**Problem**

The simplest formula for butane is C

_{2}H

_{5}and its molecular mass is about 60. What is the molecular formula of butane?

**Solution**

First, calculate the sum of the atomic masses for C

_{2}H

_{5}. Look up the atomic masses for the elements from the Periodic Table. The atomic masses are found to be:

H is 1.01

C is 12.01

Plugging in these numbers, the sum of the atomic masses for C

_{2}H

_{5}is:

2(12.0) + 5(1.0) = 29.0

This means the formula mass of butane is 29.0. Compare the formula mass (29.0) to the approximate molecular mass (60). The molecular mass is essentially twice the formula mass (60/29 = 2.1), so the simplest formula must be multiplied by 2 to get the molecular formula:

molecular formula of butane = 2 x C

_{2}H

_{5}= C

_{4}H

_{10}

**Answer**

The molecular formula for butane is C

_{4}H

_{10}.