C_{2}H_{6} + 3O_{2} -> 2CO_{2} + 3H_{2}O C_{2}H_{6} + 5O_{2} -> 2CO_{2} + 3H_{2}O 2C_{2}H_{6} + 5O_{2} -> 4CO_{2} + 4H_{2}O 2C_{2}H_{6} + 7O_{2} -> 4CO_{2} + 6H_{2}O
I gave my students this multiple choice question: Which of these chemical equations demonstrates the law of conservation of mass as it applies to the combustion of ethane?

Only one answer is correct among these 4 choices:
- C_{2}H_{6} + 3O_{2} -> 2CO_{2} + 3H_{2}O
- C_{2}H_{6} + 5O_{2} -> 2CO_{2} + 3H_{2}O
- 2C_{2}H_{6} + 5O_{2} -> 4CO_{2} + 4H_{2}O
- 2C_{2}H_{6} + 7O_{2} -> 4CO_{2} + 6H_{2}O

Could you tell me which one is correct?

2C_{2}H_{6} + 7O_{2} -> 4CO_{2} + 6H_{2}O