Question

if you wanted to make 2 moles of h2o (water) using hydrogen (h2) and oxygen (o2) gas, how many moles of oxygen (o2) would you need?
stoichiometric calculations
equation: 2 h2(g) + o2(g) → 2 h2o(l)
molecules: 2 molecules h2 + 1 molecule o2 → 2 molecules h2o
mass (amu): 4.0 amu h2 + 32.0 amu o2 → 36.0 amu h2o
amount (mol): 2 mol h2 + 1 mol o2 → 2 mol h2o
mass (g): 4.0 g h2 + 32.0 g o2 → 36.0 g h2o
the coefficients in the balanced equation give the ratio of moles of reactants and products
4.0 moles oxygen
32.0 moles oxygen
36.0 moles oxygen
1 mole oxygen

Explanation:

Step1: Identify the balanced equation

The balanced chemical equation is $2\ce{H_2}(g) + \ce{O_2}(g)
ightarrow 2\ce{H_2O}(l)$. From this, the mole ratio of $\ce{O_2}$ to $\ce{H_2O}$ is $1:2$.

Step2: Use the mole ratio to calculate moles of $\ce{O_2}$

We want to make 2 moles of $\ce{H_2O}$. Let $x$ be the moles of $\ce{O_2}$ needed. Using the ratio $\frac{\text{moles of } \ce{O_2}}{\text{moles of } \ce{H_2O}} = \frac{1}{2}$, we substitute moles of $\ce{H_2O} = 2$: $\frac{x}{2} = \frac{1}{2}$. Solving for $x$, we get $x = 1$ mole.

Answer:

1 mole oxygen