Expt 050 -- Precipitation Stoichiometry

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. 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.
  2. 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.
  3. Mix each of the wells with a toothpick.
    !!!Click here to See Movie.
  4. 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.
  5. 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

  1. 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.
  2. 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.
  3. 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.)
  4. 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. Oleates are one of the compounds found in soaps. The precipitate studied here is commonly called soap scum.
  2. 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.
  3. 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.
  4. Allow to settle for 20 minutes. (Heating the strip in the steam from a hot water bath sometimes speeds the settling process.)
  5. 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. Write balanced equations for the formation of the three precipitates studied.
  2. Predict the effect of replacing calcium with silver in these reactions. (Silver oleate, silver oxalate, and silver phosphate are insoluble salts.)

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 050 Precipitation Stoichiometry

Watch the movies and carefully record your observations.

H1. Identify the well with the most precipitate for each experiment. Calculate the ratio of Ca2+ to the anion tested.

Anion maximum ppt
ratio of [Ca2+]
/[anion]
observed theoretical
oxalate
phosphate
oleate

H2. What are the major sources of errors in this experiment? Do you see any obvious error in the phosphate experiment shown?

Answer the questions.

Curriculum-

This experiment may be introduced when solution stoichiometry is taught. The concept of limiting reagents may be introduced with this experiment. This experiment works well as one that precedes Experiment 048 on copper(II) sulfide formation.

Safety-

Oxalates are toxic. Sodium phosphate is very basic and consequently corrosive. Wear goggles and apron. Do not ingest the chemicals. Wash spills with water. Wash hands after the experiment.

Time-

Teacher Preparation: 20 minutes

Class Time: 40 minutes (There is a 20 minute waiting period for the solids to settle.)

Materials-

Disposal-

The materials used in this experiment may be discarded safely at the sink using large volumes of water.

Lab Hints-

One way to teach this is to do the calcium phosphate first and then pose the "determination of the charge of the oxalate ion" as a question for the class.

Drop sizes must be uniform in this experiment. See the special technique in which a thin stem pipet and a plastic pipet tip are combined to make a device that delivers uniform drops.

Answers-

Q1. Write balanced equations for the formation of the three precipitates studied.
A1. CaCl2 + 2 NaO2CC17H33 --> Ca(O2CC17H33)2 + 2 NaCl
CaCl2 + Na2C2O4 --> CaC2O4 + 2 NaCl
3 CaCl2 + 2 Na3PO4 --> Ca3(PO4)2 + 6 NaCl
Q2. Predict the effect of replacing calcium with silver in these reactions. (Silver oleate, silver oxalate, and silver phosphate are insoluble salts.)
A2. The ratios change. The 1:1 ratio shows up for silver oleate. A 3:1 ration of drops silver per drop of phosphate appears.

Handout Ans.-

HA1. Anion maximum ppt ratio of [Ca2+]/[anion]

Anion maximum ppt
ratio of [Ca2+]
/[anion]
observed
theoretical
oxalate
5
5/5=1
1
phosphate
3
6/3=2
3/2
oleate
6
3/6=0.5
1/2

HA2. Counting the drops and delivering uniform drops are the major errors here. Only 8 wells were used so some mistakes were made in counting drops.

CoopLearn-

It is possible to divided the labor among two-person groups such that each group works on two or three reactions. For cooperation to be effective, the groups must share their results.

Key Words 1-

solution stoichiometry, precipitate, combining ratio, solutions, insoluble, limiting reagent

Elements-

Ca P C O