Areaction of 0.028 g of magnesium with excess hydrochloric acid generated 31.0 ml of hydrogen gas. the gas was collected by water displacement in a 22o c water bath. the barometric pressure in the lab that day was 746 mm hg. use dalton's law and the vapor pressure of water at 22o c (appendix b. to calculate the partial pressure of the hydrogen gas in the gas collecting tube. 726 mmhg

Here Magnesium is reacted with hydrochloric acid and the hydrogen evolved is collected at top. Hydrochloric acid will be present in a solution (HCl + Water). The solvent will be water. Due to evaporation some amount of water will have evaporated and would be present in vapour phase. Hence when the reaction occurs, only hydrogen will not be present in vaporphase but, will be accompanied by water vaporizes as well

Hence. Dalton's law the total pressure of the system will be sum of pressure exerted by hydrogen gas and pressure exerted by water vapours

Let partial pressure of hydrogen gas be "PH2"

The partial pressure of water will be nothing but the vapor pressure of water. 

Vapour pressure of water = Pwater = 19.8 mmHg;

Total pressure of the system = 746 mmHg;

Total pressure = PH2 + Pwater;

746 = PH2 + 19.8;

From above equation PH2 = 746-19.8 = 726.2 mmHg

the answer is 0.8m.

by: nicholas