Question

how many moles are in 2.11 x 10^24 molecules of bef2?
1 mol bef2 / 6.02 x 10^23 molecules bef2 = x / 2.11 x 10^24 molecules bef2
? mol bef2

Explanation:

Step1: Recall Avogadro's number

Avogadro's number is $6.02\times 10^{23}$ molecules/mol. We set up a proportion to find the number of moles. Let $x$ be the number of moles of $BeF_2$. The proportion is $\frac{1\ mol\ BeF_2}{6.02\times 10^{23}\ molecules\ BeF_2}=\frac{x}{2.11\times 10^{24}\ molecules\ BeF_2}$.

Step2: Solve for $x$

Cross - multiply: $x=\frac{2.11\times 10^{24}\ molecules\ BeF_2\times1\ mol\ BeF_2}{6.02\times 10^{23}\ molecules\ BeF_2}$.
$x=\frac{2.11\times 10^{24}}{6.02\times 10^{23}}$. Using the rule of exponents $\frac{a^m}{a^n}=a^{m - n}$, we have $\frac{2.11}{6.02}\times10^{24 - 23}$. $\frac{2.11}{6.02}=0.35$, and $10^{24 - 23}=10^1$. So $x = 3.5\ mol$.

Answer:

$3.5\ mol$