Question

Benzene is 92.3% carbon and 7.7% hydrogen. In anexperiment,

the molecular mass of benzene was determined to be 78.0grams per
mole. What is the molecular formula forbenzene?

Answer 1

The molecular formula of benzene =
`C_(6)H_(6)`

Step-by-step explanation:

`Moles =\frac {Given\ mass}{Molar\ mass}`

% of C = 92.3

Molar mass of C = 12.0107 g/mol

% moles of C =
`(92.3)/(12.0107)` = 7.6848

% of H = 7.7

Molar mass of H = 1.00784 g/mol

% moles of H =
`(7.7)/(1.00784)` = 7.6401

Taking the simplest ratio for C and H as:

7.6848 : 7.6401

= 1 : 1

The empirical formula is =
`CH`

Molecular formulas is the actual number of atoms of each element in the compound while empirical formulas is the simplest or reduced ratio of the elements in the compound.

Thus,

Molecular mass = n × Empirical mass

Where, n is any positive number from 1, 2, 3...

Mass from the Empirical formula = 12+ 1 = 13 g/mol

Molar mass = 78.0 g/mol

So,

Molecular mass = n × Empirical mass

78.0 = n × 13

⇒ n ≅ 6

The molecular formula of benzene =
`C_(6)H_(6)`