COPPERBELT UNIVERSITY
CHEMISTRY DEPATMENT
CH110 TUTORIAL SHEET - STOICHIOMETRY
Question 1
a) If a person asks you to buy 500 toffee sweets, how would you count the sweets using the
concept of “counting by weighing”?
b) Explain why atomic masses on the periodic table are not whole numbers.
c) An element consists of 1.40% of an isotope with mass 203.973 amu, 24.10% of an
isotope with mass 205.9745 amu, 22.10% of an isotope with mass 206.9759 amu, and
52.40% of an isotope with mass 207.9766 amu. Calculate the average atomic mass and
identify the element.
d) Given that the average atomic mass of silicon is 28.09, complete the following table:
Isotope
28
Si
29
Si
30
Si
Mass (amu)
Abundance Percentage
27.98
4.70
29.97
3.09
e) Nitrogen is made up of two isotopes, N-14 and N-15. Given nitrogen's atomic mass of
14.007, what is the percent abundance of each isotope?
Question 2
Glycerol (C3H8O3) is sold in drug stores as glycerine and is commonly found in soaps and
shampoos.
a) What is the molar mass of glycerol?
b) What is the mass in grams of 1.00 mole of glycerol?
c) How many molecules are in one mole of glycerol?
d) How many grams are in 0.217 moles of glycerol?
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Question 3
a) Ammonia (NH3) is the active ingredient in many kitchen cleansers. How many atoms are in
i.
one molecule of ammonia?
ii.
one mole of ammonia?
iii.
3.40 grams of ammonia?
b) How many moles are in the following:
i.
1.29 x 1024 hydrogen atoms in HF
ii.
7.36 x 1024 free oxygen atoms
iii.
3.28 x 1023 Na atoms in salt (NaCl)
c) Ammonia and oxygen react to form nitrogen and water.
i.
Write a balanced equation for the reaction.
ii.
How many moles of oxygen are needed to react with 8 moles of ammonia?
iii.
How many grammes of nitrogen gas can be produced when 6.5g of oxygen
react?
iv.
How many grammes of water can be produced from the reaction of 34g of
ammonia?
Question 4
a) Sodium nitrite is a controversial food preservative added to processed meat and thought
to form cancer-causing compounds when heated.
i.
Write the formula of sodium nitrite.
ii.
What are the mass percentages of each element in sodium nitrite?
b) In 1987 the first substance to act as a superconductor at a temperature above that of liquid
nitrogen (77 K) was discovered. The approximate formula of this substance is
YBa2Cu3O7. Calculate the percent composition by mass of this material.
c) Fungal laccase, a blue protein found in wood-rotting fungi, is 0.390% Cu by mass. If a
fungal laccase molecule contains four copper atoms, what is the molar mass of fungal
laccase?
d) Hemoglobin is the protein that transports oxygen in mammals. Hemoglobin is 0.347% Fe
by mass, and each hemoglobin molecule contains four iron atoms. Calculate the molar
mass of hemoglobin
Question 5
a) A compound consists of 40.7% C, 5.1% H, and 54.2% O?
i. What is its empirical formula?
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ii. The molar mass of this compound is 118 grams/mole. What is the molecular
formula of this compound?
b) A 25.0 gram sample of a compound made up of magnesium, carbon and oxygen contains
7.20 grams magnesium and 3.55 grams carbon.
i. Find the empirical formula of this compound.
ii. Find the mass percentage of each element in this compound.
iii. What is the mass of magnesium in a 13.9 gram sample of this compound?
iv. What is the mass of this compound that contains 0.290 moles of carbon?
c) A sample of zinc is heated in air to form zinc oxide. Assuming all of the zinc is
converted to the oxide, use the data table below to calculate the empirical formula of zinc
oxide.
mass of crucible
= 32.00 g
mass of crucible + zinc (before heating)
= 33.64 g
mass of crucible + oxide (after heating)
= 34.04 g
Question 6
a) Balance the following equations:
i.
B2H6(l) + O2(g)  B2O3(s) + H2O(l)
ii.
PH3(g) + O2(g)

H2O(l) + P4O10(s)
iii.
iv.
v.
b) Write a balanced equation for each of the following reactions:
i.
the reaction of solid lithium with nitrogen to form solid lithium nitride.
ii. the reaction between aqueous solutions of cobalt(III) nitrate and sodium hydroxide to
form aqueous sodium nitrate and solid cobalt(III) hydroxide.
iii. the reaction between solid zinc and aqueous hydrochloric acid in a single replacement
reaction.
iv.
Solid iron (III) sulphide reacts with gaseous hydrogen chloride to form solid iron (III)
chloride and hydrogen sulphide gas.
v. Carbon disulphide reacts with ammonia gas to produce hydrogen sulphide gas and
ammonium thiocynate (NH4SCN).
Question 7
a) Hydrogen sulfide, given off by decaying organic matter, is converted to sulfur dioxide in
the atmosphere by the reaction:
2H2S(g) + 3O2(g)  2SO2(g) + 2H2O(l)
i.
How many moles of H2S are required to form 8.20 moles of SO2?
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ii.
How many grams of O2 are required to react with 1.00 mole of H2S?
iii.
How many grams of water are produced from 6.82 g H2S?
iv.
If 12.0 grams of SO2 are formed from 7.98 g of H2S, what is the percent yield?
v.
How many grams of SO2 are produced starting from 2.66 g H2S and 3.00 g O2?
vi.
Which reactant is a limiting reagent?
b) Coke is an impure form of carbon that is often used in the industrial production of metals
from their oxides. If a sample of coke is 95% carbon by mass, determine the mass of coke
needed to react completely with 1.0 ton of copper(II) oxide.
2CuO(s) + C(s) → 2Cu(s) + CO2(g)
Question 8
a) A gaseous mixture containing 7.50 mol H2(g) and 9.00 mol Cl2(g) reacts to form
hydrogen chloride (HCl) gas.
i.
Write a balanced equation for the reaction.
ii.
Which reactant is a limiting reagent?
iii.
If all the limiting reactant is consumed, how many moles of hydrogen chloride are
formed?
iv.
How many moles of the excess reactant remain unreacted?
b) A vessel contains O2(g) and SO2(g) that react to form SO3(g). If six moles of O2 react
with five moles of SO2 in a reaction that goes to completion, what is the limiting
reactant? If 96.0 g of SO2 react with 32.0 g of O2, what mass of the product will be
formed?
Question 9
a) The combustion of 40.10 g of a compound which contains only C, H, Cl and O yields
58.57 g of CO2 and 14.98 g of H2O. Another sample of the compound with a mass of
75.00 g is found to contain 22.06 g of Cl. What is the empirical formula of the
compound?
b) A compound with a known molecular weight (146.99 g/mol) that contains only C, H, and
Cl was studied by combustion analysis. When a 0.367 g sample was combusted, 0.659 g
of CO2 and 0.0892 g of H2O formed. What are the empirical and molecular formulas?
c) A 95.6 mg sample of menthol (molar mass = 156 g/mol) was burnt in excess oxygen gas
to give 269 mg CO2 and 110 mg H2O. What is menthol's empirical and molecular
formula?
d) A 0.487 grams sample of quinine (molar mass = 324 g/mol) is combusted and found to
produce 1.321 g CO2, 0.325 g H2O and 0.0421 g nitrogen. Determine the empirical and
molecular formulas of quinine.
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