(1 points) What is the pressure of a 20.5 L helium balloon containing 5.00 g of He at 25.0 ºC (298 K)? Solution moles = mass / molar mass moles of He = 5 g / ( 4 g/mol) moles of He = 1.25 mol now for gases PV = nRT P = nRT / V given V = 20.5 L n = 1.25 mol R = 0.0821 L atm / mol K T = 25 C = 298 K plug in the values P = 1.25 mol x (0.0821 L atm / mol K) x 298 K / 20.5 L P = 1.492 atm so the pressure of the balloon is 1.492 atm .