Question

Please help, I don't know the formulas to do these and I'd like to know HOW y'all get the answers...thank you so much.
A technician produced H2 gas by reacting zinc (Zn) with sulfuric acid (H2SO4). The reaction was: 2Zn (s) + H2SO4 (aq) ----> Zn2SO4 (aq) + H2
He collected the hydrogen in a flask by water displacement. Atmospheric pressure was 737.2 torr, and room temperature was 25.0 degrees celsius.
a. What was the vapor pressure in the water flask?
b. What was the pressure of H2 in the flask?
c. Suppose that a leak in the gas collection tubes accidentally let a small amount of air into the flask. The unwanted air exerts 25.2 torr of pressure in the flask. How much pressure is the H2 exerting in the flask?

Answer 1

The vapour pressure in the water flask is 23.686 torr.

The pressure of $H_2$ in the flask was 713.514  torr.

Pressure is the $H_2$ exerting in the flask is 688.314 torr

What is vapour pressure?

Vapour pressure is a measure of the tendency of a material to change into the gaseous or vapour state, and it increases with temperature.

a.The vapour pressure of water is given by:

$Log_{10} P =A- \frac{B}{C+T}$, where T is in degree Celcius, P is in torr.

For 0-100 degree celcius, A=8.07131, B=1730.63, C=233.426

$Log_{10} P = 8.07131 - \frac{1730.63}{233.426+25} =1.3745$

As T =25 degree celcius

$P = 10^{1.3745} torr$ =23.686 torr

b.We know that $P_{total} =P_{H_2} + P_{H_2O}$

$P_{total}$ = Atmospheric pressure

$737.2 torr = P_{H_2} +23.686 torr$

$P_{H_2} = 713.514 torr$

c.After there is a leak in the gas collection tube, the amount at pressure due to water vapour remains unchanged as it is a quantity dependent on temperature.

$P_{total} =P_{H_2} + P_{H_2O} + P_{leaking \;air}$

$737.2 torr = P_{H_2} + 23.686 torr + 25.2 torr$

$P_{H_2} =688.314 torr$

For calculating molar mass of any gas:

M= Density X (RT ÷ P)

M= 0.714 g X ($8.314 J mol^{-1}K^{-1}$273 K ÷ 101325 Pa)

M= 15.994 g/mol = 16 g/mol.

Learn more about vapour pressure here:

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