Chem 121 Problem set III - 1
Problem Set III Stoichiometry
Molar mass and numbers of atoms
1. What is the mass of 2.25 moles of iron?
2. How many zinc atoms are present in 20.0 g Zn?
3. How many molecules of ethane (C2H6) are present in 50.3 g of ethane?
4. Calculate the mass of carbon in 125.0 grams of aniline, C6H7N. This compound is the starting material for
many dyes.
5. A 40.0 mg sample of the compound X4O10 contains 22.5 mg of oxygen atoms. What is the atomic mass of
element X?
6. A sample of a compound with the formula MCl2.2H2O has a mass of 0.642 g. When the compound is heated
to remove the water of hydration (represented by 2H2O in the formula), 0.0949 g of water is collected. What
element is M?
7. A 1.443 g sample of a metal is reacted with excess oxygen to yield 1.683 g of the oxide M2O3. Calculate the
atomic mass of the element M.
% Composition
8. Calculate the weight percent of each element in each of the following compounds:
(a) C2H5OH
%C = 52.144, %H = 13.127, %O = 34.729
(b) Ca(HCO3)2
%ca = 24.72, %C = 14.82, %H = 1.24, %O = 59.22
(c) MgNH4PO4
%Mg = 17.70, %N = 10.20, %H = 2.936, %P = 22.56, %O = 46.61
9. When a 0.761 g sample of a compound of C and H is burned in an combustion apparatus, 2.23 g CO2 and
1.37 g H2O are produced. Determine the % composition of the compound.
Empirical Formula and Molecular Formula
10. An organic compound (0.1014 g) which contains only carbon, hydrogen and oxygen, was combusted to give
0.1486g of carbon dioxide and 0.0609 g of water. Find the empirical formula of the compound.
11. What is the empirical formula of the compound with the following composition:
32.79% Na, 13.02% Al, 54.19% F.
12. An oxide of lanthanum (LaxOy) was reduced with hydrogen gas to yield the pure metal.
? LaxOy(s) + ? H2(g) → ? La(s) + ? H2O(l)
A 0.3167 g sample of lanthanum oxide gave 0.05254 g of water. Calculate the empirical formula of lanthanum
oxide. Show all your work and be careful with significant figures.
13. In an experiment, titanium was reacted with excess sulphur to form titanium sulphide. Amazingly, 100% of
the titanium reacted formed the desired product. The following data were obtained:
Weight of crucible
11.120 g
Weight of titanium
8.820 g
Weight of crucible and product 31.700 g
a) What is the percent composition of titanium sulphide?
b) What is the empirical formula for titanium sulphide?
Chem 121 Problem set III - 2
14. An oxide of molybdenum with the chemical formula Mo2O3(s) is converted completely to another oxide. The
Mo2O3(s) had a mass of 12.64 g and the new oxide had a mass of 13.48 g. Determine the empirical formula of
the new oxide.
Balancing Equations
15. Balance the following equations:
N2O5(g) + H2O(l) → HNO3(aq)
Mg2C3(s) + H2O(l) → Mg(OH)2(s) + C3H4(g)
PCl5(l) + H2O(l) → H3PO4(aq) + HCl(aq)
Cr(s) + S8(s) → Cr2S3(s)
Au2S3(s) + H2(g) → Au(s) + H2S(g)
NH4ClO4(s) + Al(s) → Al2O3(s) + N2(g) + HCl(g) +
H2O(g)
16. Balance the reaction in which sodium metal, reacts with bromine molecules, to make ionic sodium bromide,
NaBr.
17. Write and balance the equation for the complete combustion of butane, C4H10, a common fuel.
Calculations based on equations
18. Gaseous ammonia is reacted with red-hot copper(II)oxide (CuO) to form N2 and metallic copper.
a) How many moles of CuO react with 0.445 moles of ammonia?
b) How many grams of N2 can form from 3.18 moles of CuO in the presence of excess ammonia?
c) How many grams of ammonia must react in the presence of excess CuO to produce 55.0 grams of copper?
19. For the chemical reaction below, if 100. g of WO3 is used, what mass of hydrogen is required and what
masses of tungsten and water are formed ?
WO3 + 3 H2 → W + 3 H2O
20. The solid fuel in the booster stage of the space shuttle is a mixture of ammonium perchlorate and aluminium
powder, which react as follows:
NH4ClO4(s)
+
Al(s) →
Al2O3(s)
+
N2(g)
+
HCl(g)
+
H2O(g)
Balance the equation.
What mass of aluminium should be mixed with 5.0 x 103 kg of ammonium perchlorate?
21.
Camels store the fat tristearin (C57H110O6) in the hump. As well as being a source of energy the fat is a
source of water, because when it is used the reaction below takes place. What mass of water is available from
1.0 kg of fat? The equation is not balanced.
C57H110O6(s)
+
O2(g)
→
CO2(g)
+
H2O(l)
22. Barium peroxide, (BaO2), and barium carbonate both decompose on heating:
BaO2(s)
→ BaO(s)
+
BaCO3(s)
→ BaO(s)
+ CO2(g)
O2(g)
The reactions are not balanced. If 14.53 g of a mixture of BaO2 and BaCO3 are heated and after decomposition
is complete, 12.37 g of BaO remains, what is the composition of the original mixture?
Limiting reactants (reagents)
23. Nitrogen gas (N2) can be prepared by passing gaseous ammonia (NH3) over solid copper (II) oxide at high
temperatures. The other products of the reaction are solid copper and water vapour. If 18.1 g of ammonia is
Chem 121 Problem set III - 3
reacted with 90.4 g of copper (II) oxide, how many grams of nitrogen gas will be formed and what weight of the
excess reagent remains?
24. When Xenon difluoride (XeF2) dissolves in water, it slowly reacts with water to produce xenon gas, hydrogen
fluoride and oxygen:
XeF2(s) + H2O(l) →
Xe(g) + HF(g) + O2(g)
Which is the limiting reagent when 1.00 g of xenon difluoride is dissolved in 50.0 g of water? What mass of
hydrogen fluoride can be produced and how much of the excess reagent remains?
25. Ammonia reacts with oxygen to produce nitrogen oxide and water, according to the following unbalanced
equation:
NH3 (g) + O2(g) → NO(g) + H2O(l)
If 2.00 g of ammonia is reacted with 4.50 g of oxygen
a) which is the limiting reagent?
b) how much NO is formed?
c) what mass of the excess reagent remains at the completion of the reaction?
26. Magnesium sulfate forms a hydrate, MgSO4 . nH2O, which is 51.2 % water by mass. Calculate the number of
water molecules in the molecular formula (find the value of n).
Percent yields
27. The theoretical yield of sodium perxenate (Na4XeO6) in a reaction was 1.25 g, but only 1.21 g was obtained.
What was the percentage yield of the reaction?
28. In the reaction of 4.0 moles of nitrogen with 6.0 moles of hydrogen, a chemist obtained 1.6 moles of
ammonia. What is the percent yield of ammonia?
29. A student reacts benzene, C6H6, with bromine, Br2, in an attempt to prepare bromobenzene, C6H5Br:
C6H6(l) + Br2(l) → C6H5Br(l) + HBr(g)
a) What is the theoretical yield of bromobenzene in this reaction when 30.0 g of benzene reacts with 65.0 g of
Br2? Which is the limiting reagent and which reagent is in excess?
b) Dibromobenzene, C6H4Br2, is produced as a by-product in the synthesis of bromobenzene. If the actual yield
of bromobenzene was 56.7 g, what is the percentage yield?
30. When we inhale oxygen, we expire it as water vapour. We can regenerate oxygen by reacting the water
vapour with potassium superoxide, KO2 with KOH as a byproduct. When we react 25.0 grams of H2O with 25.0
grams of KO2, we obtain 15.3 grams of KOH. What are the theoretical and percent yields of KOH?
Sequential reactions
31. We use carbon, in a form called coke, in smelting to convert metal oxide ores such as hematite, Fe2O3, to
the metal in a 2-step process involving carbon monoxide, as an intermediate product.
2C + O2 → CO
Fe2O3 + 3CO → 2Fe + 3CO2
How many grams of carbon must we use to produce 750.0 grams of iron?
Molarity and Mass of Solute
32. How many grams of sodium chloride are present in 125.0 g of solution that is 12.0% by mass NaCl?
33. The density of a 15.0% solution of NaCl is 1.108 g/mL. How many millilitres of this solution must we use to
obtain 75.0 g of NaCl?
34. What is the molarity of potassium nitrate, KNO3, in a solution prepared by dissolving 3.765 g of KNO3 in
enough water to make 375 mL of solution?
35. What is the molarity of a 20.00% solution of NaNO3? The solution density is 1.143 g/mL.
Chem 121 Problem set III - 4
Dilution of Solutions
36. If 25.00 mL of 0.3447 M NaCl is diluted to 1000.0 mL. What is the concentration of the diluted solution?
37. What is the molarity of potassium sulphate, K2SO4, in the solution made when we dilute 14.5 mL of aqueous
2.00 M K2SO4 solution with water to a final volume of 300.0 mL?
38. How many mL of water must we add to 5.00 mL of 12.0 M HCl to prepare a hydrochloric acid solution that is
0.600 M? Assume that the volumes are additive.
Solution Stoich
39. How many grams of silver nitrate, AgNO3, must we use to react completely with 75.00 mL of 0.420 M
potassium chromate, K2CrO4? The chemical equation is:
2AgNO3 +K2CrO4 → Ag2CrO4 + 2KNO3
40. A 40.0 mL aliquot of an iron (II) solution was found to react completely with 22.4 mL of 0.0200 M aqueous
potassium permanganate according to the equation below. What is the concentration of the iron (II) solution ?
MnO4−(aq) + 8 H+(aq) + 5 Fe2+(aq) → 5 Fe3+(aq) +
Mn2+(aq) + 4 H2O(l)
41. We dissolve 3.778 g of a sample that contains some sodium oxalate, Na2C2O4, in water and acidify the
solution with excess sulphuric acid. The sample requires 18.74 mL of 0.08395 M KMnO4, potassium
permanganate, for complete reaction according to the equation below. What is the percent Na2C2O4 in the
sample? Assume that no other component reacts with the KMnO4 solution.
8H2SO4 + 2KMnO4 + 5Na2C2O4(aq) → 8H2O + 2MnSO4 + 10CO2 + 5Na2SO4 + K2SO4
42. A 10.00 mL sample of sulphuric acid (H2SO4) from an automobile battery requires 32.74 mL of 2.15 M NaOH
for complete neutralization. What is the concentration of the sulphuric acid? The unbalanced equation is:
H2SO4(aq) + 2 NaOH(aq) → Na2SO4(aq) + 2 H2O(l)
43. In a standardization of an NaOH solution, 0.7284 g of KHP (molar mass = 204.2 g/mol) is dissolved in water.
This KHP solution requires 34.58 mL of the NaOH solution for neutralization. Calculate the molarity of the
sodium hydroxide solution.
Mixture Problems
44. 1.000 g of a mixture of CuO and Cu2O is reduced quantitatively to give 0.8390 g of Cu. What is the
composition of the original mixture of copper oxides ?
45. A mixture of Mg and Zn having a mass of 1.000 g when burned in oxygen gas (O2) gave a mixture of MgO
and ZnO, which had a mass of 1.409 g. What is the percentage of Zn in the mixture?
46. A 0.502 g mixture of KCl and MgCl2 was treated with sulphuric acid. The evolved HCl gas was passed
through a water bath, which was subsequently neutralized with 75.82 mL of a standard 1.054 M NaOH solution.
What was the composition of the initial mixture of halides? (This is a toughie for coneheads!)