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AP Chem Problem Set # 3

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Atomic and Molecular Weights

1. (a)What isotope is used as the standard in establishing the atomic mass scale? (b) The atomic weight of magnesium is reported as 24.3, yet no atom of magnesium has the mass of 24.3 amu. Explain.

2. Only two isotopes of copper occur naturally, 63Cu (mass = 62.9298 amu); abundance 69.09 percent) and 65Cu (mass = 64.9278 amu; abundance 30.91 percent). Calculate the average atomic mass (atomic weight) of copper.

3. Determine the formula weights of each of the following compounds: (a) N2O5; (b) FeCO3; (c) Ca(C2H3O2)2; (d) (NH4)3PO4; (e) sodium nitrate; (f) copper(II) sulfate; (g) disilicon hexabromide.

4. Calculate the percentage by mass of oxygen in the following compounds: (a) NO2; (b) CH3COOCH3; (c) Cr(NO3)3; (d) (NH4)2CO3.

The Mole

5. (a) Define the term mole. (b) What is Avogadros number, and how is it related to the mole? (c) What is the relationship between the formula weight of a substance and its molar mass?

6. The molecular formula of aspartame, the artificial sweetener marketed as NutraSweet, is C14H18N2O5. (a) What is the molar mass of aspartame? (b) How many moles of aspartame are present in 1.00 mg of aspartame? (c) How many molecules of aspartame are present in 1.00 mg of aspartame? (d) How many hydrogen atoms are present in 1.00 mg of aspartame?

7. A sample of glucose, C6H12O6, contains 2.03 X 1021 atoms of carbon. (a) How many atoms of hydrogen does it contain? (b) How many molecules of glucose does it contain? (c) How many moles of glucose does it contain? (d) What is the mass of this sample in grams?

8. (a) How many moles of chloride ions are in 0.0750 g of magnesium chloride? (b) What is the mass, in grams, of 3.50 X 10-3 mol of aluminum sulfate? (c) What is the mass, in grams, of 1.75 X 1020 molecules of caffeine, C8H10N4O2? (d) What is the molar mass of cholesterol if 0.00105 mol weighs 0.406 g?

9. Calculate the number of molecules in (a) 0.0666 mol propane, C3H8, a hydrocarbon fuel; (b) a 50.0-mg tablet of acetaminophen, C8H9O2N, an analgesic sold under the name Tylenol; (c) a tablespoon of table sugar, C12H22O11, weighing 10.5 g.

Empirical Formulas

10. (a) What is the difference between an empirical formula and a molecular formula? (b) Styrene, a hydrocarbon used to make Styrofoam cups and insulation, has an empirical formula of CH and a molar mass of 104 g/mol. What is its molecular formula?

11. Determine the empirical formulas of the compounds with the following compositions by mass: (a) 10.4 percent C, 27.8 percent S, and 61.7 percent Cl; (b) 21.7 percent C, 9.6 percent H, and 68.7 percent F.

12. What is the molecular formula of each of the following compounds? (a) empirical formula CH2, molar mass = 84 g/mol (b) empirical formula NH2Cl, molar mass = 51.5 g/mol.

13. Determine the empirical and molecular formulas of each of the following substances: (a) ibuprofen, a headache remedy contains 75.69 percent C, 8.80 percent H, and 15.51 percent O by mass; molar mass about 206 g; (b) epinephrine (adrenaline), a hormone secreted into the bloodstream in times of danger or stress: 59.0 percent C, 7.1 percent H, 26.2 percent O, and 7.7 percent N by mass; MW about 180 amu.

14. (a) Combustion analysis of toluene, a common organic solvent, gives 5.86 mg of CO2 and 1.37 mg of H2O. If the compound contains only carbon and hydrogen, what is its empirical formula? (b) Menthol, the substance we can smell in mentholated cough drops, is composed of C, H, and O. A 0.1005-g sample of menthol is combusted, producing 0.2829 g of CO2 and 0.1159 g of H2O. What is the empirical formula for menthol? If the compound has a molar mass of 156 g/mol, what is its molecular formula?

15. Epsom salts, a strong laxative used in veterinary medicine, is a hydrate, which means that a certain number of water molecules are included in the solid structure. The formula for Epsom salts can be written as MgSO4 xH2O, where x indicates the number of moles of water per mole of MgSO4. When 5.061 g of this hydrate is heated to 250 C, all the water of hydration is lost, leaving 2.472 g of MgSO4. What is the value of x?

Calculations Based on Chemical Equations

16. The fermentation of glucose, C6H12O6, produces ethyl alcohol, C2H5OH, and CO2: C6H12O6(aq) 2C2H5OH(aq) + 2CO2(g)

(a) How many moles of CO2 are produced when 0.300 mol of C6H12O6 reacts in this fashion? (b) How many grams of C6H12O6 are needed to form 2.00 g of C2H5OH? (c) How many grams of CO2 form when 2.00 g of C2H5OH are produced?

17. Aluminum sulfide reacts with water to form aluminum hydroxide and hydrogen sulfide. (a) Write the balanced chemical equation for this reaction. (b) How many grams of aluminum hydroxide are obtained from 10.5 g of aluminum sulfide?

18. Automotive air bags inflate when sodium azide, NaN3, rapidly decomposes to its component elements: 2NaN3(s) Na(s) + 3N2(g)

19. (a) How many moles of N2 are produced by the decomposition of 1.50 moles of NaN3? (b) How many grams of NaN3 are required to form 5.00 g of nitrogen gas? (c) How many grams of NaN3 are required to produce 10.0 ft3 of nitrogen gas if the gas has a density of 1.25 g/L?

20. A piece of aluminum foil 0.550 mm thick and 1.00 cm square is allowed to react with bromine to form aluminum bromide. (a) How many moles of aluminum were used? (The density of aluminum is 2.699 g/cm3.) (b) how many grams of aluminum bromide form, assuming that the aluminum reacts completely?

Limiting Reactants; Theoretical Yields

21. A manufacturer of bicycles has 5050 wheels, 3013 frames, and 2455 handlebars. (a) How many bicycles can be manufactured using these parts? (b) How many parts of each kind are left over? (c) Which part is like a limiting reactant in that it limits the production of bicycles?

22. The fizz produced when an Alka-Seltzer tablet is dissolved in water is due to the reaction between sodium bicarbonate, NaHCO3, and citric acid, H3C6H5O7:

3NaHCO3(aq) + H3C6H5O7(aq) 3CO2(g) + 3H2O(l) + Na3C6H5O7(aq)

In a certain experiment 1.00 g of sodium bicarbonate and 1.00 g of citric acid are allowed to react. (a) Which reactant is the limiting reactant? (b) How many grams of carbon dioxide form? (c) How much of the excess reactant remains after the limiting reactants is completely consumed?

23. When hydrogen sulfide gas is bubbled into a solution of sodium hydroxide, the reaction forms sodium sulfide and water. How many grams of sodium sulfide are formed if 2.50 g of hydrogen sulfide is bubbled into a solution containing 1.85 g of sodium hydroxide, assuming that the limiting reagent is completely consumed?

24. Solutions of sulfuric acid and lead(II) acetate react to form solid lead(II) sulfate and a solution of acetic acid. If 10.0 g of sulfuric acid and 10.0 g of lead(II) acetate are mixed, calculate the number of grams of sulfuric acid, lead(II) acetate, lead(II) sulfate, and acetic acid present in the mixture after the reaction is complete.

25. A student reacts benzene, C6H6, with bromine, Br2, to prepare bromobenzene, C6H5Br: C6H6 + Br2 C6H5Br + HBr

(a) What is the theoretical yield of bromobenzene in this reaction when 30.0 g of benzene reacts with 65.0 g of bromine? (b) If the actual yield of bromobenzene was 56.7 g, what was the percentage yield?

Additional Exercises

26. The molecule pyridine, C5H5N, adsorbs on the surfaces of certain metal oxides. A 5.0-g sample of finely divided zinc oxide, ZnO, was found to adsorb 0.068 g of pyridine. (a) How many pyridine molecules are adsorbed? (b) What is the ratio of pyridine molecules to formula units of zinc oxide? (c) If the surface area of the oxide is 48 m2/g, what is the average area of surface per adsorbed pyridine molecule?

27. The fat stored in the hump of a camel is a source of both energy and water. Calculate the mass of H2O produced by metabolism of 1.0 kg of fat, assuming the fat consists entirely of tristearin (C57H110O6), a typical animal fat, and assuming that during metabolism, tristearin reacts with O2 to form only CO2 and H2O.

Integrative Exercises

28. How many kilograms of CO2 are obtained when a sample of 0.15 m3 of C8H18(l), whose density is 0.69 g/mL, is combusted in air?

Solution Composition; Molarity

29. A certain volume of 0.50 M solution contains 4.5 g of a certain salt. What mass of the salt is present in the same volume of a 2.50 M solution?

30. (a) Calculate the molarity of a solution that contains 0.0345 mol NH4Cl in 400 mL of solution. (b)How many moles of HNO3 are present in 35.0 mL of a 2.20 M solution of nitric acid? (c) How many milliliters of 1.50 M KOH solution are needed to supply 0.125 mol of KOH?

31. Calculate (a) the number of grams of solute in 0.250 L of 0.150 M KBr; (b) the molar concentration of a solution containing 4.75 g of Ca(NO3)2 in 0.200 L; (c) the volume of 1.50 M Na3PO4 in milliliters that contains 5.00 g of solute.

32. Which will have the highest concentration of potassium ion: 0.20 M KCl; 0.15 M K2CrO4, or 0.080 M K3PO4?

33. Indicate the concentration of each ion or molecule present in the following solutions: (a) 0.14 M NaOH; (b) 0.25 M CaBr2; (c) 0.25 M CH3OH; (d) a mixture of 50.0 mL of 0.20 M KClO3 and 25.0 mL of 0.20 M Na2SO4. Assume the volumes are additive.