What is the molar mass of a gas that has a density of 1.25 g/L? If the gas is comprised of a single element, what would that element be?

Assume that the density is measured at STP, i.e. 1 atm and 273 K (0°C).
Molar volume of gas at STP = 22.4 L/mol

1.25 g of the gas occupy a volume of 1.00 L.
Moles of 1.25 g of the gas = (1.00 L) / (22.4 L/mol) = 1/22.4 mol
Molar mass of the gas = (1.25 g) / (1/22.4 mol) = 28.0 g/mol

Refer to the Periodic Table, the molar mass of N = 14.0 g/mol
The molar mass of N₂ = (14.0 g/mol) × 2 = 28.0 g/mol

Hence, the gas is nitrogen (N₂).

Molar mass is just the mass of a substance divided by the moles of substance. So it's grams/moles. And remember that all gasses are 22.4 L/mol. So just get liters to cancel out by multiplying 22.4 L/mol by 1.25 g/L = 28.0 g/mol. Molar masses are given on a periodic table under the elements' abbreviations.

At STP, that gas will occupy 22.4 Liters of space for every mole of gas.

22.4 liters * 1.25 g/liter =>
22.4 * 1.25 grams =>
22.4 * 5/4 grams =>
11.2 * 5/2 grams =>
5.6 * 5 grams =>
28 grams

https://www.lenntech.com/periodic/mass/atomic-mass.htm

The only element with an amu of 28, or anything close to that, is Silicon, which isn't a gas at STP. However, Nitrogen has an amu of 14, so N2 would have a mass of 28 grams per mole.