2 g of a gas collected over water at `20^(@)C` and under a pressure of 770mm Hg occupied 800ml. Calculate the volume of dry gas at S.T.P. condition. Vapour pressure of water at `20^(@)C` is 15mm Hg.

Explanation

To solve the problem, we need to calculate the volume of dry gas at Standard Temperature and Pressure (STP) conditions using the given data. Here’s a step-by-step solution:

### Step 1: Determine the pressure of the dry gas
The pressure of the gas collected over water is given as 770 mm Hg. However, we need to account for the vapor pressure of water at 20°C, which is 15 mm Hg.

\[ P_{\text{dry}} = P_{\text{collected}} - P_{\text{water}} = 770 \, \text{mm Hg} - 15 \, \text{mm Hg} = 755 \, \text{mm Hg} \]
...