What mass of water is produced when 8.0g of hydrogen reacts with excess oxygen?

1. Analyze the question

We are asked to calculate the mass of water ($H_2O$) produced when 8.0g of hydrogen gas ($H_2$) reacts completely with excess oxygen.

2. Explain the concept

To solve this, we use Stoichiometry. We must:

• Write the balanced chemical equation for the reaction.
• Determine the molar masses of the substances involved.
• Use the mole ratio from the balanced equation to find the mass of the product.

The reaction between hydrogen and oxygen is:

3. Show the solution

Step 1: Identify the molar masses.

• Relative atomic mass of $H = 1.0$, $O = 16.0$.
• Molar mass of $H_2 = 2 \times 1.0 = 2.0 \text{ g/mol}$.
• Molar mass of $H_2O = (2 \times 1.0) + 16.0 = 18.0 \text{ g/mol}$.

Step 2: Use the balanced equation. From the equation $2H_2 + O_2 \rightarrow 2H_2O$:

• 2 moles of $H_2$ produce 2 moles of $H_2O$.
• This means 1 mole of $H_2$ produces 1 mole of $H_2O$.

Step 3: Calculate the moles of hydrogen used.

Step 4: Calculate the mass of water produced. Since the ratio of $H_2$ to $H_2O$ is 1:1, 4.0 moles of $H_2$ will produce 4.0 moles of $H_2O$.

4. Identify the answer

Comparing our result to the given options, the correct mass is 72.0g. This matches option D.

5. Tip

Always check that your chemical equation is balanced before starting your calculation. A common mistake is forgetting to account for the coefficients (the numbers in front of the molecules). In this case, the 2:2 ratio simplified to 1:1, but if you had missed the coefficients, your calculation would have been incorrect.