SCH3U
Name:
Unit 4 Review
Percentage Composition
Element mass ÷ compound mass x 100%
Calculating molecular formula from simplest formula and
molar mass
E.g. in H2O, H = 11% (2 g ÷ 18 g x 100%)
Balancing Chemical
Balancing equations by inspection
Significant Digits
All digits to right of the first # are significant
In scientific notation all digits are significant
For addition/subtraction: line up decimal
For multiplication/division: # digits = fewest
Average atomic mass is equal to the sum of individual
isotope masses multiplied by their %
The factor label method
Creating conversion factors. How to use the factor label
method
Stoichiometry
grams x → moles x → moles y → grams y
Factor label method to solve stoichiometry
Molar mass is calculated from periodic table
Limiting reagents
Actual/Ideal chart for limiting reagents. The limiting
reagent is the “given quantity” . Shortcut method of
determining limiting reagent
Simplest and molecular formulae
Definitions of simplest & molecular formula
Percentage yields
Percentage yield = actual/theoretical x100%
Determining simplest formula from % composition, grams
of reactants, or moles
Actual yield is given, theoretical is calculated
The mole
There are 6.02 x 1023 particles in one mole
4 reasons why actual yield falls short
For all questions calculate molar masses to two decimal places and give answers with the correct number of
significant digits (remember: do not round your values until writing the final answer).
1. Give the percentage composition for each compound: a) H2SO4, b) Ca(OH)2.
2. Calculate the molar mass of a) H2SO4, b) Fe2(Cr2O7)3.
3. a) How many moles are in 16 grams of CuCl2? b) How much does 70 moles of NaCl weigh?
4. a) How many molecules are in exactly 4 moles of H2O? b) How many hydrogen atoms are in exactly 4
moles of H2O? c) How many hydrogen atoms are in 0.173 moles of H2O?
5. What mass of magnesium oxide results when 56.3 g O2 combines with excess magnesium?
6. Label as simplest formula, molecular formula, or both: a) CuCl2, b) CO2, c) O2, d) C4H10.
7. A substance is 80% C and 20% hydrogen by mass. a) What is the simplest formula? b) What is the
molecular formula of the compound if the molar mass is 30 g/mol?
8. Balance these equations: a) C40H82 + O2 → CO2 + H2O, b) H2O + Al4C3 → CH4 + Al(OH)3
9. What four things may cause actual yields to differ from theoretical yields?
SCH3U
Name:
Stoichiometry Review
Review and be able to work all problems from your Stoichiometry Notes, Stoichiometry Worksheet, and Limiting and
Excess Reactant Worksheet.
Complete the following problem:
1. A 0.1153gram sample of a pure hydrocarbon was burned in a C-H combustion train to produce 0.3986 grams of CO2
and 0.0578 grams of H2O. Determine the empirical formula for this hydrocarbon. (Answer: C7H5)
2. An elemental analysis showed that an unknown sample contained only C, H, and O. A 0.2028g sample of the
compound was burned in a C-H combustion train to produce 0.2972g of CO2 and 0.1218g of H2O. Determine the
empirical formula for this compound. (Answer: CH2O)
3. Sodium and water react. How many grams of sodium will react with water to produce 4.00 moles of hydrogen? (184
grams)
4. How many moles of lithium chloride will be formed by the reaction of chlorine with 4.00 grams of lithium bromide?
(0.0460 mol or 4.60X10-2 mol)
5. Aluminum will react with sulfuric acid. a. How many moles of H2SO4 will react with 18 mol of aluminum? b. How
many moles of each product will form? (a. 27 mol; b. 27 mol hydrogen and 9.0 mol aluminum sulfate)
6. What mass of acetylene, C2H2, will be produced from the reaction of 90. grams of calcium carbide with water in the
following reaction? (37 grams)
CaC2(s) + 2H2O(l)  C2H2(g) + Ca(OH)2(s)
7. What mass of ZrCl4 can be produced if 862 grams of ZrSiO4 and 950. grams of chlorine are available to react according
to the following equation? (1.10X103 grams)
ZrSiO4 + 2Cl2  ZrCl4 + SiO2 + O2
8. Heating zinc (II) sulfide in the presence of oxygen yields zinc (II) oxide and sulfur dioxide. If 1.72 moles of ZnS is
heated in the presence of 3.04 moles of O2, which reactant will be used up? (ZnS is the limiting reactant)
9. Aluminum and oxygen react. a. Which reactant is limiting if 0.32 moles of aluminum and 0.26 moles of oxygen are
available? b. How many moles of aluminum oxide are formed from the reaction of 0.00638 moles of oxygen and
0.00915 moles of aluminum? c. If 3.17 grams of aluminum and 2.55 grams of oxygen are available, which reactant is
limiting? a. aluminum is limiting; b. 4.25 X 10-3 mol; c. oxygen is limiting)
10. Bromine replaces iodine in magnesium iodide during a single replacement reaction. a. Which is the excess reactant
when 560 grams of MgI2 and 360 grams of Br2 react, and what mass remains? b. What mass of I2 is formed in the same
process? (a. Br2 is the excess and 38 grams will remain; b. 510 grams)
11. If 8.87 grams of As2O3 is used in the below reaction and 5.33 gram of arsenic is produced, what is the present yield?
(79.3%)
2As2O3 + 3C  3CO2 + 4As
12. Tungsten can be produced from its oxide by reacting the oxide with hydrogen at a high temperature according to
the following equation:
WO3 + 3H2  W + 3H2O
a. What is the percent yield if 56.9 grams of WO3 yields 41.4 grams of tungsten? b. How many moles of tungsten will
be produced from 3.72 grams of WO3 if the yield is 92.0%? (a. 91.8%; b. 0.0148 mol).