LAB 10.2 Mole Ratios
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The mole ratio of cations to anions in an ionic compound consists of small, whole numbers. For example, the mole ratio of Mg?+ ions to Br ions in MgBrz is 1:2. For every 1 mol of My ions present, there are 2 mol of Br" ions. The mole ratio of ions in KBr is 1:1. In water solution, one mole of KBr will produce 1 mol Br ions, but l mol MgBr, will produce 2 mol Br ions.
Suppose you have different compounds that contain CT ions. How could you determine the mole ratios in these compounds? Most chloride compounds dissolve in water, but some do not. Reacting dissolved chloride ions with a cation that forms an insoluble chloride compound can be used to determine the amount of chloride ions present. One such cation is silver. Reacting a chloride-containing solution with sufficient silver nitrate (AgNO;) solution will precipitate any dissolved chloride ions.
A solution of KCI will react with a certain amount of AgNO. The same volume of BaCI solution of the same concentration will require twice as much AgNO; to precipitate all the CT ions.
Problem
What is the ratio of cations to anions in an ionic compound? How can this ratio be determined?
Use with
Section 10.4
Objectives
* Measure the reacting ratios of silver nitrate solution with solutions of various chloride compounds.
* Calculate the ratio of positive ion to chloride ion in four chloride compounds.
* Determine the ratio of positive ion to chloride ion in an unknown compound.
Materials
0.10M silver nitrate (AgNO3)
0.10M potassium chloride
(KCI)
0.10M sodium chloride (NaCI)
0.10M barium chloride (BaCI2)
0.10M aluminum chloride (AlCI3)
dichlorofluorescein
test tubes (10)
10-mL graduated cylinder
dropper

Pre-Lab
1. What is meant by the term mole?
2. What do you need to know to calculate the number of moles of a substance?
3. Read the entire laboratory activity. Form a hypothesis about the expected ratios of reacting volumes. Form a second hypothesis about how these ratios can be used to determine the cation to anion ratio in an unknown substance. Record your hypotheses in the next column.
4. Summarize the procedures you will follow to test your hypotheses.
5. What is the net ionic equation for the reaction between AgNO;(aq) and KCI(aq)?
Procedure
Part A: Testing known solutions
1. Pour 1.00 mL of the KCI solution into a clean, dry test tube.
2. Add 2 drops of dichlorofluoroscein indicator solution to the test tube.
3. Add silver nitrate drop by drop to the solution until the dichlorofluoroscein turns from white to pink. Hold the dropper vertically as you add the drops. Carefully shake the tube from side to side as the drops are being added.
Do not spill any solution.
4. Count and record in Data Table 1 the number of drops needed to turn the solution from white to pink.
5. Repeat the procedure for a second 1-mL sample of KC1.
6. Repeat steps 1-5, using solutions of NaCl, BaCl, and AlCI, in turn, instead of KCI.
Part B: Testing a solution of unknown concentration
7. Obtain an unknown sample from your teacher. Record the number of the sample.
8. Repeat steps 1-5, using the unknown solution instead of KCI.
9.
Data and Observations
Samples: KCl, NaCl, BaCl2, AlCl3, Unknown
Trial 1 drops of AgNO3:
Trial 2 drops of AgNO3:
Average drops of AgNO3:
Cation/anion ratio: