The ratio between the molar mass of an alkene(A) and an alkyne(B) with the same number of carbon atoms is 1.05. Find the molecular formulas of the two hydrocarbons then write the reaction for how we can obtain the alkene A from the alkyne B.

We start by remembering the general formulas of alkenes and alkynes:
Alkenes - C_nH_2n
Alkynes - C_nH_2n-2
We calculate the molar mass of both of these using the fact that the atomic mass of carbon is 12 and for hydrogen 1 :
 μ_A=12n+2n=14n grams/mole
 μ_B=12n+2n-2=14n-2 grams/mole
The ratio then becomes:
μ_A / μ_B=1.05
14n / 14n - 2 = 1.05
14n =(14n - 2) * 1.05
14n = 14.7n - 2.1
14.7n - 14 = 2.1
0.7n = 2.1
n = 3
Therefore, our hydrocarbons are:
A = C₃H₆ - propene  
B = C₃H₄ - propyne
The reaction to obtain propene from the propyne is:
CH≡C-CH₃ + H₂ --^Ni---> CH₂=CH-CH₃
The reaction is a hydrogenation one and the condition needed for it Nickel.