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 1.00min , 13.1mL of water-saturated H2(g) is collected. The temperature and pressure of the system are 25 ?C and 715 torr. How many moles of hydrogen gas are produced?

T (?C) P (torr) 21 18.7 22 19.8 23 21.1 24 22.4 25 23.8 26 25.2 27 26.7 28 28.4 29 30.0 30 31.8

Explanation / Answer

The vapour pressure of water at 25 c is 23.8 Torr ( from table)

Now to determine the moles of hydrogen we will use

PV = nRT

Pressure of dry hydrogen = 715 - 23.8 = 691.2 torr

Now PV =nRT

691.2 / 760 atm X 0.0131 L = n X 0.08206 L atm / mol K X 298 K

n = 0.000487 moles

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