4 g of hydrogen reacts with 20 g of oxygen to form water. The mass of water formed is

Options

A. 24 g
B. 36 g
C. 22.5 g
D. 40 g

Answer

To solve the problem of determining the mass of water formed when 4 g of hydrogen reacts with 20 g of oxygen, we can follow these steps:

### Step 1: Write the balanced chemical equation
The reaction between hydrogen and oxygen to form water can be represented by the following balanced equation:
\[ 2H_2 + O_2 \rightarrow 2H_2O \]

### Step 2: Calculate the number of moles of hydrogen and oxygen
To find the number of moles, we use the formula:
\[ \text{Number of moles} = \frac{\text{Given mass}}{\text{Molar mass}} \]

- For hydrogen (H₂):
- Given mass = 4 g
- Molar mass of H₂ = 2 g/mol
\[ \text{Number of moles of } H_2 = \frac{4 \text{ g}}{2 \text{ g/mol}} = 2 \text{ moles} \]

- For oxygen (O₂):
- Given mass = 20 g
- Molar mass of O₂ = 32 g/mol (16 g/mol × 2)
\[ \text{Number of moles of } O_2 = \frac{20 \text{ g}}{32 \text{ g/mol}} = 0.625 \text{ moles} \]

### Step 3: Identify the limiting reagent
The limiting reagent is the reactant that will be completely consumed first in the reaction. From the balanced equation, we see that:
- 2 moles of H₂ react with 1 mole of O₂.

From our calculations:
- We have 2 moles of H₂ and 0.625 moles of O₂.
- The stoichiometric ratio shows that 2 moles of H₂ would require 1 mole of O₂. Therefore, 0.625 moles of O₂ can react with:
\[ 0.625 \text{ moles of } O_2 \times 2 = 1.25 \text{ moles of } H_2 \]
Since we have 2 moles of H₂ available, O₂ is the limiting reagent.

### Step 4: Calculate the moles of water produced
According to the balanced equation:
- 1 mole of O₂ produces 2 moles of H₂O.

Thus, 0.625 moles of O₂ will produce:
\[ 0.625 \text{ moles of } O_2 \times 2 = 1.25 \text{ moles of } H_2O \]

### Step 5: Calculate the mass of water formed
To find the mass of water produced, we use the formula:
\[ \text{Mass} = \text{Number of moles} \times \text{Molar mass} \]
- Molar mass of H₂O = 18 g/mol
\[ \text{Mass of } H_2O = 1.25 \text{ moles} \times 18 \text{ g/mol} = 22.5 \text{ g} \]

### Final Answer
The mass of water formed is **22.5 grams**.

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