Problem 3

A 10 cm

^3

sample of an unknown gaseous hydrocarbon was mixed with 70 cm

^3

of oxygen and the mixture was set on fire by means of an electric spark. When the reaction was over and water vapour was liquefied, the final volume of gases decreased to 65 cm

^3

. This mixture then reacted with a potassium hydroxide solution and the volume of gases decreased to 45 cm

^3

. What is the molecular formula of the unknown hydrocarbon, all volumes being measured at standard temperature and pressure (STP)?

Step 2 of 3: Balance the combustion

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\ce{CxHy} + (x + y/4)\,\ce{O2} -> x\,\ce{CO2} + (y/2)\,\ce{H2O}

Analysis

For 10 cm3 of $\ce{CxHy}$ : CO2 formed = 10x = 20 cm3, so $x = 2$ . O2 used = $10(x + y/4) = 25$ cm3, giving $y/4 = 0.5$ , i.e. $y = 2$ .