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Question

By the reaction of carbon and oxygen, a mixture of CO and CO2 is obtained. What is the composition of the mixture by mass obtained when 20 g of O2 reacts with 12 g of carbon?

A
CO : CO2 =21:11
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B
CO : CO2 =11:21
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C
CO : CO2 =21:22
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D
CO : CO2 =42:11
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Solution

The correct option is A CO : CO2 =21:11
The balanced chemical equations for the combustion reaction are as follows:
2xC+xO22xCO
yC+yO2yCO2
Total number of moles of C =2x+y
Total number of moles of oxygen = x+y
The molar masses of C and oxygen are 12 g/mol and 32 g/mol respectively.
12 g of C corresponds to 1 mole and 20 g of oxygen corresponds to 2032=0.625 moles.
Hence, 2x+y=1......(1)
and x+y=0.625......(2)
Substracting equation (2) from (1), we get
(2x+y)(x+y)=10.625
x=0.375......(3)
Substituting equation (3) in equation (2), we get
0.375+y=0.625
y=0.250
The number of moles of CO formed are 2x=2(0.375)=0.75
The number of moles of CO2 formed are y=0.250
The molar masses of CO and CO2 are 28 g/mol and 44 g/mol respectively.
The mass of CO formed is 0.75×28=21g
The mass of CO2 formed is 0.250×44=11g.
Hence, the composition of the mixture is CO:CO2=21:11.

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