Suggested problems Chapter 3
3.23 Calculate the formula weight of the following molecules to three significant figures.
a) b)
Answer: a) 64.1 amu, b) 137 amu
3.25 Ammonium nitrate, NH4NO3, is used as a nitrogen fertilizer and in explosives. What is the molar mass of NH4NO3? Answer: 80.05 g/mol
3.31 Calculate the mass in grams of each of the following.
a) 0.15 mol Na b) 0.594 mol S
c) 2.78 mol CH2Cl2 d) 38 mol (NH4)2S
3 Answer: a) 3.4 g Na, b) 19.0 g S, c) 236 g CH2Cl2, d) 2.60 x 10 g (NH4)2S
3.35 Obtain the moles of substance in the following.
a) 2.86 g Cu b) 7.05 g Cl2
c) 76 g C4H10 d) 26.2 g Al2(CO3)3
Answer: 0.238 mol, b) 9.94 x 10‐2 mol, c)1.3 mol, d) 0.112 mol
3.39 Calculate the following.
a) number of atoms in 8.21 g Li
b) number of atoms in 32.0 g Br2
c) number of molecules in 45 g NH3
d) number of formula units in 201 g PbCrO4
2‐ e) number of SO4 ions in 14.3 g Cr2(SO4)3
23 23 24 23 22 ‐2 Answer: a) 7.12 x 10 atoms, b) 2.41 x 10 atoms, c) 1.6 x 10 molecules, d) 3.75 x 10 units, e) 6.59 x 10 SO4 ions
3.43 A 1.836 g‐sample of coal contains 1.584 g C. Calculate the mass percentage of C in the sample.
Answer: 86.27%
3.47 A fertilizer is advertised as containing 14.0% nitrogen (by mass). How much nitrogen is there in 4.15 kg of fertilizer?
Answer: 0.581 kg 3.49 A sample of an alloy of aluminum contains 0.0898 mol Al and 0.0381 mol Mg. What are the mass percentages of Al and Mg in the alloy?
Answer: 72.35% Al, 27.7% Mg
3.55 Which contains more carbon, 6.01 g of glucose, C6H12O6, or 5.85 g of ethanol, C2H6O?
Answer: ethanol
3.57 Ethylene glycol is used as an automobile antifreeze and in the manufacture of polyester fibers. The name glycol stems
from the sweet taste of this poisonous compound. Combustion of 6.38 mg of ethylene glycol gives 9.06 mg CO2 and
5.58 mg H2O. The compound contains only C, H and O. What are the mass percentages of the elements in ethylene glycol?
Answer: 38.7% C, 9.79% H, 51.5% O
3.59 An oxide of osmium (symbol Os) is a pale yellow solid. If 2.89 g of the compound contains 2.16 g of osmium, what is its empirical formula?
Answer: OsO4
3.61 Potassium manganate is a dark green, crystalline substance whose composition is 39.6% K, 27.9% Mn, and 32.5% O, by mass. What is the empirical formula?
Answer: K2MnO4
3.63 Acrylic acid, used in the manufacture of acrylic plastics, has the composition 50.0% C, 5.6% H, and 44.4% O. What is its empirical formula?
Answer: C3H4O2
3.65 Two compounds have the same composition: 92.25% C and 7.75% H.
a) Obtain the empirical formula corresponding to this composition.
b) One of the compounds has a molecular weight of 52.03 amu; the other, of 78.05 amu. Obtain the molecular formulas of both compounds.
Answer: a) CH, b) C4H4, C6H6
3.69 Oxalic acid is a toxic substance used by laundries to remove rust stains. Its composition is 26.7% C, 2.2% H, and 71.1% O (by mass), and its molecular weight is 90 amu. What is its molecular formula?
Answer: C2H2O4
3.73 Butane, C4H10, burns with oxygen in air to give carbon dioxide and water.
2 C4H10(g) + 13 O2(g) → 8 CO2 (g) + 10 H2O(g)
What is the amount (in moles) of carbon dioxide produced from 0.30 mol C4H10?
Answer: 1.2 mol CO2 3.81 The following reaction, depicted using molecular models, is used to make carbon tetrachloride, CCl4, a solvent and starting material for the manufacture of fluorocarbon refrigerants and aerosol propellants.
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Calculate the number of grams of carbon disulfide, CS2, needed for a laboratory‐scale reaction with 62.7 g of chlorine.
Answer: 22.4 g CS2
3.83 When dinitrogen pentoxide, N2O5, a white solid, is heated, it decomposes to nitrogen dioxide and oxygen.
∆ 2 N2O5 (g) 4 NO2(g) + O2(g)
If a sample of N2O5 produces 1.315 g O2, how many grams of NO2 are formed?
Answer: 7.563 g NO2
3.85 Potassium superoxide, KO2, is used in rebreathing gas masks to generate oxygen.
4KO2(s) + 2H2O(l) 4KOH(s) + 3O2(g)
If a reaction vessel contains 0.25 mol KO2 and 0.15 mol H2O, what is the limiting reactant? How many moles of oxygen can be produced?
Answer: KO2 is the limiting reactant; 0.19 mol O2
3.87 Methanol, CH3OH, is prepared industrially from the gas‐phase catalytic balanced reaction that has been depicted here using molecular models.
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In a laboratory test, a reaction vessel was filled with 35.4 g CO and 10.2 g H2. How many grams of methanol would be produced in a complete reaction? Which reactant remains unconsumed at the end of the reaction? How many grams of it remain?
Answer: 40.5 g CH3OH; H2 remains, 5.1 g H2
3.89 Titanium, which is used to make airplane engines and frames, can be obtained from titanium tetrachloride, which in turn is obtained from titanium dioxide by the following process:
3TiO2(s) + 4C(s) + 6Cl2(g) 3TiCl4(g) + 2CO2(g) + 2CO(g)
A vessel contains 4.15 g TiO2, 5.67 g C, and 6.78 g Cl2. Suppose the reaction goes to completion as written. How many grams of titanium tetrachloride can be produced?
Answer: 9.07 g
3.91 Aspirin (acetylsalicylic acid) is prepared by heating salicylic acid, C7H6O3, with acetic anhydride, C4H6O3. The other
product is acetic acid, C2H4O2. What is the theoretical yield (in grams) of aspirin, C9H8O4, when 2.00 g of salicylic acid is heated with 4.00 g of acetic anhydride? If the actual yield of aspirin is 1.86 g, what is the percentage yield?
Answer: 2.61 g is the theoretical yield; 71.3%