Question

When the metal sample reacts with acid, the gas evolved will be collected over water; the gas is said to be "wet". what is the composition of "wet" gas? how can partial pressure of hydrogen gas be obtained from the total pressure of wet gas? how will you obtain the neccessay data ro determine the pressure of hydrogen gas?

Answer 1

1) The gas evolved will be collected over water. Since, over water there is water vapor, the gas collected is a mixture of the gas evolved of the reaction and the water vapour. This mixture of the gas with vapour water (or other vapour if the liquid were other substance) receives the name of wet gas because the gas obtained from the reaction is mixed with vapor from the liquid over which it gas collected.

2) The pressure read for the wet gas is the total pressure (p total), which is the sum of partial pressure of the gas evolved (pH2) plus the partial pressure of vapou (pH2O)

p total = pH2 + pH2O

Then, the partial pressure of the hydrogen is: pH2 = ptotal - pH2O.

So, to find the partial pressure of hydrogen, you need to find the partial pressure of water vapor in a table and subtract it from the total pressure.

3) The data necessary fo find the partial pressure of pH2 is, beside the total pressure read by the instruments, the partial water vapor pressure.

To obtain the partial pressure of water vapor you need the temperature of the system because the data of water vapor pressure are recorded in tables whose entrance is the temperature.