# Question #05d9c

${A}_{r}$ of phosphorus = 30.97. This means that 1 mole of phosphorus atoms weighs 30.97g
So 1 mole of ${P}_{4}$ molecules weighs $4 \times 30.97 = 123.88 \text{g}$
The number of moles $n$ = mass/mass of 1 mole:
$n = \frac{m}{M} _ r = \frac{92.91}{123.88} = 0.75$