Final answer:
Methane (CH4) has the lowest density at STP when compared to N2, Cl2, N2O, and F2 because it has the lowest molar mass of 16.06 g/mol.
Step-by-step explanation:
The gas with the lowest density at standard temperature and pressure (STP) can be determined by comparing the molar masses of the options provided since at STP, 1 mole of any ideal gas will occupy the same volume (22.4 L). The molar masses of the gases are as follows: N2 (28.02 g/mol), Cl2 (70.90 g/mol), N2O (44.02 g/mol), CH4 (16.06 g/mol), and F2 (38.00 g/mol). Given these molar masses, methane (CH4) has the lowest molar mass, and therefore has the lowest density at STP.
In determining the number of molecules, Avogadro's constant (6.022 x 1023 molecules/mol) is utilized. For each substance, you would divide the mass provided by its molar mass to find the number of moles and then multiply it by Avogadro's constant to find the number of molecules. The least number of molecules corresponds to the substance with the least moles.