Question

how many grams of water are produced if we react 3 moles of hydrogen with 3 moles of oxygen?
$2h_2(g) + o_2(g) \
ightarrow +2h_2o(g)$
about 80 grams
about 60 grams
about 40 grams
about 20 grams

Explanation:

Step1: Relate moles of $\text{H}_2$ to $\text{H}_2\text{O}$

From the balanced equation $2\text{H}_2(g) + \text{O}_2(g)
ightarrow 2\text{H}_2\text{O}(g)$, 2 moles of $\text{H}_2$ produce 2 moles of $\text{H}_2\text{O}$, so mole ratio $\frac{n_{\text{H}_2\text{O}}}{n_{\text{H}_2}} = \frac{2}{2} = 1$.
$n_{\text{H}_2\text{O}} = n_{\text{H}_2} = 3 \text{ moles}$

Step2: Calculate molar mass of $\text{H}_2\text{O}$

Molar mass of $\text{H}_2\text{O} = 2\times1 + 16 = 18 \text{ g/mol}$

Step3: Compute mass of $\text{H}_2\text{O}$

Mass = moles $\times$ molar mass
$m_{\text{H}_2\text{O}} = 3 \times 18 = 54 \text{ grams}$ (≈60 grams)

Answer:

About 60 grams