Question

1. if you start with 2.1 moles of cr, how many molecules of pb will be produced?

$4\\ cr + 3\\ pb(no_3)_4 \
ightarrow 4\\ cr(no_3)_3 + 3\\ pb$

Explanation:

Step1: Find mole ratio of Cr to Pb

From the balanced equation, the mole ratio is $\frac{3\ \text{mol Pb}}{4\ \text{mol Cr}}$.

Step2: Calculate moles of Pb

Multiply starting moles of Cr by the ratio:
$2.1\ \text{mol Cr} \times \frac{3\ \text{mol Pb}}{4\ \text{mol Cr}} = 1.575\ \text{mol Pb}$

Step3: Convert moles to molecules

Use Avogadro's number $6.022 \times 10^{23}\ \text{molecules/mol}$:
$1.575\ \text{mol Pb} \times 6.022 \times 10^{23}\ \frac{\text{molecules}}{\text{mol}}$

Answer:

$9.48 \times 10^{23}$ molecules of Pb (rounded to three significant figures)