Question

What is the mass of H2O produced when 10.0 grams of H2 react completely with 80.0 grams of O2

Answer 1

$2 \ H_2 + O_2 \to 2 \ H_2O$
2 moles of H₂ : 1 mole of O₂ : 2 moles of H₂O

number of moles of H₂ : number of moles of H₂O = 1 : 1

$m_{H_2}=10 \ g \\ M_{H_2}=2 \ \frac{g}{mol} \\ n_{H_2}=\frac{10 \ g}{2 \frac{g}{mol}}=5 \ mol$

There are 5 moles of hydrogen so there must be 5 moles of water too.
$n_{H_2O}=5 \ mol \\ M_{H_2O}=18 \ \frac{g}{mol} \\ m_{H_2O}=5 \ mol \times 18 \ \frac{g}{mol}=90 \ g$

The mass of H₂O is 90 g.