Name _____________
THE REACTION OF A METAL WITH HYDROCHLORIC ACID
Pre-Lab Questions:
1. Write a brief, no more than one-half page, summary of the procedure for this experiment.
2. Hydrogen gas was collected over water in a eudiometer at 26 oC when the atmospheric pressure was 756 mm Hg.
What was the pressure of the hydrogen gas? _________atm
3. Why is it necessary to clean the magnesium strip before reacting it with hydrochloric acid?
__________________________________
4. 3.94 g of magnesium metal was used to produce hydrogen gas. Write the balanced chemical
equation for this reaction. __________________________________________
How many moles of hydrogen gas would be produced by the complete reaction of the magnesium?
_____________________ moles hydrogen gas
5. A student weighed .0367 g of magnesium to produce hydrogen gas collected over water at
24 oC and 746 mm Hg atmospheric pressure. If the volume of the collected gas was 36.3 mL, what is the value of the
gas constant, R, determined by the student? Assume that the percent yield is 100%. What is the % error (when
compared with the true value)?
THE REACTION OF A METAL WITH HYDROCHLORIC ACID
Abstract: The reaction of magnesium metal with an aqueous hydrochloric acid solution is carried to completion
to verify a stoichiometric relationship.
Objective: Objectives in this lab are to verify the relationship between the number of moles of the reactant
magnesium and moles of hydrogen product, to determine the molar volume of an ideal gas at STP and to
calculate a value for the idea gas constant, R.
Introduction: One mole of any substance is 6.02 x 1023 units of that substance. If that substance is a gas, one
mole occupies 22.4 liters at Standard Temperature and Pressure (STP: 273.15 K and 1.00 atm). In this
experiment you will react a measured mass of magnesium metal with an aqueous solution of hydrochloric acid,
producing soluble magnesium chloride and hydrogen gas.
Mg (s) + 2 HCl (aq)  MgCl2 (aq) + H2 (g)
The volume of hydrogen gas, under non-standard conditions, will be determined by reacting a measured mass of
magnesium metal with aqueous hydrochloric acid solution. After correction to STP, if the volume of H2 per
mole of Mg is close to 22.4 L, then the stoichiometry of the reaction, as indicated in the above equation, can be
considered verified.
Materials:
1.
2.
3.
4.
5.
6.
7.
8.
6 eudiometer (Gas collecting Tube)
6 ringstands
6 clamps
6 10 ml graduated cylinders
Mg ribbon in vials
distilled water in squirt bottle
6.00 M HCl 10 ml/ experiment
4.0L nalgene beaker.
Procedure:
1. Put on a pair of goggles and apron.
2. Mass a strip of Mg metal (approximately 5 cm long) on an analytical balance. Handle the metal with a
Kimwipe to avoid depositing grease from fingers. Then clean with steel wool.
3. Insert metal strip into small wooden splint in stopper with glass tubing so that Mg will fit inside
eudiometer.
4. Use a 50.00 ml gas-measuring tube (evdiometer) for the measurement of the volume of wet gas
produced by collecting the gas by displacement of water. Be careful not to drop or hit the gas measuring
the gas measuring tube against anything (the rim chips easily).
5. Incline the tube slightly from an upright position and pour in 10 ml of 6.0M HCl. The acid is the excess
reactant so it isn’t critical that you use exactly 10 ml of the acid.
6. With the tube still inclined, slowly and completely will the tube with water from a wash bottle. Use the
water to 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. Avoid mixing the acid layer in the bottom of t he tube with the less dense water
above it. Tapping the tube gently with your finger can dislodge some of the bubbles on the sides of the
tube.
7. While holding the stopper with the Mg, insert the metal so that about 3 cm down into the tube. Place
your index finger over the glass tubing of the stopper. Invert the tube in a 1000 ml beaker with 800 ml of
tap water. Rest the mouth of the tube on the bottom of the beaker and clamp the tube to the ring stand.
The acid is denser than water and will flow down through it and react with the magnesium metal.
8. After the reaction is completed (bubbles stop forming), wait 5 minutes to allow the tube to come to room
temperature. Dislodge any bubbles clinging to the sides of the tube. (You will probably not be able to
get rid of all of them).
9. Cover the open end of the test tube with your finger and transfer the inverted tube to a large water bath
is almost filled with water (40 L beaker). Raise or lower the tube until the level of the liquid inside the
tube is the same as the level outside the tube. This allows you to measure the volume of gasses in the
tube (hydrogen gas and water vapor) under the conditions where the total pressure inside the tube equals
atmospheric pressure. Read the bottom of the meniscus with your eye at the same level as the liquid
inside the tube. Read the volume graduations to the nearest 0.05 ml.
10. Remove the gas measuring tube from the water and pour the dilute acid and magnesium chloride
solution into the sink. Rinse the tube and the beaker with tap water.
11. Record the room temperature, the water bath temperature and atmospheric pressure (Ptotal = barometer
reading).
12. Repeat the experiment, checking the barometric pressure, room temperature, and the water bath
temperature. If both temperatures are the same, then you may use this temperature to look up the vapor
pressure of water as well as in the volume correction to STP. Otherwise the room temperature is used in
the volume calculation, since the gas will be at room temperature. The bath temperature should be used
to determine the vapor pressure of water, since the water inside the gas measuring tube is at the water
bath temperature.
13. According to Dalton’s Law of Partial Pressures Ptotal= PH2 + PH20 . Calculate the partial pressure of the
hydrogen.
14. Using the Combined Gas Law with T1 = the room temperature of the gas, V1 = the volume of gas at
room temperature, and P1 = the partial pressure of the dry hydrogen, calculate the volume of the
hydrogen at STP.
15. Convert the volume of the hydrogen at STP to liters
16. The moles of H2, are equal to the moles of Mg since they are 1:1 mole ratio in the balanced equation.
Calculate the molar volume of H2 = at STP by dividing the liters of hydrogen at STP by the moles of
Mg.
17. Calculate the percent error in the molar volume:
%Error = Experimental Value – 22.4 L/mol
X100
22.4 L/mol
18. Calculate an experimental value of R in L atm/mol K using PV=nRT with T = the room temperature of
the gas converted to K, V = the volume of the gas at room temperature converted to L. P= the partial
pressure of the dry hydrogen converted to atm, and n = moles of H2.
Data Table:
Volume of gas in liters: ______________________________
Temperature of H2O (and gas): _________________________
Temperature of the Room: ____________________
Barometric Pressure: _________________________
Water Vapor Pressure in room:________________________
Pressure of Dry Gas: _________________________
Post-Lab Questions:
1. If an undetected bubble of air were trapped inside the gas collection tube, what would be its effect on the
percent yield in your experiment? Would the actual yield, and thus the percent, increase, decrease, or be
unaffected by the bubble of air? Explain.
2. In your reaction, the magnesium is necessarily the limiting reactant, so that it is completely consumed in the
reaction. The amount of hydrochloric acid added to the reaction was more than enough to consume all of the
magnesium. Calculate the exact amount of 12 M hydrochloric acid necessary to consume 0.040 grams of
magnesium metal.
3. If you hadn’t added quite enough hydrochloric acid to consume all of the magnesium metal, what would be
the effect of this error on the percent yield in your experiment? Would the actual yield, and thus the percent,
increase, decrease, or be unaffected by not adding enough HCl? Explain.