One litre of an unknown gas weight 1.25 g at STP. Which one of the following can be the formula for the gas ?

Options

A. `O_(2)`
B. CO
C. `CO_(2)`
D. `SO_(2)`

Explanation

To determine the formula of the unknown gas weighing 1.25 g per liter at STP (Standard Temperature and Pressure), we can follow these steps:

### Step-by-Step Solution:

1. **Understanding STP**: At STP, 1 mole of any gas occupies 22.4 liters. This means that if we know the weight of 1 liter of gas, we can calculate the weight of 1 mole (22.4 liters) of that gas.

2. **Calculating the weight of 1 mole of gas**:
- Given that 1 liter of the gas weighs 1.25 g, we can find the weight of 22.4 liters (which is 1 mole) using the unitary method.
- Weight of 22.4 liters = Weight of 1 liter × 22.4
- Weight of 22.4 liters = 1.25 g × 22.4 = 28 g

3. **Identifying possible gases**:
- Now that we know that 1 mole of the gas weighs 28 g, we need to identify which gas could have a molar mass of 28 g.
- We will compare the molar masses of common gases to find a match.

4. **Calculating molar masses of potential gases**:
- **Carbon Monoxide (CO)**:
- Carbon (C) = 12 g/mol
- Oxygen (O) = 16 g/mol
- Molar mass of CO = 12 g + 16 g = 28 g
- **Other gases**:
- **Oxygen (O2)**: 16 g × 2 = 32 g (not a match)
- **Carbon Dioxide (CO2)**: 12 g + 16 g × 2 = 44 g (not a match)
- **Sulfur Dioxide (SO2)**: 32 g + 16 g = 48 g (not a match)

5. **Conclusion**:
- The only gas that matches the calculated molar mass of 28 g is Carbon Monoxide (CO).
- Therefore, the formula for the unknown gas is **CO**.

### Final Answer:
The formula for the gas is **CO**.