Lactic acid, the substance that builds up in muscles during exercise, has a molar mass of about 90 g and has a composition
of 40.0% C, 6.71% H, and the rest is oxygen. What is the
molecular formula of lactic acid?

THE MOLECULAR FORMULA FOR THE LACTIC ACID IS C3H6O3

Explanation:

Lactic acid is composed of carbon, hydrogen and oxygen.

It has molar mass of 90 g/mol

Constituents composition:

Carbon = 40.0 %

Hydrogen = 6.71 %

Oxygen = 100 - (40 + 6.71) = 53.29 %

First we calculate the empirical formula

1. Divide the percentage composition by its atomic mass of each element.

(C = 12 , H = 1, O = 16)

Carbon = 40 / 12= 3.333

Hydrogen = 6.71/1 = 6.71

Oxygen = 53.29 / 16 = 3.33

2. Divide each value of the element by the smallest of them.

carbon = 3.333/ 3.33 = 1

Hydrogen = 6.71 / 3.33 = 2.015

Oxygen = 3.33 / 3.33 = 1

3. The empirical formula is therefore

C H2 O

4. Calculate the molecular formula

(CH2O)n = Molar mass

( 12 + 1 *2 + 16)n = 90

(30)n = 90

n = 90 / 30

n = 3

The molecular formula is written as

(CH2O) 3

= C3H6O3

Hence, the molecular formula for the lactic acid is C3H6O3.