Step 1: Relationship between Molecular Weight and Density
At STP (Standard Temperature and Pressure), gas density is directly proportional to its molar mass. Higher molar mass results in higher density.
Step 2: Molar Masses of Gases
The molar masses are:
- \( \text{O}_2 \): \( 32 \, \text{g/mol} \)
- \( \text{N}_2 \): \( 28 \, \text{g/mol} \)
- \( \text{CO}_2 \): \( 44 \, \text{g/mol} \)
- \( \text{H}_2 \): \( 2 \, \text{g/mol} \)
Step 3: Density Comparison
\( \text{CO}_2 \) has the highest molar mass, thus the highest density at STP.
Answer: Option (3): \( \text{CO}_2 \).