Paper 3 · stoichiometry

A mixture of equal volumes of methane and ethane was burned in excess oxygen. After cooling to room temperature, the residual gas absorbed in $KOH_{(aq)}$ was found to be 30 $cm^3$ . What was the volume of methane at the beginning?

A10

cm^3

B15

cm^3

C20

cm^3

D25

cm^3

Explanation: The marking scheme gives answer A. Let volume of

CH_4

= x and

C_2H_6

= x.

CH_4 + 2O_2 \to CO_2 + 2H_2O

;

2C_2H_6 + 7O_2 \to 4CO_2 + 6H_2O

. CO2 from methane = x, CO2 from ethane = 2x. Total CO2 = 3x = 30, so x = 10

cm^3

ZIMSEC Chemistry Paper 3, June 2009, Q2

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