Dumbledore is with your progress. snape is, as always, somewhat indifferent. however, they both feel a gas phase density problem should come next. calculate the density of helium gas at 25.0◦c and at a pressure of 1521 mmhg. express your answer in g/l.

Answer is: 0,327 g/l.
Ideal gas law: pV = nRT.
V - volume, the amount of space occupied by the gas.
p - pressure ,1 atm = 760 torr = 760 mmHg.
n - amount of substance.
T - temperature, 273 K = 0°C. T = 25 + 273 = 298K.
R - ideal gas constant, R = 0,08206 L·atm/mol·K.
d(He) = M·p ÷ R·T.
d - density.
M - molar mass.
d(He) = 4g/mol · 2atm ÷  298K · 0,08206 L·atm/mol·K = 0,327 g/l.