Question

How many chlorine atoms are there in seven sodium chloride formula units?

Answer 1

The number of formula units of NaCl are 7. In 1 mole of NaCl there are $6.023\times 10^{23}$ formula unit are present.

Thus, 1 formula unit will be there in $\frac{1}{6.023\times 10^{23}}=1.66\times 10^{-24}$ moles.

and, 7 formula unit will be there in $1.16\times 10^{-23}$ moles.

There is 1 mol of Cl present in 1 mol of NaCl. thus, $1.16\times 10^{-23}$ of Cl will be there in  $1.16\times 10^{-23}$  mol of NaCl.

Converting number of moles into number of atoms as follows:

In 1 mole of a substance there are $6.023\times 10^{23}$ atoms of chlorine thus, in $1.16\times 10^{-23}$  mol there will be 7 atoms of chlorine.

Therefore, number of atoms of chlorine will be 7.