Question

1.Water can be decomposed into its elements by applying an external voltage. Hydrogen gas form sat one electrode and oxygen at the other, according to the following overall equation : 2H2O(l)→2H2(g) + O2(g)The hydrogen gas evolved by this reaction was collected in a water-filled tubeat 25 °C. The water levels were equalized to make the pressure of the trapped gas equal to the ambient pressure.However, the trapped gas is actually a mixture of hydrogen gas and water vapor. The partial pressure of water (its “vapor pressure”) depends only on temperature, and is 23.8 mmHg at 25 °C.

(a)If the ambient pressure is 748.0 Torr, what is the partial pressure of H2(g) in this tube?

(b)Use the pressures from part a to calculate the mole fractions of H2(g) and water vapor.

(c)Calculate the total moles of trapped gas (hydrogen and water vapor) if the measured total volume is 75.0 mL.

(d)How many moles ofH2(g) were trapped?

Answer 1

a) The law of partial pressures says that the total pressure is the sum of partial pressures. Since there's only water and hydrogen, we can calculate the partial pressure of hydrogen as:

b) We will use Dalton's law, that says that the partial pressure of a gas in a mixture is equal to the product of the total pressura and the molar fraction of the gas in question. In this case:

Same thing for water:

c) For the total moles, we use the ideal gas equation:

Re-arranged for the number of moles:

We are converting Torr to Atm and have converted mL to L.

4: For the number of moles of H₂, we use: