Name: Date: M O L A R M A S S & P E R C E N T C O M P O S I T I O N

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1 Name: Date: M O L A R M A S S & P E R C E N T C O M P O S I T I O N I. Molar Masses Given a periodic table, you should be able to calculate the molecular mass (in amu s) or the molar mass (in grams) for any element or compound. Examples: (Give answers to four significant figures.) H 2 SO 4 Cl 2 Ca(OH) 2 HC 2 H 3 O 2 CO 2 N 2 O NaOCl Al 2 S 3 II. Fraction and Percent Composition It is useful to determine how much of a compound s mass is made up of each element. Water, H 2 O, for example has a molar mass of g. The H s mass is 2(1.008) = g. The O s mass is g. We can set up fractions for each element: H = = = 11.19%. O = = = 88.79% This is called the percent composition. The fraction composition is a good in-between step. Determine the fraction and percent composition of each element below: 1. H 2 SO 4 2. Ca(OH) 2 3. HC 2 H 3 O 2 4. CO 2 5. N 2 O 6. NaOCl 7. Al 2 S 3

2 Name Date: C O M B U S T I O N E Q U A T I O N S For burning to occur, you need a fuel, an oxidizer, and heat. When hydrocarbons are the fuel and O 2 in the air is the oxidizer, then CO 2 and H 2 O are the products. Example: Solution: Write the balanced equation for the complete combustion of propane, C 3 H 8, in air. First, set up the basic equation. You memorize the + O 2 CO 2 + H 2 O part. C 3 H 8 + O 2 CO 2 + H 2 O Next, balance. 3 C s in C 3 H 8 result in 3CO 2 s; 8 H s in C 3 H 8 result in 4 H 2 O s; C 3 H 8 + O 2 3 CO H 2 O Total O s on the product side = 10 [(3 x 2) + (4 x 1)] = total O s on the reactant side. This would mean that 5 O 2 s were involved. Tip: If an UNEVEN number of O s need to be represented, a fraction should be used. 7 O s = 7 / 2 O 2 Tip: Take into account fuels that contain oxygen. Subtract the O s from that represented as O 2 s Practice: Write the balanced combustion equations for the following substances. 1. CH 4 2. C 5 H C 9 H C 2 H 6 5. C 8 H C 4 H C 2 H 5 OH 8. C 3 H 7 OH 9. HC 2 H 3 O CH 3 COCH 3

3 Name Date: B A L A N C I N G E Q U A T I O N S 1. ZnS + HCl ZnCl 2 + H 2 S 2. HCl + Cr CrCl 2 + H 2 3. Al + Fe 3 O 4 Al 2 O 3 + Fe 4. H 2 + Br 2 HBr 5. Na 2 S 2 O 3 + I 2 NaI + Na 2 S 4 O 6 6. LaCl 3 + Na 2 CO 3 La 2 (CO 3 ) 3 + NaCl 7. NH 4 Cl + Ba(OH) 2 BaCl 2 + NH 3 + H 2 O 8. Ca(OH) 2 + H 3 PO 4 Ca 3 (PO 4 ) 2 + H 2 O 9. La 2 (CO 3 ) 3 + H 2 SO 4 La 2 (SO 4 ) 3 + H 2 O + CO Na 2 O + (NH 4 ) 2 SO 4 Na 2 SO 4 + H 2 O + NH C 4 H 10 + O 2 CO 2 + H 2 O 12. C 7 H 6 O 2 + O 2 CO 2 + H 2 O 13. P 4 O 10 + H 2 O H 3 PO FeS 2 + O 2 Fe 2 O 3 + SO NH 3 + O 2 NO + H 2 O 16. Fe + HCl H 2 + FeCl PbO 2 + HCl H 2 O + PbCl 2 + Cl Fe 2 O 3 + H 2 SO 4 Fe 2 (SO 4 ) 3 + H 2 O 19. NO 2 + H 2 O NO + HNO C 2 H 6 S + O 2 CO 2 + H 2 O + SO 2 Complete combustion: 21. C 6 H C 2 H 5 OH 23. C 3 H 7 OH 24. C 6 H C 17 H 35 COOH

4 Name Date: S T O I C H I O M E T R Y P R O B L E M S General Stoichiometry 1. Several brands of antacid tablets use aluminum hydroxide to neutralize excess acid. Al(OH) 3 (s) + 3 HCl(aq) AlCl 3 (aq) + 3 H 2 O(l) What quantity of HCl, in grams, can a tablet with g of Al(OH) 3 consume? What quantity of water is produced? 2. If 10.0 g of carbon is combined with an exact, stoichiometric amount of oxygen (26.6 g) to produce carbon dioxide, what mass, in grams, of CO 2 can be obtained? That is, what is the theoretical yield of CO 2? 3. The equation for one of the reactions in the process of reducing iron ore to the metal is Fe 2 O 3 (s) + 3 CO(g) 2 Fe(s) + 3 CO 2 (g) (a) What is the maximum mass of iron, in grams, that can be obtained from 454 g (1.00 lb) of iron(iii) oxide? (b) What mass of CO is required to reduce the iron(iii) oxide to iron metal? 4. Burning coal and oil in a power plant produces pollutants such as sulfur dioxide, SO 2. The sulfur-containing compound can be removed from other waste gases, however, by the following reaction: 2 SO 2 (g) + 2 CaCO 3 (s) + O 2 (g) 2 CaSO 4 (s) + 2 CO 2 (g) (a) Name the compounds involved in the reaction. (b) What mass of CaCO 3 is required to remove 155 g of SO 2? (c) What mass of CaSO 4 is formed when 155 g SO 2 is consumed completely? 5. Your body deals with excess nitrogen by excreting it in the form of urea, NH 2 CONH 2. The reaction producing it is the combination of arginine (C 6 H 14 N 4 O 2 ) with water to give urea and ornithine (C 5 H 12 N 2 O 2 ). C 6 H 14 N 4 O 2 + H 2 O NH 2 CONH 2 + C 5 H 12 N 2 O 2 If you excrete 95 mg of urea, what quantity of arginine must have been used? What quantity of ornithine must have been produced?

5 Limiting Reactants 6. The reaction of methane and water is one way to prepare hydrogen: CH 4 (g) + H 2 O(g) CO(g) + 3 H 2 (g) If you begin with 995 g of CH 4 and 2510 g of water, what is the maximum possible yield of H 2? 7. Disulfur dichloride, S 2 Cl 2, is used to vulcanize rubber. It can be made by treating molten sulfur with gaseous chlorine: S 8 (l) + 4 Cl 2 (g) 4 S 2 Cl 2 (l) Starting with a mixture of 32.0 g of sulfur and 71.0 g of Cl 2, which is the limiting reactant? What mass of S 2 Cl 2 (in grams) can be produced? What mass of the excess reactant remains when the limiting reactant is consumed? 8. Aspirin (C 9 H 8 O 4 ) is produced by the reaction of salicylic acid (C 7 H 6 O 3 ) and acetic anhydride (C 4 H 6 O 3 ) (page 163). C 7 H 6 O 3 (s) + C 4 H 6 O 3 (l) C 9 H 8 O 4 (s) + CH 3 CO 2 H(aq) If you mix 100. g of each of the reactants, what is the maximum mass of aspirin that can be obtained? Percent Yield 9. Diborane, B 2 H 6, is a valuable compound in the synthesis of new organic compounds. One of several ways this born compound can be made is by the reaction 2 NaBH 4 (s) + I 2 (s) B 2 H 6 (g) + 2 NaI(s) + H 2 (g) Suppose you use g of NaBH 4 with an excess of iodine and obtain g of B 2 H 6. What is the percent yield of B 2 H 6? 10. Disulfur dichloride, which has a revolting smell, can be prepared by directly combining S8 and Cl2, but it can also be made by the following reaction: 3 SCl 2 (l) + 4 NaF(s) SF 4 (g) + S 2 Cl 2 (l) + 4 NaCl(s) Assume you begin with 5.23 g of SCl 2 and excess NaF. What is the theoretical yield of S 2 Cl 2? If only 1.19 g of S 2 Cl 2 is obtained, what is the percent yield of the compound?

6 Name Date: CHEMICAL ANALYSIS Do all of your work on another sheet of paper. Chemical Analysis 1. A mixture of CuSO 4 and CuSO 4 5H 2 O has a mass of g, but, after heating to drive off all the water, the mass is only g. What is the weight percent of CuSO 4 5H 2 O in the mixture? 2. A 1.25-g sample contains some of the very reactive compound Al(C 6 H 5 ) 3. On treating the compound with aqueous HCl, g of C 6 H 6 is obtained. Al(C 6 H 5 ) 3 (s) + 3HCl(aq) AlCl 3 (aq) + 3C 6 H 6 (l) Assuming that Al(C 6 H 5 ) 3 was converted completely to products, what is the weight percent of Al(C 6 H 5 ) 3 in original g sample? Determination of Empirical Formulas 3. Styrene, the building block of polystyrene, is a hydrocarbon, a compound consisting only of C and H. If g of styrene is burned in oxygen and produces g of CO 2 and g of H 2 O, what is the empirical formula of styrene? 4. Menthol, from the oil of mint, has a characteristic cool taste. The compound contains only C, H, and O. If 95.6 mg of menthol burns completely in O 2, and gives 269 mg of CO 2 and 110 mg of H 2 O, what is the empirical formula of menthol? 5. Silicon and hydrogen form a series of compounds with the general formula Si x H y. to find the formula of one of them, a 6.22-g sample of the compound is burned in oxygen. On doing so, all of the Si is converted to g of SiO 2 and all of the H to g of H 2 O. What is the empirical formula of the silicon compound? AP Question An organic compound was synthesized and found to contain only C, H, N, O, and Cl. It was observed that when a g sample of the compound was burned, it produced g of CO 2 and g of H 2 O. All the nitrogen in a different g sample of the compound was converted to NH 3, which was found to have a mass of g. Finally, the chlorine in a g sample of the compound was converted to AgCl. The AgCl, when dried, was found to weigh g. (a) (b) Calculate the percent by mass of each element in the compound. Determine the empirical formula for the compound.