Question

A sample of hydrogen (H2) gas is collected over water at 35oC and 725 mm. The volume of the gas collected is 72.0 ml. How many moles of H2 gas has been collected? How many grams of H2 gas has been collected?

Answer 1

Answer:

2.56 × 10⁻³ mol

5.17 × 10⁻³ g

Explanation:

Given data

Temperature = 35°C + 273 = 308 K

Pressure = 725 mmHg

Volume = 72.0 mL = 0.0720 L

The vapor pressure of water at 35°C is 42.20 mmHg. The total pressure is equal to the sum of the partial pressures.

P = pH₂ + pH₂O

pH₂ = P - pH₂O = 725 mmHg - 42.20 mmHg = 683 mmHg

pH₂ = 683 mmHg × (1 atm / 760 mmHg) = 0.899 atm

We can find the moles of H₂ using the ideal gas equation.

P × V = n × R × T

0.899 atm × 0.0720 L = n × (0.08206 atm.L/mol.K) × 308 K

n = 2.56 × 10⁻³ mol

The molar mass of H₂ is 2.02 g/mol. The mass of H₂ is:

2.56 × 10⁻³ mol × 2.02 g/mol = 5.17 × 10⁻³ g