What is the formula of the substance, the constituent fractions of the elements of which are W (C) -75%; W (H) -25%.

Mass ratio of elements in an unknown compound:

w (C): w (H) = 75: 25

Taking into account the atomic masses of the elements, let’s move on to the quantitative ratio:

n (C): n (H) = w (C) / M (C): w (H) / M (H) = 75/12: 25/1 = 6.25: 25 = 1: 4

This means that in an unknown compound there are 4 hydrogen atoms per 1 carbon atom, whence the simplest formula of this compound is CH4 (methane).

Answer: CH4.