Question

A sample of hydrogen is collected by displacing 175 mL of water. The atmospheric pressure is 725 mm Hg. The temperature of the water is 25.5 degrees Celsius and that of the gas is 312 Kelvin. How many moles of hydrogen have been evolved?

Answer 1

pressure of gas = pressure observed - aqueous tension

aqueous tension = vapor pressure of water at the given temperature (here 25.5 degree c) =24.47mm Hg

therefore , the pressure of hydrogen =(725-24.47) mm Hg =700.53 mm Hg

also , 700.53 mm Hg = (700.53/ 760) atm = 0.921 atm, volume = 175ml = 0.175 L

now by using the ideal gas equation pv = nRT

we get, n=pv/RT

n= (0.921x0.175)/(0.082x 312) = 6.304 x 10^-3  mol

Hence  6.304 x 10^-3  mol of hydrogen is evolved.