Question

A common way to make hydrogen gas in the laboratory is to place a metal such as zinc in hydrochloric acid (see the figure). The hydrochloric acid reacts with the metal to produce hydrogen gas, which is then collected over water. Suppose a student carries out this reaction and collects a total of 153.6 mL of gas at a pressure of 742 mmHg and a temperature of 25∘C. What mass of hydrogen gas (in milligrams) does the student collect? (The vapor pressure of water is 23.78 mmHg at 25∘C.)

Express the mass to three significant figures and include the appropriate units.

Answer 1

total pressure = partial pressure of gas+ saturated vapor pressure of water

742= partial pressure of gas +23.78

partial pressure of gas, P= 742-23.78=718.22 mm Hg, 1 since 1 atm= 760 mm Hg, partial pressure of H2= 718.22/760 atm =0.945 atm

volume of gas , V= 153.6ml= 153.6/1000 L= 0.1536 L, R= gas constant =0.0821 L.atm/mole.K, T= 25 deg.c= 25+273= 298K

n= no of moles of hydrogen = PV/RT = 0.945*0.1536/(0.0821*298) =0.005933

mass of hydrogen = moles of hydrogen* molar mass= 0.005933*2= 0.011866 gm

1gm =1000mg, mass of hydrogen =1000*0.011866 gm =11.866 mg