At STP (standard temperature and pressure), the temperature is 273 K and the pressure is 1 atm. The molar volume of a gas at STP is 22.4 L/mol.
To calculate the number of moles of gas in a volume of 3.5 L at STP, we can use the following formula:
moles = volume (in L) / molar volume (in L/mol)
moles = 3.5 L / 22.4 L/mol
moles = 0.15625 mol
Therefore, the number of moles of gas in a volume of 3.5 L at STP is approximately 0.15625 mol.