Chemistry 108 Lab #3
Name_____________________________
Lab # 3: Gases
Percent Yield of Hydrogen Gas from
Magnesium and Hydrochloric Acid
Introduction
For chemical reactions involving gases, gas volume measurements provide a convenient means of
determining stoichiometric relationships. A gaseous product is collected in a long, thin graduated glass
tube, called a eudiometer, by displacement of a liquid, usually water. Magnesium reacts with hydrochloric
acid, producing hydrogen gas:
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
Note: for every mole of Mg (s) that is reacted, one mole of H2(g) is produced. If we know the
mass of Mg(s) we can convert to moles of Mg(s). Then, since we get 1 mole of H2(g) for every mole of
Mg(s), we can predict how many moles of H2(g) could be made (theoretical yield). We use an excess of
HCl so that we would react all the Mg(s) before we used all of the HCl.
When the magnesium reacts with the acid, the evolved hydrogen gas is collected by water
displacement and its volume is measured. The temperature of the gas is taken to be the same as the
temperature of the water it is in contact with because, given a sufficient amount of time, the two will reach
thermal equilibrium. The level of water in the eudiometer is adjusted so that it is equal to the level of water
outside the eudiometer. This insures that the pressure in the eudiometer is equal to the prevailing
atmospheric pressure. Because the hydrogen gas was collected above water, and water has a significant
vapor pressure, to get the pressure of pure hydrogen (dry hydrogen), we must subtract the vapor pressure of
water. The pressure of the dry hydrogen gas (PH2) is calculated from Dalton's Law of Partial Pressures:
Ptotal = PH2 + PH2O
so
PH2 = Ptotal - PH2O
where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH2O (the water vapor
pressure) is the pressure exerted by water vapor that has evaporated into the eudiometer. We will get the
vapor pressure of water from the table below of vapor pressure vs. temperature.
1
Chemistry 108 Lab #3
The number of moles of hydrogen gas collected can then be calculated from the ideal gas law:
(n= # moles H2)
n = PV
RT
(Use PH2 here, not Ptotal)
This will give you the experimental # moles of hydrogen gas collected. The theoretical # of moles
of H2(g) can be calculated by converting the mass of Mg to moles Mg, and understanding that we get 1
mole of H2 from every mole of Mg(s). From the theoretical yield and the experimental yield, one can
calculate the percent yield:
PROCEDURE:
Percent Yield = experimental # moles H2 ×
theoretical #mole H2
100%
1. PUT ON RUBBER GLOVES. Fill the largest beaker in your drawer
(400 ml or a 600mL beaker) about 2/3 full of water and allow it to sit on the
base of a ring stand so that the temperature of the water may adjust to room
temperature. Place a buret clamp on the ring stand well above the beaker.
2. Obtain a 4-5 cm length of magnesium ribbon from the back counter of the
lab room. Weigh and record the mass of the magnesium ribbon in your
data table. Your magnesium should have a mass no larger than 0.0450 g. Roll
the magnesium ribbon into a loose coil. Obtain a piece of thread 25 cm in
length, and tie it to one end of the magnesium ribbon in such a way that all the
loops of coil are tied together (see Figure 1).
3. Get a eudiometer. Always carry a eudiometer in a
vertical position. The eudiometer will contain water.
Empty out the water into your sink and temporarily
attach it to the buret clamp, open end up.
Figure1
4. Using a glass funnel, add about 10 mL of hydrochloric acid to your eudiometer.
Remove the eudiometer from the buret clamp, hold it on a slight slant, and add
enough water to the eudiometer to fill it completely. Try to mix the water and the
acid as little as possible. Reattach the eudiometer to the buret clamp, open end up
(see Figure 2).
5. Obtain a one-hole rubber stopper from the back counter. Take your magnesium
coil and lower it into the water of the eudiometer to a depth of about 5 cm. Have the
thread attached to the coil hang over the lip and out of the eudiometer. Insert the onehole rubber stopper into the eudiometer so the thread is held firmly against the edge,
and when water squirts out of the hole in the stopper, cover the hole firmly with your
thumb (see Figure 1).
Figure2
2
Chemistry 108 Lab #3
6. Taking care that no air enters, remove the eudiometer from the buret clamp, invert it, and place its open end
underwater in the beaker. Re-clamp the eudiometer to the buret clamp so that the bottom of the eudiometer
is about 1 cm below the surface of the water in the beaker. The acid will flow down the eudiometer and
react with the magnesium.
7. When
the
magnesium
has
disappeared entirely and the reaction
has stopped, tap the tube with your
finger to dislodge any bubbles you see
attached to the side of the eudiometer.
Measure the temperature of the water in
your beaker; this will be the temperature
of the hydrogen gas in the eudiometer.
Record this value, to the nearest
0.1oC, in your data table. Because
your thermometer reads to a tenth of a
degree Celsius, add 273.2 when
converting to Kelvin.
8. Place your finger over the hole in the
stopper and remove the eudiometer
from the beaker. Lower the eudiometer
into the leveling tank and remove your
finger. Raise or lower the eudiometer
until the water level inside the
eudiometer is the same as the water
level in the leveling tank. Read the
volume of gas in the eudiometer and
record it in your data table. Record
the volume to the nearest 0.01 mL.
Figure3.Invertedeudiometerillustratinggas
collectionandwaterdisplacement.
9. Your instructor will write the
barometer for today’s atmospheric
pressure on the board. Record this value
in your data table. The water vapor
pressure can be found in the table on
page 2. Record this value for the
vapor pressure of water in your data
table.
10. When finished with the experiment, clean the eudiometer with deionized water, dry the outside, fill it
with deionized water, and return it.
3
Chemistry 108 Lab #3
DATA TABLE:
Mass of Magnesium Metal
(must be less than 0.0450 g)
=
g
Volume of Gas
=
mL
Temperature of Gas = Temperature of the water =
=
°C
Atmospheric Pressure
=
torr
Water Vapor Pressure
(at the above temperature, see table on last page)
=
torr
CALCULATIONS:
1. Theoretical (calculated) yield of H2 gas (# moles H2).
a) Convert mass Mg to #moles Mg
b) Convert # moles Mg reacted to moles of H2 that could be produced (1 mole H2 is produced for every 1
mole Mg reacted- this comes from the balanced chemical equation):
Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)
# moles H2 = _____________________________________ (theoretical yield of H2)
2. Experimental yield of H2 gas (# moles H2).
a) Determine pressure of dry H2 (PH2) by subtracting the vapor pressure of water from the total
pressure,
Ptotal = PH2 + PH2O
so
PH2 = Ptotal - PH2O
where Ptotal (the pressure in the eudiometer) is atmospheric pressure, and PH2O (the water
vapor pressure) is the pressure exerted by water vapor that has evaporated into the
eudiometer. Use the table provided on the last page to find the vapor pressure of water as a
function of temperature.
PH2 = _____________________torr
b) Convert this pressure from torr to atm (760.0 torr = 1atm)
PH2 = _____________________atm
4
Chemistry 108 Lab #3
c) Use the Ideal Gas Equation to calculate the number of moles (n) of H2 that you produced
in your experiment (experimental yield). Make sure to use the correct units so that they
match the units in the gas constant (R).
3. Calculate the Percent Yield.
% Yield = experimental # moles H2 ×
theoretical #mole H2
100%
% Yield =______________________________
4. What are the possible sources of error in this experiment?
5