The solubility of a salt in water is `40 g` at `30^(@)C`. The amount of water required to dissolve `120 g` at the same temperature is:

Options

A. (a) `400 g`
B. (b) `4 litre`
C. (c ) `300 g`
D. (d) `500 g`

Explanation

To solve the problem of how much water is required to dissolve 120 g of a salt at 30°C, given that the solubility of the salt is 40 g in 100 g of water, we can follow these steps:

### Step 1: Understand the solubility
The solubility of the salt is given as 40 g per 100 g of water. This means that to dissolve 40 g of salt, you need 100 g of water.

### Step 2: Determine the ratio of salt to water
From the solubility information:
- 40 g of salt requires 100 g of water.

### Step 3: Set up a proportion for 120 g of salt
We need to find out how much water is required to dissolve 120 g of salt. We can set up a proportion based on the solubility:
\[ \frac{40 \text{ g salt}}{100 \text{ g water}} = \frac{120 \text{ g salt}}{x \text{ g water}} \]

### Step 4: Cross-multiply to solve for x
Cross-multiplying gives us:
\[ 40 \cdot x = 120 \cdot 100 \]
\[ 40x = 12000 \]

### Step 5: Solve for x
Now, divide both sides by 40:
\[ x = \frac{12000}{40} = 300 \text{ g} \]

### Conclusion
The amount of water required to dissolve 120 g of salt at 30°C is **300 g**.