[Duzzy]

The hydrocarbon contains 25% of hydrogen (g/g) is:

a) C2H2 b) CH4 c) C2H6 D) C3h8 E) C2H4

For this question the answer is "b" according to the test key but it doesn't make any sense. When going through these tests I do the questions first and check the answers afterwards. Shouldn't this be C since 24 g/mol and 6g/mol is equal to 25%? All B is saying is that there are 4 hydrogens to one carbon.

[Sophia7X]

The best way to approach these type of problems is to assume 100 grams. So 25% hydrogen by mass = 25 grams.
That means there must be 75 grams carbon, right?

Then you should be able to get B as the answer with a little conversion.

[Borek]

The best way to approach these type of problems is to assume 100 grams.

No need to assume any mass here.

C₂H₂ has a molar mass of 2*12+2*1=26 g. There are two moles of atomic hydrogen in the mole of C₂H₂, that means 2*1 g of hydrogen, so mass percentage of hydrogen is 2/26*100%=7.7%.

Shouldn't this be C since 24 g/mol and 6g/mol is equal to 25%?

Your mistake is that you compare mass of hydrogen to mass of carbon, while you should compare mass of hydrogen to the total mass of the molecule.