A Quantitative Investigation of the Reaction
of a Metal with Hydrochloric Acid
The reaction between an active metal, such as magnesium, Mg, and an acid can be an
explosively dangerous event. Hydrochloric acid is a very corrosive acid. Since hydrogen
chloride is gaseous, the acid can burn the eyes, nasal passages and irritate your skin.
Because hydrogen gas, H2 (g), is a product of the reaction, care must be taken to prevent
the combustion of this highly flammable gas.
The balanced equation for this reaction is: Mg (s) + 2 HCl (aq)  H2 (g) + MgCl2 (aq)
Magnesium chloride, awater-soluble salt, is produced when hydrochloric acid oxidizes
the metal. This investigation concerns the volume of hydrogen gas that can be generated
from a given mass of magnesium. For example, how many liters of dry hydrogen gas at
room conditions can be produced per mole of magnesium metal?
By employing a 50-mL gas measuring tube, a 1-hole stopper, a short strip of magnesium
ribbon, 6.0 M HCl, and a short length of thin copper wire, we can investigate this matter.
What volume of 6.0 M HCl will be needed to in generate a maximum of 50 mL of
gaseous H2? How much magnesium will be required? Stoichiometric calculations
will allow us to specify the quantities of magnesium and hydrochloric acid used in the
reaction.
Some procedural tips to promote your efficient use of time are as follows:
1. Incline the gas measuring tube slightly from an upright position and pour in a
sufficient volume of 6.0 M HCl that will completely consume the Mg sample.
2. With the tube in the same position, slowly fill the tube with distilled water from
a beaker. While pouring, rinse any acid that may be on the sides of the tube so that
the liquid in the top of the tube will contain very little acid. Try to avoid stirring up
the acid layer in the bottom of the tube.
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3. Roll up the Mg ribbon and thread the copper wire through the opening; use the copper
wire as a twist tie to hold the roll of Mg together. There should be enough copper
wire beyond the Mg to hold the sample of magnesium about two inches into the end
of the gas measuring tube. Hook the copper wire over the edge of the tube and hold it
there by inserting the rubber stopper. The tube should be completely filled so that the
stopper displaces a little water when put in place. See the figure provided below (far
left).
4. Cover the hole(s) in the stopper with your finger and invert the tube in the beaker of
water, as shown in the figure below (second picture from left). Allow no air in the
tube. Clamp the tube in place.
5. When the reaction stops, wait for about 5 minutes to allow the tube to come to room
temperature. Dislodge any bubbles clinging to the sides of the tube.
6. Record the volume of gas in the tube to the nearest 0.1 mL, the temperature of the
water to the nearest 0.1 °C, and the room pressure to the nearest 1 mm Hg.
7. Cover the hole(s) in the stopper with your finger and transfer the tube to a large
cylinder, which is almost filled with water at room temperature. Raise or lower the
tube until the level of the liquid inside the tube is the same as the level outside the
tube. See figure below (far right). Read the volume with your eye at the same level as
the bottom of the meniscus. The figure on the next page illustrates how to avoid a
parallax error. Record the volume of gas in the tube now.
Post-lab Questions
1. The 6.0 M HCl diffused down the inverted tube and consumed the magnesium
sample. What property of the acid enables this to happen?
2. Does the copper wire generate any hydrogen gas when it is exposed to hydrochloric
acid? If so, will this complicate matters?
3. When the hydrogen gas is generated in the tube in this manner, it’s called wet
hydrogen. Water in the tube evaporates into the gaseous space and generates a vapor
pressure. Does this complicate matters? If so, how do we determine the pressure
exerted by the hydrogen gas alone?
4. Why does the volume of trapped gases (H2 and H2O) change when transferring the
tube to the large cylinder?
4. What is the molar volume of hydrogen gas at STP (0°C, 1 atm)? What is hydrogen’s
density at STP? Make sure that you always show your calculational work to back up
your answers.
This project is funded by a grant awarded under the President’s Community Based Job Training Grant as implemented by
the U.S. Department of Labor’s Employment and Training Administration (CB-15-162-06-60). NCC is an equal
opportunity employer and does not discriminate on the following basis:

against any individual in the United States, on the basis of race, color, religion, sex, national origin, age
disability, political affiliation or belief; and

against any beneficiary of programs financially assisted under Title I of the Workforce Investment Act of 1998
(WIA), on the basis of the beneficiary’s citizenship/status as a lawfully admitted immigrant authorized to work in
the United States, or his or her participation in any WIA Title I-financially assisted program or activity.
This product was funded by a grant awarded under the President’s High Growth Job Training Initiative, as implemented by
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