Which one of the following is the lightest ?

Options

A. 0.2 mole of hydrogen gas.
B. `6.023 xx 10^(23)` molecules of nitrogen.
C. 0.1 mole of oxygen gas.
D. 1 g of water.

Explanation

To determine which of the given options is the lightest, we will calculate the weight of each substance based on the number of moles and their respective molecular weights.

### Step-by-Step Solution:

1. **Calculate the weight of Hydrogen Gas (H₂)**:
- Given: 0.2 moles of H₂
- Molecular weight of H₂ = 2 (since the atomic weight of hydrogen is 1 and there are 2 hydrogen atoms in H₂).
- Weight of H₂ = Number of moles × Molecular weight = 0.2 moles × 2 g/mol = 0.4 grams.

2. **Calculate the weight of Water (H₂O)**:
- Given: 0.2 moles of H₂O
- Molecular weight of H₂O = 2 (for H) + 16 (for O) = 18 g/mol.
- Weight of H₂O = Number of moles × Molecular weight = 0.2 moles × 18 g/mol = 3.6 grams.

3. **Calculate the weight of Nitrogen Gas (N₂)**:
- Given: 1 mole of N₂ = 28 grams (since the atomic weight of nitrogen is 14 and there are 2 nitrogen atoms in N₂).
- Since we are given 6.023 × 10²³ molecules of N₂, this corresponds to 1 mole.
- Weight of N₂ = 28 grams.

4. **Calculate the weight of Oxygen Gas (O₂)**:
- Given: 0.1 moles of O₂
- Molecular weight of O₂ = 2 × 16 = 32 g/mol.
- Weight of O₂ = Number of moles × Molecular weight = 0.1 moles × 32 g/mol = 3.2 grams.

5. **Compare the weights**:
- Weight of H₂ = 0.4 grams
- Weight of H₂O = 3.6 grams
- Weight of N₂ = 28 grams
- Weight of O₂ = 3.2 grams

From the calculations, we can see that the lightest substance is **0.2 moles of Hydrogen gas (H₂)**, which weighs **0.4 grams**.

### Final Answer:
The lightest substance is **0.2 moles of Hydrogen gas (H₂)**.