Question

1. (a) how many moles is 1.20 x 10²⁵ molecules nh₃ ( ammonia)? 19.9 mol nh₃ (b) what mass is this number of molecules? answer: 338 g nh₃

Explanation:

Step1: Recall Avogadro's number

Use $n=\frac{N}{N_A}$, where $N = 1.20\times10^{25}$, $N_A=6.022\times 10^{23}\text{ mol}^{-1}$. So $n=\frac{1.20\times 10^{25}}{6.022\times 10^{23}}\approx19.9\text{ mol}$.

Step2: Calculate molar - mass of $NH_3$

$M(NH_3)=14 + 3\times1=17\text{ g/mol}$. Then $m = nM$, so $m=19.9\text{ mol}\times17\text{ g/mol}=338.3\text{ g}\approx338\text{ g}$.

Answer:

(a) 19.9 mol
(b) 338 g