Question

The simplest formula for butane is ${C_2}{H_5}$ and its molecular mass is about 60. What is the molecular formula of butane $?$

Answer 1

To determine the molecular formula from the empirical formula. We need to find the empirical formula mass then compare the empirical formula mass to the molecular formula mass. Divide the molecular formula mass by the empirical formula mass. The result is the whole number by which the subscripts in the empirical formula will be multiplied to determine the subscripts of the molecular formula.

Complete answer:
A molecular formula consists of the chemical symbols for the constituent elements followed by numeric subscripts describing the number of atoms of each element present in the molecule. The empirical formula represents the simplest ratio of the number of atoms in a compound.
To determine the empirical formula mass, multiply the subscript of each element by its atomic weight on the periodic table. Since the atomic weight of carbon is $12.011\,u$ and that of hydrogen is $1.008\,\;u$ .
Then the empirical formula mass of ${C_2}{H_5}$ $= 2\left( {12.011\,} \right) + 5\left( {1.008\,} \right) = 29.062\;u$
Given the molecular formula mass of ${C_2}{H_5}$ $= 60\;u$ . Then divide the molecular formula mass by the empirical formula mass, we get
$\dfrac{{60}}{{29.062}} = 2.0645$ -----(1)
Since the ratio must be the whole number. So, the equation (1) written as
$\dfrac{{60}}{{29.062}} = 2$ (Rounded to one significant figure)
Then multiply the subscripts of the empirical formula by $2$ . We get
The molecular formula of ${C_2}{H_5}$ $= {C_{\left( {2 \times 2} \right)}}{H_{\left( {2 \times 5} \right)}} = {C_4}{H_{10}}$
Hence the molecular formula for butane is ${C_4}{H_{10}}$ .

Note:
The molecular formula gives the actual number of each different atom present in a molecule. The molecular formula is a multiple of the empirical formula. Also note that the total weight of an atom is called the atomic weight and it is approximately equal to the number of protons and neutrons.