Question

A 20g sample of P4O10 contains how many molecules of phosphorus pentaoxide?

Answer 1

1. Find moles of $P_4O_{10}$

$$n=\frac{m}{M}=\frac{20g}{283.89\frac{g}{mol}}=0.070 mol$$

As 1 molecule of $P_4O_{10}$ cobtains 2 molecules of $P_2O_5$ , then moles of $P_2O_5$ :

$$n(P_2O_5)=n(P_4O_{10})\times 2 = 0.070\times 2= 0.14 mol$$

2.Find number of molecules of $P_2O_5$

$$N=N_A\times n=6.022\times10^{23}\times 0.14 =8.4\times 10^{22} molecules$$