Step 1: Recall the formula for gas density
Gas density at STP (Standard Temperature and Pressure) is calculated as: \[ \text{Density} = \frac{\text{Molar mass}}{\text{Molar volume at STP}} \] The molar volume of any ideal gas at STP is approximately \( 22.4 \, \text{L/mol} \).
Step 2: Compare gas molar masses
- \( \text{CO}_2 \) molar mass = \( 12 + 2 \times 16 = 44 \, \text{g/mol} \),
- \( \text{O}_2 \) molar mass = \( 2 \times 16 = 32 \, \text{g/mol} \),
- \( \text{N}_2 \) molar mass = \( 2 \times 14 = 28 \, \text{g/mol} \),
- \( \text{CH}_4 \) molar mass = \( 12 + 4 \times 1 = 16 \, \text{g/mol} \).
Step 3: Identify the densest gas
At STP, gas density is directly proportional to molar mass. \( \text{CO}_2 \) has the highest molar mass, thus it is the densest at STP.
Answer:
The gas with the highest density at STP is \( \text{CO}_2 \). This corresponds to option (1).