Water-displacement method?

I collected hydrogen by reacting Mg with HCl. I can calculated the pressure of dry H2 by subtacting the pressure of water vapor at that particular temperature. However, does the water vapor have volume ? And how could I calculate it ?

1 Answer
Ernest Z.
Feb 14, 2018

Yes, the water vapour has volume.

Explanation:

I am guessing that your experiment was like that shown in the diagram below.

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You collected the hydrogen over water in the graduated cylinder.

The water vapour occupied the same volume as the hydrogen.

You don't need the volume of the water vapour for the experiment,

However, Here's how you would calculate it if you could separate the vapour from the hydrogen.

Example

Assume that you collected 45.3 mL of gas at 18 °C and 1.040 bar. The vapour pressure of water at 18 °C is 0.0204 bar. What is the volume of pure water vapour in the gas?

Solution

We can use Boyle's Law to calculate the volume of the water vapour by itself.

#color(blue)(bar(ul(|color(white)(a/a)p_1V_1 = p_2V_2color(white)(a/a)|)))" "#

#p_1 = "0.0204 bar"; V_1 = "45.3 mL"#
#p_2 = 1.040color(white)(l) "bar";color(white)(ll) V_2 = ?#

#V_2 = V_1 × p_1/p_2 = "45.3 mL" × (0.0204color(red)(cancel(color(black)("bar"))))/(1.040 color(red)(cancel(color(black)("bar")))) = "0.89 mL"#

Thus, if you could separate the water vapour from the hydrogen, its volume would be 0.89 mL.

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