Reaction of a Metal with Hydrochloric Acid
After studying how to apply Daltonʼs Law of Partial Pressures in class, I wonder:
under the conditions of temperature and pressure in our lab, is the correction for
the presence of water vapor significant to the volume of gas collected? This is a very
practical question. If makes no difference, then we can use a simpler model for our
PV=nRT calculations by ignoring the water vapor. But if it does make a difference, it
would be important to use the more complex model.
This is a team lab with a hypothesis. The “spotlight” is DATA.
PROCEDURE:
1. Cut 2 cm of Mg ribbon and mass it to the nearest 0.001 g.
2. Cut a piece of thin Cu wire about 12” long. Wrap one end 8-10 times around the
3. Mg ribbon & leave the rest as a “handle”.
4. Get a eudiometer and a one-hole stopper. Set up a ringstand and clamp to hold the
eudiometer. Place a 400-600 mL beaker full full of water nearby.
5. Wearing chemical gloves, fill the eudiometer with about 15 mL of 3M HCl (aq).
6. The acid is the excess reagent, so you do not have to be precise nor record the
volume used. A colored indicator has been added to the acid to make it easier to
distinguish from water.
7. With the eudiometer tilted, gently add water on top of the acid until the tube is
completely full. The denser acid should stay in a layer at the bottom of the tube.
8. (Youʼll need to make the eudiometer upright to finish filling it.) The eudiometer
must be totally full of water.
9. Holding the copper-caged Mg by its handle, insert the Mg into the tube. Fold the
end of the handle over the top of the tube. Then stopper the tube: water should
pop out of the hole in the stopper, showing there are no air bubbles in the tube. If
you donʼt see water squirt out of the hole in the stopper, remove it, add more
water to the eudiometer, and try again.
10. Holding one finger over the stopper, invert the eudiometer into the beaker of
water and then clamp in place. You should not see an air bubble at the top of the
tube.
11. The denser acid will now flow down the eudiometer and react with the Mg,
producing hydrogen gas. While the reaction happens, measure room temperature
and room pressure.
12. When the reaction stops, check that the Mg is completely gone. Then let the tube
sit for about 5 minutes, occasionally tapping the sides with your finger or a pencil.
This will help to loosen any H2 bubbles that are stuck to the glass.
13. There should be a very large ungraduated cylinder nearby filled with water. Put
your finger over the hole in the stopper of your eudiometer, and carefully move
the eudiometer from your beaker to the ungraduated cylinder.
14. Adjust the height of the eudiometer inside the cylinder until the water levels in
both tubes match. Then read the volume of gas in the eudiometer.
15. REPEAT the entire experiment for a second trial.
NO DIAGRAMS HAVE BEEN PROVIDED. DRAW AND LABEL YOUR OWN!!
CALCULATIONS:
1. The full reaction is: solid magnesium reacts with hydrochloric acid [HCl (aq)] to
form aqueous magnesium chloride plus hydrogen gas. Write a balanced equation
for this reaction. Then calculate the theoretical yield (in moles) of hydrogen gas
produced from your starting mass of Mg.
2. Determine the partial pressure of the hydrogen gas collected, using your table of
water vapor pressures.
3. Using the Ideal Gas Law, calculate the moles of hydrogen gas you collected.
4. Do this TWO WAYS: (a) assuming no water vapor was mixed with the H2 gas,
and (b) correcting for the presence of water vapor.
5. Calculate the %yield of H2 both ways, and share with the class.