The vapor pressure of water changes with temperature, as shown here. A student d

Question

The vapor pressure of water changes with temperature, as shown here. A student designs an ammeter (a device that measures electrical current) that is based on the electrolysis of water into hydrogen and oxygen gases. When electrical current of unknown magnitude is run through the device for 4.00 min 16.9 mL of water-saturated H_2(g) is collected. The temperature and pressure of the system are 25 degree C and 715 torr. How many moles of hydrogen gas are produced? Express your answer to three significant figures and include the appropriate units.

Explanation / Answer

P = 715 torr = 715 * 0.001316 atm = 0.941 atm

V = 16.9mL = 0.0169 L

T = 25oC = 298K

PV = nRT

or, n = PV/RT

        = 0.941 atm * 0.0169L /0.082Latm/K/mol * 298 K

         = 6.51 *10^-4 moles

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