Volume of N2 in air=375×0.8=300ml
Volume of O2 in air=375×0.2=75ml
CxHy15ml0+(x+4y)O215(x+4y)0⟶xCO2(g)15x+2yH2O(ℓ)−
After combustion, total volume
345=VN2+VCO2
345=300+15x
x=3
Volume of O2 used
15(x+4y)=75
x+4y=5
y=8
So, hydrocarbon isC3H8.