The mass fraction of potassium in the compound is 56.6%, carbon – 8.7%, oxygen – 34.8%. Determine the formula for this substance.

Given:

w% (K) = 56.6%

w% (C) = 8.7%

w% (O) = 34.8%

To find:

Formula -?

Decision:

1) Find the molecular weight of a substance using the following formula:

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

Find Mr using potassium data:

Mr (substance) = Ar (element): w% (element) = 39: 0.566 = 69.

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

69 – 100%

x – 56.6%,

x = 56.6% * 69: 100% = 39

N (K) = 39: Mr (K) = 39: 39 = 1 atom.

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

69 – 100%

x – 34.8%,

x = 34.8% * 69: 100% = 24

N (O) = 24: Mr (O) = 24: 16 = 1.5 atoms

4) Then the molecular weight of carbon accounts for 69 – 24 – 39 = 6

N (C) = 6: Mr (C) = 6: 12 = 0.5 atoms

We have obtained the simplest formula of the substance KC0.5O1.5, a substance with such a formula does not exist, so we multiply the indices by 2: KCO3 – calcium carbonate.

Answer: KCO3 is calcium carbonate.