Question

#1. Zinc metal is added to hydrochloric acid to generate hydrogen gas and is collected over a liquid whose vapor pressure at 18.4 °C is 59.7 torr. The volume of the mixture is 7.7 L and its total pressure is 0.86 atm. Determine the partial pressure of the hydrogen gas in this mixture. Respond with the pressure to the nearest tenth of a torr.

#2. Zinc metal is added to hydrochloric acid to generate hydrogen gas and is collected over a liquid whose vapor pressure at 22.7 °C is 35.0 torr. The volume of the mixture is 5.2 L and its total pressure is 1.04 atm. Determine the number of moles of the hydrogen gas in this mixture. Respond with the nearest hundredth of a mol.

Answer 1

1) When we collect gas over certain liquid , it is not a pure gas. It is a mixture of gas and vapors of liquid. Hence, total pressure we measured is the sum of partial pressure of that gas and vapor pressure of liquid.

i e P _total = P _gas + P _{liquid Vapor.}

P _gas = P _{total -} P _{liquid Vapor.}

Pressure of hydrogen gas =  P _{total -} P _{liquid Vapor.}

We have, vapor pressure of liquid = 59.7 torr, total pressure = 0.86 atm . We have relation, 1 atm = 760 torr .

Total pressure = 0.86 atm ( 760 torr / 1 atm ) = 653.6 torr

Pressure of hydrogen gas = 653.6 torr - 59.7 torr = 593.9 torr

ANSWER : Pressure of hydrogen gas = 593.9 torr

2) Given :

Total pressure = 1.04 atm = 1.04 atm ( 760 torr / 1 atm ) = 790.4 torr

Vapor pressure of liquid = 35.0 torr

Temperature = 22. 7 ⁰ C = 295.7 K

Volume of gas = 5.2 L

We have,  Pressure of hydrogen gas =  P _{total -} P _{liquid Vapor.}  

Pressure of hydrogen gas = 790.4 torr - 35.0 torr = 755.4 torr

We have relation, 1 atm = 760 torr

Pressure of hydrogen gas = 755.4 torr ( 1 atm / 760 torr ) = 0.9939 atm

We have, P V = n R T

where, P is a pressure of a gas, V is a volume of a gas, n is no. of moles of gas, R is a gas constant and T is temperature of gas.

Number of moles of Hydrogen gas = P V / R T

Number of moles of Hydrogen gas = 0.9939 atm 5.2 L / ( 0.082057 L atm mol ^-1 K ^-1 295.7 K )

Number of moles of Hydrogen gas = 0.21299 mol

ANSWER : Number of moles of Hydrogen gas in the mixture = 0.21 mol