One of the half-reactions for the electrolysis of water is 2H+(aq) + 2e− → H2(g) If 0.845 L of H2 is collected at 25°C and 782 mmHg, how many faradays of electricity had to pass through the solution?

0.0725 faradays of electricity had to be passed through the solution.

Explanation:

Find how many hydrogen moles are produced.

1 mole of any gas consumes 24 liters at room temperature  

atmospheric pressure 1 environment is 760 mm Hg  

temperature is 25 deg C  

so if you reduced the pressure down to 760 mm Hg on that 0.845 liters you've gathered, you would discover the volume expanded to  

0.845 x 782/760 = 0.869 liters  

which equals to 0.0362 moles (divide by 24)  

since every atom of hydrogen required 2 electrons to be made, hence we need 0.0725 moles of electrons or 0.0725 faradays.