LAB 5: A VOLUMETRIC ANALYSIS OF A CHLORIDE MIXTURE
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This lab does NOT require a lab report.
You will work with a partner for this lab.
I Introduction
In Lab 2, we found the silver content in a sterling silver necklace by using gravimetric
analysis. We added a chloride salt to form a silver chloride precipitate, as shown in
the equation below:
AgNO3(aq) + NaCl(aq)  AgCl(s) + NaNO3(aq)
We then used the mass of solid silver chloride to determine the amount of silver in
the necklace. This time around, we’ll do almost the opposite: we will analyze a
chloride mixture by adding silver nitrate. Although we will again form a precipitate,
this time we will ignore it. Instead, we will use the volume of silver nitrate solution
added to determine the amount of chloride present.
Because this method is based on the volume of reagent used instead of the mass of
product formed, this is an example of a volumetric analysis. All titrations (including
this one) are volumetric analyses. Although volumetric analyses are typically quicker
and easier than their gravimetric counterparts, the main drawback is that it is often
difficult to determine the exact equivalence point of the reaction; in other words, it’s
difficult to tell when you’ve added exactly the right volume to react with the analyte
present. For this reason, many titrations require a third substance, or indicator, to be
added. The indicator is usually chosen so that it will produce a color change when the
equivalence point of the titration has occurred.
In this experiment, the indicator of choice will be potassium chromate, K2CrO4. Silver
ions preferentially react with chloride ions in solution because silver chloride is
especially stable:
Ag+(aq) + Cl-(aq)  AgCl(s)
white
As noted in the equation above, the silver chloride compound is white in color.
Once the chloride ions are exhausted, however, the silver ions will then react with
the chromate ion:
Ag+(aq) + CrO4-2(aq)  Ag2CrO4(s)
red
As noted in the equation above, the silver chromate compound is red in color. This
provides a convenient endpoint for our titration: when the analyte solution turns from
a milky white color to a red color, our titration is over.
In this experiment, you will use a silver nitrate, AgNO3(aq), solution to titrate a
sample containing magnesium chloride and inert impurities. Your goal is to determine
the percent, by mass, of magnesium chloride in the sample.
Equipment and Reagents
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Buret
Erlenmeyer flask
Magnetic stirrer and stirbar
Analytical balance
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0.10 M silver nitrate, AgNO3(aq)
0.50 M potassium chromate, K2CrO4(aq)
Unknown chloride mixture
Distilled water
! Warnings!
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Solutions containing the silver ion (Ag+) are slightly toxic and solutions
containing the chromium(VI) ion (Cr+6) are HIGHLY TOXIC (think Erin
Brokovich). Treat these solutions with care and do not pour them down the
drain. Wash your hands when you have completed the lab.
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Silver nitrate will stain skin and clothing as the silver ions are photochemically
reduced to silver metal. Immediately clean up any spills.
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Because of the above hazards, you MUST wear safety goggles and aprons.
Procedure
1. Put on your safety goggles and apron and set up your lab notebook.
2. Fill your buret with 0.10 M AgNO3(aq).
3. Weigh about 0.5 grams of the chloride sample and record the exact mass used in
yoru lab notebook. Carefully transfer the entire sample to the Erlenmeyer flask and
dissolve it in about 25 mL of distilled water. Add 3-4 drops of 0.50 M K2CrO4(aq).
4. Titrate the sample until its original yellowish color has turned red. Record the
volume of AgNO3(aq) used.
5. Pour your analyte solution into the collection beaker. Leave the excess AgNO3
solution in your buret for later collection by Mr. Benter.
6. Record your results on the board for class comparison.
Data and Calculations (to be recorded on your lab Data Sheet)
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Title and Purpose
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Brief outline of procedure used
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Amounts of reagents used (to the correct number of sig figs) and observations
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Balanced equations of all reactions occurring
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Data Analysis (to the correct number of sig figs): mols AgNO3 added, mols
chloride present in sample, mols MgCl2 present in sample, mass MgCl2 present
in sample, mass percent of MgCl2 in sample, and class average
? Questions to Answer
1) What are some possible sources of error in your results? How would they affect your
results: too high or too low?