Final answer:
The partial pressure of H2 gas collected over water when Cr reacts with dilute HNO3 at 25°C is determined by subtracting the vapor pressure of water from the total pressure, yielding a partial pressure of H2 at 738.2 torr.
Step-by-step explanation:
To determine the partial pressure of H₂ when a small piece of Cr metal reacts with dilute HNO3, we need to account for the vapor pressure of water at the given temperature. Since the reaction is collected over water at 25°C, we need to subtract the vapor pressure of water at this temperature from the total pressure to find the partial pressure of H₂.
At 25°C, the vapor pressure of water is typically around 23.8 torr (this value can slightly vary in different references). Subtracting this from the total pressure in the flask (762 torr), we get:
Partial pressure of H₂ = Total pressure - Vapor pressure of water
Partial pressure of H₂ = 762 torr - 23.8 torr
Partial pressure of H₂ = 738.2 torr
In this scenario, the partial pressure of H₂ gas collected over water at 25°C is 738.2 torr. This value is crucial for further calculations involving the Ideal Gas Law or determining the amount of H₂ gas produced.