THE PROBLEM: Devise and carry out an experiment to determine how much magnesium hydroxide is in 1.0 mL of milk of magnesia. Describe the method you developed to solve this problem.

The problem tests student understanding of titrations involving a dibasic material, simple experimental design, and the manipulation of titration data.

1. The student measures out a known volume of milk of magnesia, either by counting drops into a small graduated cylinder or by using a graduated Beral-type pipe.

2. The student adds a drop of indicator and titrates with the given acid solution, counting the drops from a micro-tip pipet or by using a graduated pipet, until a clear and colorless endpoint is reached.

3. The student determines the volume of acid used to neutralize the 1.0 mL of milk of magnesia by either counting the number of drops in 1.0 mL of acid or by determining the volume of a drop of acid.

4. The student repeats the titration and also repeats the volume determination.

5. The student calculates the number of moles in the average volume of acid used and uses proportions to determine how much Mg(OH)₂ is present in 1.0 mL of milk of magnesia.

1. Measurement of a known volume of milk of magnesia, either by using a graduated Beral-type pipet or by counting drops into a graduated cylinder 1.0 pt

2. Titration to a phenolphthalein endpoint with acid of known concentration 1.5 pt

a) The student reports only a single value for the number of drops used. 1.0 pt

b) The student reports at least two determinations for the number of drops.
1.5 pt

NOTE: It is not strictly necessary to use any indicator, as the disappearance of the white precipitate is a reasonably accurate endpoint in itself.

3. Determination of the volume of acid used 1.5 pt

a) The student reports only a single value for the number of drops in one mL or for the volume of a drop of acid. 1.0 pt

Rationale: Reproducibility of results is important in this determination.

b) The student reports at least two determinations for the number of drops in one mL or for the volume of a drop of acid. 1.5 pt

4. Calculating the number of moles in the average volume of acid used and uses proportions to determine Mg(OH)₂ present in 1.0 mL 1.0 pt

a) The student calculation neglects that Mg(OH)₂ is dibasic. 0.5 pt

b) The student follows the likely approach calculation. 1.0 pt

Extra credit could be awarded if the student

a) includes a net ionic equation for the reaction taking place.

b) discusses the possible sources of error in the determination.

Chemicals Equipment Possible Distracters Milk of Magnesia HCl solution (concentration should be between 0.1 M and 1.0 M, and indicated on the label) phenolphthalein solution reaction place, 24-well micro-tip Beral-type pipets graduated Beral-type pipets standard base solution

1. "Milk of magnesia" is not a true solution; it is mostly a suspension of the relatively insoluble magnesium hydroxide in water.

2. The problem can be made more challenging and also longer by supplying a standard base solution and an acid solution of unknown concentration. Then students will have to determine the acid's concentration before they proceed with the titration of the magnesium hydroxide.

3. See Activity C.2 for a related activity involving milk of magnesia.

1. Be sure to follow usual safety rules for working with acids and bases.

DATE CLASS NOTES