Final answer:
Equal partial pressures of helium and nitrogen over water at equilibrium imply equal amounts of moles for both gases, according to Dalton's Law and the ideal gas law, assuming no reaction between the gases.
Step-by-step explanation:
When helium and nitrogen are present in a gas at equal partial pressures over water at equilibrium, it means that the total number of molecules for both gases is proportional to their respective partial pressures. This is in line with Dalton's Law which states that the total pressure exerted by a mixture of non-reacting gases is equal to the sum of the partial pressures of individual gases. Considering the ideal gas law, at a given temperature and volume, the moles of helium will be equal to the moles of nitrogen if their partial pressures are equal.
It is also important to note that if gases are collected over water, the observed pressure is a combination of the gas's partial pressure and the water vapor pressure at the given temperature. Hence, the actual amount of the gas is calculated by subtracting the water vapor pressure from the total pressure observed.