Chemistry 151 Week 10 – Generating Hydrogen Gas
College of the Canyons Fall 2021
Page 1 of 12

Name __________________________________

Date ____________________________ Section_________________________________


For gas forming reactions, such as magnesium metal reacting with hydrochloric acid,

Mg(s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g)

the amount of hydrogen gas that is produced depends on stoichiometric quantities of magnesium metal
and hydrochloric acid used. For example, if 0.0750 g sample of magnesium metal reacts with excess
hydrochloric acid, the number of moles of hydrogen gas can be determined:

0.0750 g Mg ∗ 1 mol Mg
24.31 g Mg ∗ 1 mol H2
1 mol Mg = 𝟎𝟎. 𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎𝟎 𝐦𝐦𝐦𝐦𝐦𝐦 𝐇𝐇𝟐𝟐

We see from the balanced chemical equation above, that 1 mol of magnesium reacts to produce 1 mol of
hydrogen gas. Therefore, when 0.0750 g of magnesium are used, less than 1 mol of magnesium reacts
and less than 1 mol of hydrogen gas can be formed, or in this case, 0.00309 mol of hydrogen can form.

A classic experiment is to collect the gas produced from a chemical reaction over water, Figure 1. Using
the graduated cylinder, we can measure exactly the volume of hydrogen gas produced.

The gas that is collected will be mixed with water vapor, and is called a “wet gas”. According to Dalton’s
Law of Partial Pressures, the equation found below, the total pressure is the sum of the partial pressures
of the gases that make up the mixture. For gases that are collected over water, the total pressure is the
related to the pressure of the gas collected and the pressure of the water vapor.

Figure 1: An inverted graduated cylinder with a gas evolution apparatus and syringe
filled with hydrochloric acid. The base of the graduated cylinder has been removed.

Chemistry 151 Week 10 – Generating Hydrogen Gas
College of the Canyons Fall 2021
Page 2 of 12

The pressure due to the water vapor will depend on the temperature of the water, found in Table 1 below.

Table 1. Vapor Pressure of Water

Temperature Pressure Temperature Pressure Temperature Pressure
16 °C 14 mm Hg 21 °C 19 mm Hg 26 °C 24 mm Hg
17 °C 15 mm Hg 22 °C 20 mm Hg 27 °C 25 mm Hg
18 °C 16 mm Hg 23 °C 21 mm Hg 28 °C 26 mm Hg
19 °C 17 mm Hg 24 °C 22 mm Hg 29 °C 27 mm Hg
20 °C 18 mm Hg 25 °C 23 mm Hg 30 °C 28 mm Hg

For our example of 0.0750 g of magnesium metal reacting with hydrogen gas, if we measure the
temperature and atmospheric pressure, we can determine the partial pressure from the hydrogen gas.
Suppose the hydrogen gas was collected over water at 20 °C and an atmospheric pressure of 763 mmHg,
let’s determine the partial pressure due to hydrogen gas. First, we need to look up in Table 1, the vapor
pressure of water at this temperature. According to Table 1, the vapor pressure of water at 20 °C is 18
mmHg. Next, the atmospheric pressure of 763 mmHg is the total pressure. So using Dalton’s Law of
Partial Pressures, we can find the partial pressure of hydrogen gas:

Here is a general animated visualization of collecting a gas over water in chemistry and with an
explanation of Dalton’s Law of Partial Pressures:
Standard Molar Volume

The standard molar volume of a gas is the volume occupied by one mole of a gas at standard temperature
and pressure (STP) conditions. Standard conditions are defined as 0 °C and 1 atm. Using the STP
conditions, the value for the standard molar volume can be calculated using the Ideal Gas Law:

PV = nRT
Thus, the standard molar volume of any gas at STP conditions is 22.4 L/mol. The volume occupied by 1
mole of a gas at STP is 22.4 L.
1 mol any gas = 22.4 L at STP
Combined Gas Law

Let’s go back to our magnesium solid reacting with hydrochloric acid to produce hydrogen gas. We have
already determined how many moles of hydrogen can be produced (0.00309 mol). We have also figured
out what the partial pressure of the hydrogen gas is (745 mmHg). We can measure the experimental
volume of hydrogen gas produced using the graduated cylinder.

Do you have a similar assignment and would want someone to complete it for you? Click on the ORDER NOW option to get instant services at We assure you of a well written and plagiarism free papers delivered within your specified deadline.