AP Lab #2: Determine Mass of Mg(OH)2 in antacid by Back
Titration
* Milk of Magnesia states it contains 1200 mg of magnesium hydroxide in 15 mL.
We will check to see how much is actually in 15 mL.
1. Place 15 mL of Milk of Magnesia into small Erlenmeyer flask. How many
grams of Mg(OH)2 “should” be in the flask? (we will pretend this amount is
unknown)
2. Place 1.0 M HCl into buret and record initial amount
3. Calculate the volume of 1.0M HCl needed to react with the “assumed” mass
of your sample.
4. Place 10 mL above the amount determined in step two, so that you have
gone past the endpoint and now have an excess amount of HCl in your flask.
5. Place 2-3 drops of BMB indicator into flask—what color should you see and
why?
6. Place your standardized NaOH in a buret and record initial amount.
7. titrate your excess HCl (created in step 4) with your NaOH. What color
transition should you see with this indicator?
8. Calculate the volume of HCl that reacted with the NaOH.
9. Subtract the volume of HCl from step 8 from the total volume of HCl used, to
determine the volume of HCl used to titrate the Mg(OH)2
10. Save the contents of the flask for next experiment!!!
Data:
Initial volume
HCl
NaOH
Final volume
Volume used
for NaOH
Volume used
for Mg(OH)2
N/A
N/A
Analysis
Explain and show all calculations as you work through steps 1-9
Summarize the technique of back titration
Calculate % Error from your calculated mass of Mg(OH)2 and the 1200 mg from
the manufacture’s label.
discuss the possible experimenter errors.
AP Lab #3: Determine Mass of Mg(OH)2 in antacid by
Gravimetric Analysis
* Milk of Magnesia states it contains 1200 mg of magnesium hydroxide in 15 mL.
We will check to see how much is actually in 15 mL.
1. Start with the solution from your titration in Lab #2
2. Calculate the amount of sodium dihydrogen phosphate monohydrate is
needed to react “theoretically” with 1.2 grams of magnesium hydroxide
Mg(OH)2 + 2NaH2PO4 → Mg(H2PO4)2 +2 NaOH
3. Add an “excess” amount of NaH2PO4 to the solution in the flask. (Why
excess?)
4. Allow the precipitate to settle to the bottom of flask.
5. Pour off excess supernate (liquid above the precipitate—only that amount
that you are sure contains no precipitate.
6. prepare your filter paper (fold and tear corner) mass the paper and record.
7. filter your supernate and precipitate through the filter paper.
8. set the precipitate aside to allow to dry overnight
9. mass the paper and precipitate subtract the mass of the paper
10. calculate the mass of Mg(OH)2 from your precipitate’s mass.
Data:
Mass of paper
Mass of paper
and
precipitate
Mass of
precipitate
Analysis
Explain and show all calculations as you work through steps 1-10
Summarize the technique of gravimetric analysis
Calculate % Error from your calculated mass of Mg(OH)2 and the 1200 mg from
the manufacture’s label.
discuss the possible experimenter errors.