCl solution. a. How much stock will you need? b. How much water?