Naphthalene is a soft covalent solid that is often used in mothballs. Its molar mass is 128.18 g/mol and it contains 93.75% carbon and 6.25% hydrogen. Determine the molecular formula of napthalene from this data.

C10H8

Explanation:

To answer this question, we need to know the atomic masses of carbon and hydrogen. The atomic mass of carbon is 12 amu while that of hydrogen is 1 amu. We start answering the question by dividing the percentages of these elements by their atomic masses.

H = 6.25/1 = 6.25

C = 93.75/12 = 7.8125

We proceed to divide both by the smallest number. This is 6.25

H = 6.25/6.25 = 1

C = 7.8125/6.25 = 1.25

The ratio of both is 1:1.25.

To make a whole number out of these, we multiply through by 4 and that gives 4:5

The empirical formula is thus C5H4

Now, we calculate the molecular formula:

(C5H4) n = 128.18

(5 (12) + 4 (1)) n = 128.18

(60+4) n = 128.18

64n = 128.18

n = 128.18/64

n = approximately 2

The molecular formula is thus C10H8