Question

How do I determine the vapor pressure of H2O (mmHg) using the below data and following formula:

P atmospheric= Pressure H2 + Pressure H2O + Pressure height diff

	Trial 1	Trial 2	Trial 3
Mass of Mg used (g)	0.042 g	0.044 g	0.045 g
Volume of H2 gas collected (L)	0.0435 L	0.045 L	0.048 L
Temperature of gas (Kelvin)	295.15 K	295.15 K	295.15 K
Atmospheric pressure (in Hg)	29.53	29.53	29.53
Height difference (mmH2O)	14.5 cm 145 mm	7.01cm 70.1 mm	9.50 cm 95 mm
Vapor pressure of H2O (mmHg)	?	?	?

Answer 1

According to the equation given:

We can calculate the pressure of the hydrogen by using the ideal gas equation:

The moles of H2 can be calculated from the known chemical reaction between Mg and hydrochloric acid:

From the mass of Mg used:

Notice that we used the stoichiometrical relationship between Mg an H2 in the second fraction.

With a value of R = 0.082 atm*L/mol*K, we subtitute the values in equation (2) to calculate the pressure of H2:

Converting into mmHg units:

Converting the height difference into mmHg units:

Converting atmospheric pressure into mmHg units:

Substituting these values in equation (1) and solving for pressure of H2O:

This is the vapor pressure of H2O for trial (1).

For trial (2) we will apply the same calculations shown previously to have the results:

Substituting in equation (1) and solving for pressure of H2O:

Repeating the procedure for trial (3), with the calculated values:

Substituting in equation (1) and solving for pressure of H2O: