23) B

Butane would always have 4 carbon atoms.

Mr of 4 carbon atoms = 12 x 4 = 48

80% = 48

1% = 48 ÷ 80 = 0.6

100% = 0.6 x 100 = 60

So the mass of butane = 60

Mass that must be made by deuterium and hydrogen = 60 – 48 = 12

Mass of deuterium = 2 x mass of hydrogen

Let number of hydrogen atoms = H

Let number of deuterium atoms = D

H + 2D = 12

Sum of number of hydrogen and deuterium atoms = 10

So H + D = 10

Using simultaneous equations,

H + 2D = 12

H + D = 10

D = 2

Since H + D = 10

H = 10 – 2 = 8

So the formula must be C₄H₈D₂