Reaction of alkanes

Question

C3H6 gas and an excess amount of H2 are put into a closed container. The gases react with each other until all of the C3H6 is consumed. After the reaction and at the initial temperature, the ratio of the initial pressure to the final pressure is found to be 8/7. According to this data, what is the mass percentage of C3H6 in the starting mixture?

Answer 1

C3H6 + H2 → C3H8

ratio of the initial pressure to the final pressure is found to be 8/7

The pressure is a function of the number of gas molecules

 

So, 4C3H6 + 28H2 → 4C3H8 + 24H2

 

This mixture would produce a ratio of 32/28, which is = to 8/7

 

42.08 g/mol = molar mass of C3H6

2.02 g/ mol = molar mass of 

 

4C3H6(42.08 g/mol) = 168.32g

 

28H2(2.02 g/mol) = 56.56g

168.32g/(168.32g + 56.56g) = 74.8% by mass

 

Alexa - this is a total shot in the dark.  I've never done a problem like this, or if I have, it's too long ago to recall...  There are better chemists answering questions online - maybe one will check my work...  Or, perhaps you'll recognize any errors from class practice.