Explanation:

At STP, 1 mole of any gas occupies 22.4 L. Therefore, 1.0 L of gas at STP contains 1.0 / 22.4 = 0.0446 moles of gas.

The molar mass of CH4 is 16 g/mol, the molar mass of CL2 is 70.9 g/mol, the molar mass of CO2 is 44 g/mol, and the molar mass of H2 is 2 g/mol.

Therefore, the mass of 0.0446 moles of CH4 is 0.0446 * 16 = 0.714 g.

The mass of 0.0446 moles of CL2 is 0.0446 * 70.9 = 3.16 g.

The mass of 0.0446 moles of CO2 is 0.0446 * 44 = 1.96 g.

The mass of 0.0446 moles of H2 is 0.0446 * 2 = 0.0892 g.

Therefore, the 1.0 liter sample with the greatest mass at STP is CO2g.