# An A-level chemistry question?

10cm3 of ethane is burned in 45cm3 of oxygen at a pressure of 101 kPa and a temperature of 200C. Complete combustion takes place.

What is the total volume of gas present when the reaction is complete, measured under the same conditions?

Relevance

10 mL of ethane is burned in 45 mL of oxygen at a pressure of 101 kPa and a temperature of 200ºC. Complete combustion takes place.

Ethane + Oxygen ➜ Carbon Dioxide + Water

2C₂H₆(g) + 7O₂(g) ➜ 4CO₂(g) + 6H₂O(g)

2 mol C₂H₆(g) + 7 mol O₂(g) ➜ 4 mol CO₂(g) + 6 mol H₂O(g)

volume of ideal gases at same temp and pressure is proportional to number of moles, so this is

2 mL C₂H₆(g) + 7 mL O₂(g) ➜ 4 mL CO₂(g) + 6 mL H₂O(g)

ratio of ethane to oxygen is 2/7 ... you have 10/45 or 2/9 ... so you have excess oxygen and ethane is controlling factor.

2 mL of ethane results in 4+6 = 10 mL of product

you have 10 mL of ethane so product is 50 mL

plus there is leftover oxygen. 10(7/2) = 35 mL is used, leaving 10 mL

so total volume = 60 mL