The hydrogen-oxygen fuel cell is described in the text. For the questions below,

Question

The hydrogen-oxygen fuel cell is described in the text. For the questions below, assume the following:

Assume that the air is 20 % O2 by volume and that all the O2 is consumed in the cell. The other components of air do not affect the fuel-cell reactions. Assume ideal gas behavior.

(a) What volume of H2(g), stored at 25.0°C at a pressure of 155 atm, would be needed to run an electric motor drawing a current of 8.50 A for 3.00 hr ?
(b) What volume (in liters) of air at 25.0°C and 0.85 atm will have to pass into the cell per minute to run the motor ?

Explanation / Answer

(a) 8.5 Amps * 3 hours * (3600 sec/1 hour) * (1 C/1 Amp*sec) = 91800 C * (1 mol e-/96500 C) = 0.951 mol e-

0.951 mol e- * (2 mol H2/4 mol e-) = 0.476 mol H2
V = (nRT)/P = (0.476 mol H2*0.0821*298 K)/155 atm = 0.075 L H2

(b) 0.951 mol e- * (1 mol O2/4 mol e-) = 0.238 mol O2

V=(0.238 mol O2 * 0.0821 * 298K)/ 0.85 atm = 6.85 L

now air has 20 percent of oxygen, so volume of oxygen needed= 6.86/20% = 34.25 L

time = 3 hours

so volume of air per minute neede to run motor = 34.25(2 *60) =0.285 L/min

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