A cylinder with a movable piston contains 2.00 g of helium, He, at room temperature. More helium was added to the cylinder and the volume was adjusted so that the gas pressure remained the same. How many grams of helium were added to the cylinder if the volume was changed from 2.00 L to 2.50 L ? (The temperature was held constant.) Solution Molar mass of He = 4.003 g/mol mass of He = 2.00 g we have below equation to be used: number of mol of He, n = mass of He/molar mass of He =(2.0 g)/(4.003 g/mol) = 0.4996 mol we have: n1 = 0.4996 mol V1 = 2.00 L V2 = 2.50 L we have below equation to be used: V1/n1 = V2/n2 2.0 L / 0.4996 mol = 2.5 L / n2 n2 = 0.6245 mol Molar mass of He = 4.003 g/mol we have below equation to be used: mass of He, m = number of mol * molar mass = 0.6245 mol * 4.003 g/mol = 2.50 g Answer: 2.50 g .