Question

Calculate the number of moles in 15 liters of oxygen at STP.

Answer 1

At STP, 1 mole of any ideal gas occupies exactly 22.4 L - this is known as the molar volume of a gas at STP.
n = 15.0 L * (1 mole / 22.4 L) = 0.6696 mole

Answer: 0.6696 moles in 15 liters of oxygen at STP.