Determining formula

2006 QUESTION FIVE: CALCULATING MOLAR MASSES
Calculate the molar masses of the following compounds.

Refer to the Resource Booklet provided. Show your working.
(a) CaCO3
Ca = 1 x 40 = 40
C = 1 x 12 = 12
O = 3 x 16 = 48
Molar mass = 100 g/mol

(b) Al2O3
Al = 2 x 26.9 = 53.8
O = 3 x 16 = 48
Molar mass = 101.8 g/mol

(c) (NH4)2SO4
N= 2 x 14 = 28
H = 8 x 1 = 8
S = 1 x 32 = 32
O = 4 x 16 = 64
Molar mass = 132 g /mol

2007 QUESTION FOUR: MOLAR MASSES
Calculate the relative molar masses of the following compounds. Use the relative atomic masses provided in the periodic table in the Resource Booklet.

(a) ZnO
Zn = 1 x 65.4 = 65.4
O = 1 x 16 = 16
Molar mass = 81.4 g/mol

(b) CuSO4
Cu = 1 x 63.5 = 63.5
S = 1 x 32 = 32
O = 4 x 16 = 64
Molar mass = 159.5 g/mol

(c) Pb(NO3)2
Pb = 1 x 207.2 = 207.2
N = 2 x 14 = 28
O = 6 x 16 = 96
molar mass = 331.2 g/mol


2006 QUESTION SIX: CALCULATING MASS
Ammonia, NH3 , can be prepared by heating ammonium chloride, NH4Cl, with calcium hydroxide, Ca(OH)2 .

2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O

Calculate the mass of calcium hydroxide required to react with 2.14 grams of ammonium chloride.

107g --------> 74
2NH4Cl + Ca(OH)2 → CaCl2 + 2NH3 + 2H2O
2.14g --------> ?

2.14/107 x 74 = 1.48g




2007 QUESTION FIVE: CALCULATING MASS
(a) Calculate the mass of zinc hydroxide that must be heated to produce 1.00 gram of water. Use the equation below. Show all of your working clearly.

Zn(OH)2(s) --> ZnO(s) + H2O(g)

99.4 --------> 18
Zn(OH)2 --> H2O
? ------------> 1.00

1.00 / 18 x 99.4 = 5.52g


(b) Calculate the mass of sodium hydrogen carbonate, NaHCO3, required to form 5.40 grams of carbon dioxide, CO2, when heated. Show all of your working clearly.

2NaHCO3(s) --> Na2CO3(s) + H2O(g) + CO2(g)

168 ------------> 44
2NaHCO3 ----> CO2
? ---------------> 5.40

5.40 / 44 x 168 = 20.62g

2006 QUESTION SEVEN: DETERMINING A FORMULA
Determine the formula of the compound made when 3.55 g of chlorine combines with 5.60 g of oxygen.

The molar mass of the compound is 183.0.

Refer to the Resource Booklet provided. Show your working.
Chlorine----------> oxygen

3.55g/35.5 --------> 5.60 / 16

0.1 mol/ 0.1--------------> 0.35 mol / 0.1

1 --------------------------> 3.5

2 ---------------------------> 7

Emperical formula: Cl2O7

empirical formula mass = molar mass so molecular formula = empirical formula



2007 QUESTION SEVEN: MOLECULAR FORMULA
A compound was analysed and found to contain:
• 20.2% phosphorus
• 10.4% oxygen and
• 69.4% chlorine.
It has a relative molar mass of 153.5.

Determine the molecular formula of this substance. Show all of your working clearly.

phosphorus _________ oxygen __________chlorine

20.2 / 31 ____________ 10.4 / 16 _________ 69.4 / 35.5

0.65 mol/0.65 ________0.65mol/0.65 _____1.95 mol/0.65

1 __________________ 1 ______________ 3

Emperical formula = POCl3

Emperical mass = 153.5, Molar mass = 153.5