Question

The reaction 2C2H6 + 7O2 yields 4CO2 + 6H2O has a ratio of 2 parts ethane (C2H6) and 7 parts oxygen (O2). How many parts ethane (C2H6) will be needed to react with 21 parts of oxygen (O2)? Please show your work.

Answer 1

You know that 2 parts ethane react with 7 parts oxygen .
Now you want to know how many parts ethane react with 21 parts oxygen.

Can you see that 21 is 7*3 ? Therefore if you multiply through by 3 you have your answer:
6 parts ethane react with 21 parts oxygen .

A more general equation:
Parts ethane = 21/7*2 = 6 parts ethane.