Final answer:
The partial pressure of helium gas in the mixture is 1800 mm Hg, which is calculated using Dalton's law of partial pressures by taking 0.75 of the total pressure.
Step-by-step explanation:
The partial pressure of helium gas in the mixture is obtained by finding the fraction of helium in the mixture and multiplying it by the total pressure of the mixture. Here, helium makes up 0.75 of the total number of moles (since it's 3.0 mol of He out of 1.0 mol H₂ + 3.0 mol He), and the total pressure is 2400 mm Hg. Therefore, the partial pressure of helium can be calculated as 0.75 × 2400 mm Hg, which equals 1800 mm Hg.