Teacher's notes and answers to questions in the book - Hodder Plus ...

WJEC GCSE Additional Science Teacher’s Notes
PRACTICAL Investigating the rate of a reaction
(pages 119–120)
Give students a suitable range of reactions to investigate – you could have different groups
doing different reactions and different factors then pooling the results. This Practical (and its
report outcome) has been deliberately left open to suit local conditions and time constraints.
_Chemical calculations (pages 120–29)_____________
Questions
10. Calculate the relative molecular mass, M r , for the following molecules.
a oxygen gas, O 2 = [16 + 16] = 32
b sulfur dioxide, SO 2 = [32 + (2 × 16)] = 64
c methane, CH 4 = [12 + (4 × 1)] = 16
d nitrogen dioxide, NO 2 = [14 + (2 × 16)] = 46
e carbon tetrachloride, CCl 4 = [12 + (4 × 35.5)] = 154
f ammonia, NH 3 = [14 + (3 × 1)] = 17
g ethane, C 2 H 6 = [(2 × 12) + (6 × 1)] = 30
11. Calculate the relative formular mass of the following ionic compounds.
a lithium chloride, LiCl = [7 + 35.5] = 42.5
b potassium oxide, K 2 O = [(2 × 39) + 16] = 94
c sodium sulfide, Na 2 S = [(2 × 23) + 32] = 78
d magnesium carbonate, MgCO 3 = [24 + 12 + (3 × 16)] = 84
e calcium nitrate, Ca(NO 3 ) 2 = [40 + (2 × (14 + (3 × 16)))] = 164
f beryllium oxide, BeO = [9 + 16] = 25
g rubidium carbonate, Rb 2 CO 3 = [(2 × 85) + 12 + (3 × 16)] = 230
h ammonium sulfate, (NH 4 ) 2 SO 4 = [(2 × (14 + (4 × 1))) + 32 + (4 × 16)] = 132
12. Calculate the percentage composition of each molecule or compound in Questions 10 and 11.
Molecules in Question 10:
a oxygen gas, O 2
100 % oxygen
b sulfur dioxide, SO 2
50 % sulfur, 50 % oxygen
c methane, CH 4 75 % carbon, 25 % hydrogen
d nitrogen dioxide, NO 2 30.4 % nitrogen, 69.6 % oxygen
e carbon tetrachloride, CCl 4 7.8 % carbon, 92.2 % chlorine
f ammonia, NH 3 82.4 % nitrogen, 17.6 % hydrogen
g ethane, C 2 H 6
80 % carbon, 20 % hydrogen
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