Be sure to answer all parts. The hydrogen-oxygen fuel cell is described in the t

Question

Be sure to answer all parts. 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 137 atm, would be needed to run an electric motor drawing a current of 8.50 A for 3.00 hr ?

?L

(b) What volume (in liters) of air at 25.0°C and 1.00 atm will have to pass into the cell per minute to run the motor ?

?L/min

Explanation / Answer

The reaction is H2--------> 2H+ 2e-

1 amp current gives rise to 6.241*1018 electrons/second

in 3 hours time and with 8/5 amps current it becomes 8.5*3*60*60*6.241*1018 electrons=5.78232*1023 electrons

1 mole correspond to 6.023*1023 electrons

5.2832*1023 correspond to 5.72832*1023/ 6.023*1023 = 0.95 moles of electrons

0.95/2 = 0.475 moles of hydrogen ( 2 moles of electrons give 1 mole of hydrogen)

from PV= nRT

n=0.475, P=137 atm, T=25 deg.c =25+273.15= 298.15K

V= nRT/ P= 0.475*0.0821*298.15/137=0.084 L

b) O2 +4e- -------->2O2-

4 electrons required for oxygen where as two electrons are required for hydrogen

moles of oxygen = 0.475/2= 0.2375

from PV=nRT

V= nRT/P= 0.2375*0.0821*(25+273.15)/1 =5.81 L

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