What weight of the non-volatile urea (`NH_2- CO - NH_2`) needs to be dissolved in 100g of water in order to decrease the vapour pressure of water by 25%? What will be molality of the solution?

Explanation

To solve the problem, we need to determine the weight of urea that must be dissolved in 100 g of water to achieve a 25% decrease in the vapor pressure of water. We will also calculate the molality of the resulting solution.

### Step 1: Understand the Problem
We know that the vapor pressure of water decreases by 25%. This means that if the original vapor pressure is \( P_{A0} \), the vapor pressure of the solution \( P_s \) will be:
\[ P_s = P_{A0} - 0.25 P_{A0} = 0.75 P_{A0} \]

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