How many moles of argon (Ar) are present in 5.6 liters of argon gas a STP?

To answer this, we must first know what STP is. STP (Standard Temperature and Pressure) is when the environment is at the following:

• 0oC
• 1 Atmosphere of Pressure (101.325 kPa)

To figure out any missing units in a gas equation, we use the Ideal Gas Law, PV=nRT.

• 'P' is pressure in kPa
• 'V' is volume in litres or dm3
• 'n' is the number of mol
• 'R' is the gas constant = 8.314
• 'T' is temperature in kelvin

To work out the number of mol, we substitute in the calues we have and solve for n. This gives us 101.325kPa*5.6dm3=n*8.314*273K. Therefore, n=0.25 mol.

Alternatively, we could simply use the fact that 1 mol of any gas at STP takes up 22.4 litres. So 5.6/22.4 leaves us with 0.25 mol.