A quantity of gas is collected in a gradutated tube over the mercury. The volume of the gas at `20^(@)C` is `50.0 mL` and the level of the mercury in the tube is `100 mm` above the outside mercury level. The barometer reads `750 mm`. Volume at `STP` is

Options

A. `39.8 mL`
B. `40 mL`
C. `42 mL`
D. `60 mL`

Explanation

To solve the problem of finding the volume of gas at Standard Temperature and Pressure (STP), we can follow these steps:

### Step 1: Determine the Effective Pressure
The effective pressure of the gas can be calculated by subtracting the height of the mercury column above the outside mercury level from the barometric pressure.

Given:
- Barometric pressure (P_barometer) = 750 mm Hg
- Height of mercury in the tube (h) = 100 mm Hg
...