Precipitation Stoichiometry
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Description
Solutions of a soluble calcium salt are mixed with solutions of other
soluble salts (sodium oxalate, sodium phosphate, and sodium oleate)
that form insoluble calcium salts. By varying the relative amounts
mixed so that one or the other chemical becomes limiting, the volume
ratio for the maximum formation of precipitate is determined.
Safety
Oxalates are toxic. Sodium phosphate is corrosive. Wear goggles and
apron. Do not ingest the chemicals. Wash spills with water. Wash
hands after the experiment.
Procedure
Calcium Oxalate
1.
2.
3.
4.
Select a 12-well strip. Place 4 drops of distilled water into each
of the first 9 wells of the strip. Add 1 drop of 0.1 M calcium
chloride to well-1, 2 drops to well-2, etc., and finally add 9 drops
to well-9.
Add 9 drops of 0.1 M sodium oxalate to well-1, 8 drops to well-2,
etc., and finally add 1 drop to well-9.
Mix each of the wells with a toothpick. Sorry, your browser does
not support mp4 format video. Try a different browser. Allow to settle for 5 minutes. Hold the strip up to eye level to
facilitate observations. Note the level of the precipitate in each of
the wells by looking through the side of the wells. Identify the
well with the largest amount (volume) of precipitate. Determine
the drop ratio for this well.
On the basis of the ratio of drops, write a balanced equation for
the formation of calcium oxalate.
6.
Wash the 12-well strip at the sink. Use a cotton swab to scrub
each well. Wash hands.
Calcium Phosphate
5.
1.
2.
3.
4.
Select a 12-well strip. Place 4 drops of distilled water into each
of the first 9 wells of the strip. Add 1 drop of 0.1 M calcium
chloride to well 1, 2 drops to well 2, etc., and finally add 9 drops
to well 9.
Add 9 drops of 0.1 M sodium phosphate to well 1, 8 drops to well
2, etc., and finally add 1 drop to well 9. Mix each of the wells
with a toothpick.
Allow to settle for 20 minutes. (Heating the strip in the steam
from a hot water bath (Use a 250 mL beaker.) sometimes
speeds the settling process.)
Hold the strip up and observe the level of the precipitate in each
of the wells by looking through the side of the wells. Identify the
well with the largest amount (volume) of precipitate. Determine
the ratio of drops for this well.
5.
On the basis of the ratio of drops, write a balanced equation for
the formation of calcium phosphate.
6.
Wash the 12-well strip at the sink. Use a cotton swab to scrub
each well. Wash hands.
Calcium Oleate
1.
2.
3.
4.
5.
Oleates are one of the compounds found in soaps. The
precipitate studied here is commonly called soap scum.
Select a 12-well strip. Place 4 drops of distilled water into each
of the first 8 wells of the strip. Add 1 drop of 0.1 M calcium
chloride to well 1, 2 drops to well 2, etc., and finally add 8 drops
to well 8.
Add 8 drops of 0.1 M sodium oleate to well 1, 7 drops to well 2,
etc., and finally add 1 drop to well 8. Mix each of the wells with a
toothpick.
Allow to settle for 20 minutes. (Heating the strip in the steam
from a hot water bath sometimes speeds the settling process.)
Hold the strip up and observe the level of the precipitate in each
of the wells by looking through the side of the wells. Identify the
well with the largest amount (volume) of precipitate. Determine
the ratio of drops for this well.
6.
On the basis of the ratio of drops, write a balanced equation for
the formation of calcium oleate.
7.
Wash the 12-well strip at the sink. Use a cotton swab to scrub
each well. Wash hands.
Questions
1.
2.
Write balanced equations for the formation of the three
precipitates studied.
Predict the effect of replacing calcium with silver in these
reactions. (Silver oleate, silver oxalate, and silver phosphate are
insoluble salts.)