A student designs an ammeter (a device that measures electrical current) that is

Question

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 , 12.6 mL 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?

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

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

vapor pressure (pwater-vap) of water at 25oC is 23.8 torr (as provided above)

Ptotal = pwater-vap + phydrogen

Ptotal - pwater-vap = phydrogen

phydrogen = 715 torr - 23.8 torr = 691.2 torr = (691.2/760) atm = 0.9094 atm

now applying ideal gas equation phydrogenV= nhydrogenRT

0.9094 * 12.6 = nhydrogen * 0.082 * (25 + 173)

nhydrogen = 11.458/24.436 = 0.4688 moles = 468.8 milimoles.

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