# Derive the formula of organic matter by mass fractions of elements C = 40% O = 53.3 H = 6.7%

Given:

w% (C) = 40%

w% (O) = 53.3%

w% (H) = 6.7%

Find:

Formula -?

Solution:

1) Find the molecular weight of the substance:

w% (element) = Ar (element): Mr (substance), where Ar is the atomic mass,

then Mr (substance) = Ar (element): w% (element)

You can calculate the molecular weight of a substance by one atom from its composition, if its mass fraction is known. Take the mass fraction and the atomic mass of carbon:

Mr (substances) = 12: 0.4 = 30.

2) Knowing the mass fraction of carbon, we find its molecular weight in the composition and the number of atoms:

30 – 100%

x – 40%

x = 40% * 30: 100% = 12

N (C) = 12: Mr (C) = 12: 12 = 1 atom.

3) Find the molecular weight of oxygen in the composition and the number of atoms:

30 – 100%

x – 53.3%,

x = 53.3% * 30: 100% = 16

N (O) = 16: Mr (O) = 16: 16 = 1 atom.

4) Then the molecular weight of hydrogen accounts for 30 – 12 – 16 = 2

N (H) = 2: Mr (H) = 2: 1 = 2 atoms.

We got a substance with the formula HCHO – methanal or formic aldehyde. 