The Reaction of Magnesium with Hydrochloric Acid
In this experiment you will determine the volume of hydrogen gas which is produced when a
sample of magnesium reacts with 6 M hydrochloric acid. The volume of the hydrogen gas
produced will be measured at room temperature and pressure. The data you obtain will enable
you to answer the questions:
1. How many liters of dry hydrogen gas at room temperature and pressure can be
produced per mole of magnesium metal?
2. What is the molar volume of hydrogen gas at STP?
PROCEDURE
a) Obtain a 2.5 cm piece of magnesium ribbon wrapped in copper wire. The cage should
have no large openings through which small pieces of magnesium ribbon could escape.
The piece of magnesium is uniform in thickness and width.
b) Set up a ring stand and utility clamp in position to hold a 50 mL gas-measuring tube as
shown in Figure 12-3. Fill a 400 mL beaker about two thirds full of tap water. Place it
near the ring stand.
c) Incline the gas-measuring tube slightly from an upright position and pour in about 5 mL
of 6 M hydrochloric acid.
d) With the tube in the same position, slowly fill it with tap water from a 250 mL beaker.
While pouring, rinse down any acid that may be on the side of the test 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. Bubbles clinging to the side of the tube can be dislodged
by tapping the tube gently.
e) Holding the copper coil by the handle, insert the cage about 5 cm down into the 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.
f) Cover the hole in the stopper with your finger and invert the tube in the 400 mL filled
with water, as shown in Figure 12-6. Clamp it in place. The acid, being more dense
than water, will fall down through it and eventually react with the metal.
g) After the reaction stops, wait for about 5 minutes to allow the gas in the tube to equalize
to the temperature of the water in the tube. Dislodge any bubbles clinging to the side of
the tube.
h) Cover the hole in the stopper with your finger and transfer the tube to a large cylinder or
battery jar which is almost filled with water at room temperature. See Figure 12-4.
Lower or raise the tube until the level of liquid inside the tube is the same as the
level outside the tube. This permits you to measure the volume of gases in the tube
(hydrogen and water vapor) at room pressure. Read the gas volume. Your eye should be
at the same level as the bottom of the meniscus (the lens-shaped surface taken by the
water in tube). See Figure 12-5. Record the volume of the gas as precisely as you can.
i) Remove the gas-measuring tube from the water and pour the acid solution it contains
down the sink. Rinse the tube with tap water.
j) Record water temperature and atmospheric pressure.
<Fig.12-3
1)
Figure 12-6A
Fig. 12-5>
Reduce errors by reading the
bottom of the meniscus with
your eye at the proper level
<Fig. 12-4 Measuring the volume of a gas
PROCESSING THE DATA:
(for credit must show all work including units and substance. Four steps not necessary.)
1) Determine the mass of the magnesium you used knowing the grams per meter and the
length of your piece of ribbon.
2) Determine the number of moles of magnesium used.
3) Determine the pressure of the hydrogen gas at the conditions of this experiment. (Hint:
Dalton’s Law of Partial Pressure)
4) Determine the volume of dry hydrogen gas at standard temperature and pressure. (Hint:
you can use combined gas law or ideal gas law )
5) Calculate the volume of dry hydrogen which would be produced if you had used one
mole of magnesium . (Hint: you will need a) the amount of hydrogen collected (#4) and
b) the number of moles of magnesium used (#2).
6) Write a balanced chemical equation for the reaction of magnesium and hydrochloric acid.
7) What is the amount (volume) of hydrogen theoretically you could produce from one mole
of Mg at STP?
8) Calculate the percent error of the value you obtained for the molar volume of hydrogen
compared with the accepted value (theoretical value) at STP.
9) If 1 mole of hydrogen weighs 2.0 g, what is the mass of a liter (the density) of hydrogen
at room temperature and 1 atmosphere pressure?
YOUR DATA TABLE SHOULD INCLUDE THE FOLLOWING:
Mass of magnesium ribbon in grams per 100.0 centimeters (from teacher)
Length of magnesium ribbon used
Volume of hydrogen and water vapor collected
Temperature of the water (temperature of the gas in the gas measuring tube)
Barometer reading (room pressure)
Vapor pressure of water at the above temperature (see table)
Modified from Ms. Gupta with help from: http://homepage.mac.com/ms.bearse/labhydrogen.pdf.pdf