Unit 8. Chemical Calculations

Index Unit 8. Chemical Calculations 1. - Amount of substance...2 1.1- The mole...2 1.2.- Calculating concentration...3 2.- Chemical reactions...5 2.1.- What is a chemical reaction?...5 2.2.- Balance of chemical reactions...5 2.3.- Types of chemical reactions...6 2.4.- Acid-base reactions...7 3.- Rate of Reaction...9 4.- Energy Changes in Chemical Reactions...10 5.- Fundamental laws of chemical reactions and chemical equations...10 6.- Stoichiometric calculations...11 6.1 Calculation with masses...11 6.2 Calculations with volume...13 6.3 Calculations with volume in different conditions than STP conditions...13 6.4 Calculations with reactants in solution...14 Practice exam...15 Page 1 of 15

1. - Amount of substance 1.1- The mole A mole is defined as the amount of matter that contains as many objects as the number of atoms in exactly 12 grams of 12C. Various experiments have determined that this number is... 6.0221367 x 10 23 This is usually abbreviated as 6.022 x 10 23 for simplification, and is known as Avogadro's number. One mole of atoms, cars, people, etc. contains 6.02 x 10 23 of these objects. Molar mass is the weight of one mole (or 6.022 x 10 23 molecules) of any chemical compound. It is the same number as molecular mass but with different units. Molar mass is expressed in grams. How can we calculate the molar mass? The molar mass can be calculated as the sum of the individual masses (as found in the periodic table) of all the atoms in any molecule, and expressed in grams. Calculate the molar mass of CO2 m = 1 mc + 2 mo= (1 12) + (2 16) = 12 + 32 = 44 g The relation between moles and molar mass m = n. M where m is mass n are moles M is molar mass Exercise with moles: 1.- Calculate the molar mass of: a) H2CO3 b) Mg (OH)2 c) Al2 (SO4)3 d) Cl2O5 e) O2 f) CuSO3 Data of atomic mass in u: H = 1; C = 12; O = 16; Mg = 24; Al = 27 ; S = 32 ; Cl = 35.5 ; Cu = 63.5 Solutions a) 62 g b) 58 g c) 342 g d) 151 g e) 32 g f) 143.5 g 2.- How many atoms are there in 3 moles of potassium? Sol: 1.81 10 24 atoms 3.- If a glass has 3.8 moles of water. How many molecules are there in the glass? Sol: 2.29 10 24 molecules 4.- How many atoms are there in 3 moles of Ag? Sol: 1.8 10 24 atoms Page 2 of 15

5.- How many molecules are there in 3 moles of H2SO4? How many atoms? Sol: 1.8 10 24 molecules1.26 10 25 atoms 6.- How many atoms are there in 2 moles of AgNO3? 7.-How many moles are in 3.011 10 23 atoms of Li? Sol: 6.022 10 24 atoms Sol: 0.5 moles 8.-In a laboratory glass bottle there are 300 g of CuSO4. Work out how many moles of that substance are contained in the glass. Sol 1.88 moles of CuSO 4 9.- How many moles are in 10 g of Na OH? (Na = 23 u ; O = 16 u ; H = 1 u) 10.- How many grams are in 2 moles of H 3 PO 4? ( H = 1 u; P = 31 u; O = 16 u) 11.- How many grams are in 1.8 10 24 molecules of water? ( H = 1 u ; O = 16 u) 12.- How many molecules are in 100 g of P 2 O 5? (P = 31 u ; O = 16 u) Sol: 0.25 moles Sol: 196 g Sol: 53.8 g Sol: 4.2 10 23 molecules 1.2.- Calculating concentration Percent Composition by Mass (%) Volume Percent (% v/v) Molarity (M) mass of s olut e per c ent composition by mass= mass of solution 100 %V = volume percent = v o l ume of s o lu t e v ol ume of solution 100 m ol e s of sol u t e M olar i ty= li t r e s of solution grams/litres(g/l) mass g grams/ litres = volumeof solution 13.- Determine the percent composition by mass of 20 g of salt mixed with 80 g of water. Sol: 20% Page 3 of 15

14.- What is the molarity of a solution made when water is added to 11 g CaCl2 to make 100 ml of solution? (Ca = 40 u ; Cl = 35.5 u) Sol: 1.0 M 15.- Calculate percent composition by mass of a solution containing 45 g of salt in 500 g of water. 16.- Calculate the grams of solute contained in a 250 g solution that is 6 % by mass. Sol: 8.26 % 17.- Calculate percent composition by mass of a solution containing 25 g of glucose in 100 g of water. 18.- Calculate the volume of alcohol contained in a 150 ml solution that is 60 % by volume. Sol: 15 g Sol: 20 % Sol: 90 ml 19.- 75 ml of pure ethyl alcohol is mixed with 250 ml of water. Calculate the volume percent of the solution. 20.- Calculate the molarity of a solution containing 58.8 g of CaI 2 in 1 litre solution. m Ca = 40 u m I = 127 u 21.- Calculate the mass of NaOH contained in 50 ml of a solution 0.6 M. m Na = 23 u m O = 16 u m H = 1 u Sol: 23.08% Sol: 0.2 M Sol: 1.2 g 22.- We have a 0.7 M solution of hydrochloric acid, HCl. To make a chemical reaction we need 0.05 moles of HCl. What is the volume needed? m Cl = 35.5 u m H = 1 u 23.- Calculate the molarity of a solution containing 2.8 g of KOH in 200 ml of solution. m K = 39 u m O = 16 u m H = 1 u 24.- Calculate the mass contained in 500 ml 1 M solution of hydrochloric acid, HCl. m Cl = 35.5 u m H = 1 u 25.- Calculate the molarity of a solution containing 7.9 g of H 2 SO 4 in 284 ml of solution. m S = 32 u m O = 16 u m H = 1 u Sol: 0.071 L Sol:0.25 M Sol: 18.25 g Sol: 0.28 M Page 4 of 15

26.- Calculate the volume of a solution 0.05 M of nitric acid, HNO 3 containing 0.08 moles of HNO 3. m N = 14 u m O = 16 u m H = 1 u Sol: 1.6 L 27.- 3.8 grams of sodium hydroxide, NaOH, are dissolved in methanol until the volume of solution is 20 ml. Express the concentration in grams/litre 2.- Chemical reactions 2.1.- What is a chemical reaction? sol: 190 g/l A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Reactants: The substance (or substances) initially involved in a chemical reaction. Products: The substance (or substances) obtained in a chemical reaction. 2.2.- Balance of chemical reactions When you write an equation for a chemical reaction, the two sides of the equation should balance (you need the same number of each kind of element on both sides.) 1) Cl2 + Ag AgCl 2) S + O2 SO3 3) Br2 + H2 HBr 4) C3H8 + O2 CO2 + H2O 5) H3PO4 + NaOH Na3PO4 + H2O 6) HNO3 + Fe Fe (NO3)2 + H2 7) HCl + Ca (OH)2 CaCl2 + H2O 8) P + O2 P2O5 9) Ag + HNO3 AgNO3 + H2 10) Fe + O2 Fe2O3 11) Mg + H2O Mg(OH)2 + H2 12) Pb + H 2 PbH4 13) Na + O2 Na2O 14) C6H12O6 + O2 CO2 + H2O 15) PbS + O2 PbO + SO2 16) CH4 + O2 CO2 + H2O Page 5 of 15

17) H2SO4 + LiOH Li2SO4 + H2O 18) Br2 + O2 Br2O5 19) Na2CO3 + Ca(OH)2 CaCO3 + NaOH 20) Na3P + H2O PH3 + NaOH 2.3.- Types of chemical reactions a. Depending on the reorganization of their atoms. We have: Synthesis reactions In a synthesis reaction, two or more simple substances combine to form a more complex substance. : A + B AB Cl2 + Ag 2 AgCl Decomposition reactions A decomposition reaction is the opposite of a synthesis reaction, where a more complex substance breaks down into its more simple parts. These reactions are in the general form: AB A + B CaCO3 CaO + CO2 Single replacement reactions In a single replacement reaction, a single uncombined element replaces another in a compound. 2 Ag + 2 HNO3 AgNO3 + H2 Double replacement reactions In a double replacement reaction, parts of two compounds switch places to form two new compounds. These reactions are in the general form: AB + CD AD + CB Pb(NO 3 ) 2 + 2 KI PbI 2 + 2 KNO 3 b. Depending on the mechanism of the reaction. We have: Acid-Base Reaction An acid-base reaction is type of double displacement reaction that occurs between an acid and a base. The H + ion in the acid reacts with the OH - ion in the base to form water and an ionic salt: HA + BOH H 2 O + BA The reaction between hydrobromic acid (HBr) and sodium hydroxide is an example of an acid-base reaction: HBr + NaOH NaBr + H 2 O Page 6 of 15

Oxidation-Reduction or Redox Reaction In a redox reaction the oxidation numbers of atoms are changed. Redox reactions may involve the transfer of electrons between chemical species. Ag 2 O(aq) + H 2 (aq) 2 Ag(s) + H 2 O(l) Precipitation Reactions If a reaction taking place in aqueous solution produces an insoluble product, that product will precipitate, forming a solid product at the bottom of the reaction vessel. Precipitation often happens with double replacement reactions, and one of the most dramatic of such reactions is the reaction between lead nitrate and potassium iodide: 2KI(aq) + Pb(NO 3 ) 2 (aq) -> 2KNO 3 (aq) + PbI(s) Both of the reactants form colorless solutions, but if these solutions are poured together the lead iodide product is a dramatic yellow solid which precipitates out of the solution. Combustion A combustion reaction is a type of redox reaction in which a combustible material combines with an oxidizer to form oxidized products and generate heat (exothermic reaction). Usually in a combustion reaction oxygen combines with another compound to form carbon dioxide and water. An example of a combustion reaction is the burning of naphthalene: C 10 H 8 + 12 O 2 10 CO 2 + 4 H 2 O 28.- Classify the following reactions depending on the reorganization of their atoms: a) 2 Mg + O2 2 MgO b) Zn + H2SO4 ZnSO4 + H2 c) Cu + S CuS d) H2CO3 H2O + CO2 e) Na2CO3 + Ca(OH)2 CaCO3 + 2 NaOH f) 2 NaCl 2 Na + Cl2 g) 2 K + 2 H2O 2 KOH + H2 h) 2 HCl + Zn(OH)2 ZnCl2 + H2O 29.- Balance and classify the following reactions depending on the reorganization of their atoms: a) Cl + KBr KCl + Br b) CaO + Li Ca + Li2O c) HNO3 + Ca(OH)2 Ca(NO3)2 + H2O d) H2O-> H2 + O2 e) KClO3 KCl + O2 2.4.- Acid-base reactions When an acid and a base are placed together, they react to neutralize the acid and base properties, producing a salt. The H(+) cation of the acid combines with the OH(-) anion of the base to form water. The compound formed by the cation of the base and the anion of the acid is called a salt. The combination of hydrochloric acid and sodium hydroxide produces common table salt, NaCl: Page 7 of 15

The word salt is a general term which applies to the products of all such acid-base reactions. Acids properties. When dissolved in water, acids 1.-Conduct electricity 2.-Have a sour taste 3.-React with bases to neutralize their properties 4.- React with active metals to liberate hydrogen. Base Properties When dissolved in water, bases 1. Conduct electricity 2. Have a slippery feeling 3. React with acids to neutralize their properties. ph Scale It is a measure of acidity or alkalinity of water soluble substances (ph stands for 'potential of Hydrogen'). A ph value is a number from 1 to 14, with 7 as the middle (neutral) point. Values below 7 indicate acidity which increases as the number decreases, 1 being the most acidic. Values above 7 indicate alkalinity which increases as the number increases, 14 being the most alkaline. Page 8 of 15

3.- Rate of Reaction The rate of a reaction is the speed at which a reaction happens. If a reaction has a low rate, that means the molecules combine at a slower speed than a reaction with a high rate. Some reactions take hundreds, maybe even thousands, of years while others can happen in less than one second. The rate of reaction depends on the type of molecules that are combining. If you want to think of a very slow reaction, think about how long it took dinosaur bones to become fossils through breakdown. You can thank chemical processes in bacteria for most of those dinosaur bones in the museum. There is another big idea for rates of reaction called collision theory. The collision theory says that as more collisions in a system occur, there will be more combinations of molecules bouncing into each other. If there are a higher number of collisions in a system, more combinations of molecules can occur. The reaction will go faster and the rate of that reaction will be higher. Even though they are both liquids, think about how slowly molecules move in honey when compared to your soda. There are a lower number of collisions in the honey. Reactions happen - no matter what. Chemicals are always combining or breaking down. The reactions happen over and over, but not always at the same speed. A few things affect the overall speed of the reaction and the number of collisions that can occur. Concentration: If there is more of a substance in a system, there is a greater chance that molecules will collide and speed up the rate of the reaction. If there is less of something, there will be fewer collisions and the reaction will probably happen at a slower speed. Sometimes when you are in a chemistry lab, you will add one solution to another. When you want the rate of reaction to be slower, you will add only a few drops at a time instead of the entire beaker. Temperature: When you raise the temperature of a system, the molecules bounce around a lot more because they have more energy. When they bounce around more, they are more likely to collide. That fact means they are also more likely to combine. When you lower the temperature, the molecules are slower and collide less. That temperature drop lowers the rate of the reaction. Back to the chemistry lab! Sometimes you will mix solutions in ice so that the temperature of the system stays cold and the rate of reaction is slower. Pressure: Pressure affects the rate of reaction, especially when you look at gases. When you increase the pressure, the molecules have less space in which they can move. That greater density of molecules increases the number of collisions. When you decrease the pressure, molecules don't hit each other as often. The lower pressure decreases the rate of reaction. Catalyst: Catalysis is the increase in the rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents in the chemical reaction, a catalyst is not consumed. A catalyst may participate in multiple chemical transformations. The opposite of a catalyst, a substance that reduces the rate of a reaction, is an inhibitor. Surface area.this means that the more finely divided a solid or liquid reactant the greater its surface area per unit volume and the more contact it makes with the other reactant, thus the faster the reaction Page 9 of 15

4.- Energy Changes in Chemical Reactions If two substances react and the temperature of the mixture decreases, the reaction is endothermic. If two substances react and the temperature of the mixture increases, the reaction is exothermic. A chemical reaction involves the breaking of bonds in the reactants and the forming of bonds in the products. It takes energy to break bonds. Energy is released when bonds are formed. If a reaction is endothermic, it takes more energy to break the bonds of the reactants than is released when the bonds of the products are formed. If a reaction is exothermic, more energy is released when the bonds of the products are formed than it takes to break the bonds of the reactants. 5.- Fundamental laws of chemical reactions and chemical equations 1.-The law of conservation of mass (Lavoisier, 18th century): Lavoisier was one of the first to carry out quantitatively accurate chemical measurements. He demonstrated that combustion required oxygen, and he demonstrated oxygen's role in the rusting of metals. His observations led him to deduce the following general law known as the law of conservation of mass: In every chemical transformation, an equal quantity of matter exists before and after the reaction. 2.- The law of definite proportions (Joseph Proust, shortly after Lavoisier): Proust studied metal compounds, including metal oxides, carbonates and sulfides. From the work of Robert Boyle in the 17th century, it was understood that substances that could be broken down into more fundamental components were mixtures or compounds. Substances that could not be further broken down were referred to as elements. Thus, Proust deduced the so-called law of definite proportions: In a given chemical compound, the proportion by mass of the elements that compose it are fixed, independent of the origin of the compound or its mode of preparation. This is basically saying that sodium chloride, for example, is always NaCl, no matter how it is obtained, made, or prepared. There are no ``intermediate'' compounds. 3.- The law of multiple proportions: (John Dalton, shortly after Proust): Studied gases and gaseous mixtures under different external conditions. Building on Proust's work, he noted that mathematically discrete manner in which elements combined to form different compounds. For example, in carbon monoxide (CO), the mass ratio of oxygen to carbon and in carbon dioxide (CO 2 ). Thus, since the amount of carbon is fixed in each compound, we can look at how the amount of oxygen varies, and we find that. The generalization of this idea is the law of multiple proportions: When two elements form a series of compounds, the masses of one that combine with a fixed mass of the other are in the ratio of (small) integers to each other. Page 10 of 15

4.-The law of combining volumes (Joseph Gay-Lussac, early 19th century following Dalton): Again, based on studies of gases and how they combine, the law of combining volumes has been attribued to these three. The generalization of the observations is stated as follows: When two gases are allowed to react, such that the gases are at the same temperature and pressure, the volumes of each gas consumed will be in the ratio of small integers. Moreover, the ratio of the volume of each product gas to the volume of either reacting gas will be a ratio of simple integers. Example: 3 volumes of hydrogen + 1 volume of nitrogen = 2 volumes of ammonia. 6.- Stoichiometric calculations Stoichiometry is the study of quantitative relationships involved in chemical reactions. To make stoichiometric calculations you have to follow 4 steps: 1.- Balance the equation. 2.- Convert units of a given substance to moles. 3.- Using the mole ratio, calculate the moles of substance yielded by the reaction. 4.- Convert moles of wanted substance to desired units. 6.1 Calculation with masses Example: What will be the mass of iodine needed to react completely with 7 grams of aluminium? Al + I2 AlI3 Data: mal = 27 u mi = 127 u sol: 99.06 g Activities: 30.- What will be the mass of KCl obtained when we have 5 g of KClO 3? KClO 3 KCl + O 2 Data: mk = 39 u mcl = 35.5 u mo = 16 u sol: 3 g 31.-Given the reaction Mg + O 2 MgO, work out the mass of oxygen which is consumed and the mass of magnesium oxide produced if we get 8.1 g of magnesium to react. mmg = 24 u mo = 16 u Sol: 5.3g O2 13.4g MgO 32. The following reaction is one of the methods for obtaining nitric acid: NO 2 + H 2 O HNO 3 + NO Calculate how many grammes of nitrogen dioxide must react in order to produce 8.8 g of nitric acid. mn = 14 u mo = 16 u mh= 1 u Sol: 9.6 g NO 2 Page 11 of 15

33. Carbon disulphide reacts with oxygen to produce carbon dioxide and sulphur dioxide, both in a gaseous state. Write the equation and calculate the mass of oxygen that is to be consumed so that 100g of carbon dioxide are produced. mc = 12 u mo = 16 u ms = 32 u Sol: 218 g of O 2 34. Ethyl alcohol C 2 H 5 OH burns in the presence of oxygen O 2 in the air thus producing water and carbon dioxide. Calculate the mass of oxygen needed to burn 10.5 g of ethanol. Calculate the masses of water and carbon dioxide obtained. mc = 12 u mo = 16 u mh = 1 u Sol: 21.9g; 12.3g and 20.1g 35. The process of roasting of zinc sulphide is represented by the reaction ZnS (s) + O 2 (g) ZnO (s) + SO 2 (g) a. What amount of oxygen is needed to react with 974 g of zinc sulphide? b. What amount of sulphur dioxide is emitted in the process? mzn = 65.4 u mo = 16 u ms = 32 u Sol:480g ; 640g 36. Hydrochloric acid HCl reacts with calcium carbonate, CaCO 3, and produces calcium chloride, CaCl 2, carbon dioxide CO 2 and water. What mass of water is obtained when 18 g of calcium carbonate react fully? mca= 40 u mc = 12 u mcl = 35.5 u mo = 16 u Sol:3.24 g 6.2 Calculations with volume Example:.- What will be the volume of oxygen measured at STP, obtained when we have 1 kg of KClO 3? KClO 3 KCl + O 2 Data: mk = 39 u mcl = 35.5 u mo = 16 u sol: 274.3 L 37. When zinc is attacked by hydrochloric acid HCl, a dissolution of zinc chloride, ZnCl 2 takes place, and hydrogen gas, H 2 is released. Calculate the volume of this gas as measured in normal conditions, which is obtained by allowing 20 g of Zn to react. mzn = 65.4 u mh = 1 u mcl = 35.5 u Sol: 6.72 L 38. Calculate the volume of ammonia NH 3 which is obtained from 15 L of hydrogen H 2, both at 1 atm and 0ºC, according to the reaction: N 2 (g) + H 2 (g) NH 3 (g) Sol:10 L 39. Iron in contact with the oxygen O 2 in the air is oxidised and iron oxide(iii) is obtained. Calculate the volume of oxygen which is consumed when 8 g of iron come in contact with oxygen, measured in normal conditions of pressure and temperature. mfe = 56 u mo = 16 u Sol:2.4 L Page 12 of 15

40. What volume of oxygen at STP is needed for the complete combustion of 3,3 L of propane (C 3 H 8 )? (Hint: CO 2 and H 2 O are the products as in all combustion reactions) Sol: 16.5 L 6.3 Calculations with volume in different conditions than STP conditions Example: Hydrochloric acid HCl reacts with calcium carbonate, CaCO 3, and produces calcium chloride, CaCl 2, carbon dioxide CO 2 and water. What volume of carbon dioxide, measured at 20 ºC and 0.98 atm is obtained when 18 g of calcium carbonate react fully? mca= 40 u mc = 12 u mcl = 35.5 u mo = 16 u Sol:4.41 L 42. What volume of hydrogen (H 2 ) measured at 30ºC and 780 mm Hg is obtained when 130 g of zinc reacts with an excess of sulphuric acid (H 2 SO 4 ). ZnSO 4 is also produced.? Data: m O = 16; m S = 32: m H = 1; m Zn = 65,4 Solution: 48,18 L 43. 10 g of marble (CaCO 3 ) reacts completely with hydrochloric acid. a) How many grams of carbon dioxide are obtained? b) What volume will it occupy at STP? c) And if it is at 25ºC and 2 atmospheres of pressure? In the chemical reaction also we obtain calcium dichloride and water. Data: m O = 16; m C = 12: m Ca = 40 Solution: a)4,4 g of CO 2 ; b) 2,24 L; c) 1,22 L 44. Calculate the mass of pure iron which will be dissolved in H 2 SO 4 solution if we obtain 89 L of H 2 measured at 90 º C and 710 mm Hg, according to the following chemical reaction: Fe (s) + H 2 SO 4 (aq) FeSO 4 (s) + H 2 (g) Datum: m Fe = 56 Solution: 156.4 g of Fe 45. 1.5 g of magnesium are added to HCl solution. Magnesium chloride and hydrogen are produced. Calculate the volume of hydrogen released at 25ºC and 700 mm Hg of pressure. Datum: m Mg = 24 Solution: 1,66 L of H 2 46) A determined amount of calcium carbonate (CaCO 3 ) has been broken down, by means of heat, in calcium oxide and carbon dioxide. 500 kg of calcium oxide has been obtained. a) Write the corresponding chemical equation. b) Calculate the amount of calcium carbonate before it had broken down. c) What volume of CO 2, at STP, has been obtained? Data: m O = 16; m C = 12: m Ca = 40 Solution: ; b) 892,86 kg; c) 2 10 5 L of CO 2 47. Sulphur trioxide reacts with water to give sulphuric acid. If 100 g of this acid is wanted, what volume of sulphur trioxide is necessary, measured at 40 ºC and 780 mm Hg? Data: m O = 16; m S = 32: m H = 1 Solution: 25.5 L of SO 3 Page 13 of 15

4 Calculations with reactants in solution Example: Given the equation: NaOH + HCl NaCl + H 2 O 25 ml of a 0,2 M hydrochloric acid solution was pipetted into a conical flask and titrated with sodium hydroxide. It was found that 15 ml of the sodium hydroxide was needed to neutralise the acid. Calculate the concentration of the sodium hydroxide. Exercises: 48.- Given the equation: KOH + HNO 3 KNO 3 + H 2 O Sol: 0,33 M 20 ml of a 1,3 M potassium hydroxide solution was pipetted into a conical flask and titrated with nitric acid. It was found that 17 ml of the nitric acid was needed to neutralise the base. Calculate the concentration of the nitric acid. 49.- Given the equation: Ca(OH) 2 + H 3 PO 4 Ca 3 (PO 4 ) 2 + H 2 O Sol: 1.53 M 10 ml of a 0,4 M calcium hydroxide solution was pipetted into a conical flask and titrated with phosphoric acid. It was found that 11 ml of the phosphoric acid was needed to neutralise the base. Calculate the concentration of the phosphoric acid. Sol: 0.24 M 50.- A 3,7 g sample of calcium carbonate is dissolved in water. The final solution has a volume of 500 ml. 25 ml of this solution is then pipetted into a conical flask and titrated with hydrochloric acid. It is found that 20 ml of the hydrochloric acid completely neutralises the calcium carbonate solution. a) What is the molarity of the calcium carbonate solution? b) What is the concentration of the hydrochloric acid? The equation for this reaction is: Data: Ca = 40 u C = 12 u O = 16 u CaCO 3 + HCl CaCl 2 + H 2 O+ CO 2 Sol: 0.185 M 51.- 4,9 g of sulfuric acid is dissolved in water and the final solution has a volume of 220 cm3. Using an acid-base titration, it was found that 20 ml of this solution was able to completely neutralise 10 ml of a sodium hydroxide solution. Calculate the concentration of the sodium hydroxide. The equation is: Data: S = 32u H = 1 u O = 16 u H 2 SO 4 + NaOH Na 2 SO 4 + H 2 O Sol: 0.909 M Page 14 of 15

Practice exam 1.- a) Define: reactant, product, chemical reaction, acid-base reaction and endothermic reaction b) Write the Law of Conservation of Mass and the law of definite proportions. 2. Balance the following reactions: a) N2 + O2 N2O3 b) FeS + HCl FeCl2 + H2S c) C5H12 + O2 CO2 + H2O d) H2O H2 + O2 e) HCl + Al AlCl3 + H2 3.- Classify the following chemical reactions depending on the reorganization of their atoms: a) N2 + O2 N2O3 b) FeS + HCl FeCl2 + H2S c) H2O H2 + O2 d) HCl + Al AlCl3 + H2 4.- Sodium carbonate (Na2CO3) reacts with hydrochloric acid (HCl) as seen in the following: Na2CO3 + HCl NaCl + CO2 + H2O Calculate the mass of CO2 and water that result from a complete reaction of 15 g of sodium carbonate in enough hydrochloric acid. Data: mc = 12 u mna = 23 u mo = 16 u mh= 1 u Sol: a) 6.23 g b) 2.55 g 5.- Calculate the volume of C4H10 that we need to burn to produce 5 L of carbon dioxide. All the gases are measured at STP. The chemical reaction is: C4H10 + O2 CO2 + H2O 6.- If it takes 54 ml of 0.1 M NaOH to neutralize 125 ml of an HCl solution, what is the concentration of the HCl? 7.- Find the molarity of the following solutions: 1) 0.5 moles of sodium chloride is dissolved to make 0.05 L of solution. 2) 0.5 grams of sodium chloride is dissolved to make 0.05 L of solution. Data: Cl 0 35.5 u Na 0 23 u Sol: 1.25 L Sol: 0.043 M Sol: 10 M, 0.17 M 8.- a) work out how many molecules and how many atoms are contained in 34 g of phosphorus trichloride b) What mass of this gas is contained in 98 L measured at 20º C and 780 mm Hg? Sol: a) 1.5 10 23, 6 10 23 b) 575.6 g Page 15 of 15