One of the current barriers to widespread use of solar energy is the storage of
ID: 637865 • Letter: O
Question
One of the current barriers to widespread use of solar energy is the storage of that energy. Hydrogen gas may be a solution to that problem by using the energy (electricity) generated from PV panels to perform hydrolysis (split water into oxygen and hydrogen gases). A researcher working on the pilot study performed a demonstration by collecting 275 mL of hydrogen gas over water at 25°C. The pressure in the gas collection chamber was 765 torr. What mass of hydrogen gas (mg) was collected? Hint: You need to subtract the partial pressure of water vapor from the total pressure. Look up the water vapor pressure at the given temperature.
Explanation / Answer
Volume of H2 gas collected V = 275 mL x 1L/1000mL
= 0.275 L
Temperature T = 25 + 273 = 298 K
Partial pressure of H2 = Barometric pressure - Vapor pressure of water at 25C
= 765 - 23.8
= 741.2 torr x 1 atm/760torr
= 0.9753 atm
From the ideal gas equation
Moles of H2 gas collected = PV/RT
= 0.9753 atm x 0.275 L / (0.0821 L-atm/mol-K x 298K)
= 0.0109621 mol
Mass of H2 = moles x molecular weight
= 0.0109621 mol x 2 g/mol
= 0.02192 g
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