The mass of NaCl required to prepare 0.07 m aqueous solution in 1kg water is

Options

A. 1g
B. 2g
C. 3g
D. 4g

Explanation

To find the mass of NaCl required to prepare a 0.07 molal (m) aqueous solution in 1 kg of water, we can follow these steps:

### Step-by-Step Solution:

1. **Understand the Definition of Molality**:
Molality (m) is defined as the number of moles of solute per kilogram of solvent. The formula is:
\[ \text{Molality (m)} = \frac{\text{Number of moles of solute}}{\text{Mass of solvent (kg)}} \]

2. **Identify Given Values**:
- Molality (m) = 0.07 m
- Mass of solvent (water) = 1 kg

3. **Set Up the Equation**:
Using the definition of molality, we can rearrange the formula to find the number of moles of NaCl:
\[ \text{Number of moles of NaCl} = \text{Molality} \times \text{Mass of solvent (kg)} \]
Substituting the known values:
\[ \text{Number of moles of NaCl} = 0.07 \, \text{m} \times 1 \, \text{kg} = 0.07 \, \text{moles} \]

4. **Calculate the Mass of NaCl**:
To find the mass of NaCl, we use the formula:
\[ \text{Mass of NaCl} = \text{Number of moles} \times \text{Molar mass of NaCl} \]
The molar mass of NaCl is approximately 58.5 g/mol. Thus:
\[ \text{Mass of NaCl} = 0.07 \, \text{moles} \times 58.5 \, \text{g/mol} \]
\[ \text{Mass of NaCl} = 4.095 \, \text{grams} \]

5. **Round the Result**:
Since we typically round to a reasonable number of significant figures, we can round this to:
\[ \text{Mass of NaCl} \approx 4 \, \text{grams} \]

### Final Answer:
The mass of NaCl required to prepare a 0.07 m aqueous solution in 1 kg of water is **4 grams**.