Final answer:
At STP, 1 mole of any gas occupies a volume of 22.4 L. Therefore, 4 L of O2 gas at STP will have the same number of molecules as 4 L of CO2 gas.
Step-by-step explanation:
At STP (Standard Temperature and Pressure), 1 mole of any gas occupies a volume of 22.4 L.
Given that 4 L of O2 gas is present, we can determine which of the options contains the same number of molecules by comparing their volumes at STP.
Option A: 1 L of NH3 contains 3 moles of NH3 gas.
Option B: 2 L of Cl2 contains 1 mole of Cl2 gas.
Option C: 4 L of CO2 contains 1 mole of CO2 gas.
Option D: 8 L of He contains 2 moles of He gas.
Comparing the number of moles, 1 L of NH3 and 4 L of CO2 both contain the same number of molecules as 4 L of O2 gas at STP. Therefore, the correct answer is Option C: 4 L of CO2.