Question

Small quantities of hydrogen can be prepared by the addition of hydrochloric acid to zinc. A sample of 195 mL of hydrogen was collected over water at 25°C and 753 torr. What mass of hydrogen was collected? (Pwater = 24 torr at 25°C)

Answer 1

Given : volume of H2 gas = 195 mL

T = 25 deg C= 25.0 degC + 273.15 = 298.15 K

Pressure= 753 torr

We know total pressure = pressure of H2 gas + pressure of H2O vapor

Lets subtract the pressure of water from total pressure to get pressure of H2 gas

Pressure of H2 gas = 753 torr – 24 torr

= 729 torr

Lets find moles of H2

To get mol of H2 we use ideal gas equation.

n = pV / RT

here value of R is L atm per K per mol

R = 0.0821 L atm per K per mol

T in K

We convert pressure to atm

Pressure in atm

=729 torr 8 * 1 atm / 760 torr

= 0.9592 atm

n = 0.9592 atm * 0.195 L / ( 0.0821 L atm per K per mol * 298.15 K )

= 0.00764 mol H2

We use molar mass to get mass of H2

Mass of H2 in g

= mol of H2 * molar mass of H2

= 0.00764 mol H2* 2.016 g per mol / mol H2

= 0.0154 g H2

Mass of H2 collected = 0.0154 g