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Calculations
on the mole concept
Francine
Taylor-Campbell, Contributor
Let
us try this question. 1. (a) On complete
combustion, 4.0g of a hydrocarbon
give 11.0 g of carbon dioxide and
9.0 g of water. What is the formula
of the hydrocarbon?
(b)
1.89g of zinc nitrate was heated according
to the equation
2Zn(NO3)2
(s) == 2ZnO (s) + 4NO2
(g) + O2 (g)
(i)
What is the percentage by mass of
oxygen in zinc nitrate?
(ii)
What volume of oxygen at r.t.p. is
given off when 1.89g of zinc nitrate
are heated?
ANSWERS
(a)
Mr of CO2 =
12+(16x2) = 44
Mr
of H2O = (1x2)+16 = 18
#mol
CO2 = 11 /44 = 0.25 mol
#mol
H2O = 9/18 = 0.5 mol
Based
on the formula of the hydrocarbon
CxHy
#mol
of C = 0.25 mol and #mol of H = 2x0.5
= 1 mol
Ratio
of C:H = 0.25: 1 = 1:4
Hydrocarbon
is CH4
(b)
2Zn(NO3)2 ==
2ZnO + 4NO2 + O2
Molar
mass of Zn(NO3)2
= 65 + 14x2 + 16x6 = 189g
%
O = mass of oxygen in zinc nitrate
= (96/189)x100 = 50.8%
Total
mass of zinc nitrate
(ii)
1.89g of Zn(NO3)2
represents 1.89/189 = 0.01 mol
According
to the equation 2 mol of Zn(NO3)2
gives off 1 mol O2
Therefore
0.01 mol Zn(NO3)2
will produce 0.01/2 = 0.005 mol O2
At
r.t.p. 1 mole of any gas occupies
a volume of 24dm3
0.005
mol O2 has a volume of
0.005x24 = 0.12dm3
Question
2
2.
Ammonia and carbon dioxide react to
form water and a solid, urea, CON2H4.
In the reaction, 72dm3
of carbon dioxide at r.t.p. are converted
to urea.
(i)
Write the equation for the formation
of urea.
(ii)
Calculate the volume of ammonia at
r.t.p. which reacted.
(iii)
Calculate the mass of urea formed.
ANSWERS
(i)
2NH3 (g) + CO2
(g) == H2O (l) + CON2H4
(s)
(ii)
Based on the equation 2 mol NH3 react
with 1 mol CO2 (2:1 ratio)
Hence
if 72 dm3 of CO2 is used then 2 x
72 dm3 of NH3 would react.
Volume
of ammonia = 144dm3
(iii)
1 mol of gas at r.t.p has a volume
of 24dm3
#
mol CO2 used = 72/24 =
3 mol
Using
the equation again 1 mol CO2
produces 1 mol CON2H4
(urea)
Thus
3 mol CO2 produces 3 mol
urea
Molar
mass of urea = (12 + 16 + 14x2 + 1x4)
= 60g
3
mol urea has a mass of 60 x 3 = 180g.
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Fifth-form
students at Clan Carthy High
School in Kingston.
- Norman Grindley/Chief Photographer
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Francine
Taylor-Campbell is an independent
contributor.
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