GRADE 11 CHEMISTRY
Complete the following by the beginning of next class.
You will need to record your answers on a separate piece of paper.
Please show all your work for calculations.
Numerical answers can be found on the last page.
Significant Digits (Sig Digs) and Rounding
1) What is the difference between accuracy and precision?
2) How many significant digits are in each of the following?
a) 624 students
b) 22.40 mL
c) 0.00786 g
3) Round the following measured quantities to the number of significant digits specified:
a) 9.276 x 103 m (2 sig digs)
b) 87.45 g (3 sig digs)
c) 93.951 kg (3 sig digs)
4) Express each calculation to the correct number of significant digits:
a) 10.25 g + 2.5 g
b) 5845 cm x 23.9 cm
c) 1106.9 mL – 783.687 mL
d) 64.3 g ÷ 9.52 mL
Percentage Composition
The calculations that you will be doing involve determining the
percentages of elements in compounds.
Example 1
A compound with a mass of 48.72 g is found to contain 32.69 g of zinc and 16.03 g of
sulphur. What is the percentage composition of the compound?
% Zn =
mass of Zn
mass total
X
100
=
32.69 g
48.72 g
X
100
=
32.90 %
%S
=
mass of S
mass total
X
100
=
16.03 g
48.72 g
X
100
=
32.90 %
Therefore, the percentage composition of the compound is 67.10 % zinc and 32.90 %
sulphur.
Questions
1) A sample of a compound is analyzed and found to contain 0.90 g of calcium and 1.60 g
of chlorine. The sample has a mass of 2.50 g. Determine the percentage composition of
the compound.
2) Find the percentage composition of a pure substance that contains 7.22 g of nickel, 2.53
g of phosphorus and 5.25 g of oxygen.
3) Bromine azide is a compound that is used in explosives. A sample of bromine azide was
found to contain 2.35g of Br and 1.24 g of N. Determine the percentage composition.
If masses are not given, you can determine the percentage composition
of a substance by using the masses on the periodic table.
Example 2
On the periodic table, calcium (Ca) mass
Assume that a.m.u. is the same as grams
= 40.08 atomic mass units (a.m.u.)
= 40.08 g
Try the same for chlorine (Cl)
= ___________________ a.m.u.
= ___________________ g
In the compound CaCl2, there are
1 Ca at 40.08 g
2 Cl at 35.45 g each
Therefore, the total mass is
= (40.08g) + 2(35.45g)
= 110.98 g
Now we can calculate the percentage composition of CaCl2:
% Ca =
mass of Ca
mass total
X
100
=
40.08 g
110.98
X
100
=
36.11%
% Cl =
=
2(35.45)
110.98
X
100
63.89%
Therefore, the percentage composition of CaCl2 is 36.11% Ca and 63.89% Cl.
Questions
4) Determine using the periodic table, the masses of the following compounds.
a) Na3PO4
c) Mg3(PO4)2
b) Ca(NO3)2
d) CH3COOH
5) Sulphuric acid, H2SO4, is an important acid in laboratories and industries. Determine the
percentage composition of sulphuric acid.
6) A mining company wishes to extract manganese metal from pyrolusite ore, MnO 2. What
is the percentage composition of pyrolusite ore?
7) Pyridine, C5H5N is a slightly yellow liquid with a nauseating odour. It is used in the
synthesis of vitamins and drugs and has many other uses in industrial chemistry.
Determine the percentage composition of pyridine.
Example 3
Calculate the mass of aluminum in 28 g of aluminum oxide, Al2O3.
From the periodic table
mass of Al
Mass total
= 26.98 g
= 2(26.98g) + 3(16.00g)
= 101.96 g
The ratio will be the same, in Al203. Therefore,
Mass of Al from periodic table
Total mass from periodic table
=
=
mass of aluminum in this case
total mass in this case
2(26.98g)
101.96g
=
__X____
28 g
=
15 g
X
Questions
8) Indigo, C16H10O2, is the common name of the dye that gives blue jeans their
characteristic colour. Calculate the mass of oxygen in 25.0 g of indigo.
9) Potassium perchlorate, KClO4, is used extensively in explosives. Calculate the mass of
oxygen in a 24.5 g sample of potassium chlorate.
10) The label on a box of baking soda, NaHCO3 claims that there are 137 mg of sodium per
0.500 g of baking soda. Comment of the validity of this claim.
11) A piece of magnesium reacts with 16.0 g of oxygen to form 40.3 g of a compound.
a) Calculate the percentage of magnesium in the compound.
b) Calculate the mass of magnesium that would be present in 10 000 g of the
compound.
12) a) Calculate the percentage composition of iron (III) oxide.
b) Calculate the mass or iron that could be obtained from 20 000 g of this compound.
13) A sample of a compound with K, Cl and O in it, was placed in a test tube and
decomposed by heating. After heating, a compound containing K and Cl was left (called
the residue) and oxygen gas escaped into the room. From the given data, calculate the
percent of oxygen in the compound.
Mass of empty test tube
= 18.00 g
Mass of test tube and compound
= 24.13 g
Mass of test tube and residue
= 21.73 g
Numerical Answers
Significant Digits
2)
a) 3 b) 4 c) 3
3)
a) 9.3 x 103 m
b) 87.5 g
4)
a) 12.7 g
b) 1.40 x 105 cm2
c) 94.0 kg
c) 323.2 mL d) 6.75 g/mL
Percentage Composition
1) Ca = 36%, Cl = 64%
2) Ni = 48.1%, P = 16.9%, O = 35.0%
3) Br = 65.5%, N = 35.5%
4) a) 163.94 g
b) 164.10 g
c) 231.90 g d) 60.06 g
5) H = 2.06%, S = 32.70%, O = 65.25%
6) Mn = 63.19%, O = 36.81%
7) C = 75.91%, H = 6.38%, N = 17.71%
8) 3.42 g
9) 11.3 g
11) a) 60.3% b) 6.03 x 103 g
12) a) Fe2O3 – 69.94%Fe, 30.06%O
b) 1.399 x 104g
13) 39.2%