How to identify cations and anions in an unknown solution

Purpose

The purpose of this lab is to identify anions and cations through flame tests and displacement reactions.

Procedure

After gathering all of the required materials, follow the procedure and safety guidelines to complete the lab.

1. Be sure to put on your gloves and safety goggles.

2. Place a 24 well plate in your work area. Using scissors. Cut off the tips of the six pipets labeled, Bromide, Carbonate, Chloride, Iodide, Sulfate and Sulfide. Use the well plate to hold the pipets.

3. In the well directly in front of the pipet, put 8 drops of bromide.

4. Add 1 drop of silver nitrate to the bromide. Wait a few seconds. Make and record your observations of the reaction between the bromide and silver nitrate in Data Table 1 under the column “Addition of AgNO3”.

5. In the same well, add 2 drops of HCI. Wait a few seconds. Make and record your observation in Data Table 1 under the column “Addition of HCl”.

6. Repeat steps 3 – 5 for the remaining 5 chemicals (not the unknown) and record your observations in Data Table 1.

7. Put 8 drops of the unknown in #103 in an empty well.

8. Add 1 drop of silver nitrate to the Unknown #103. Wait a few seconds. Make and record your observation in Data Table 1 under the column titles “Addition of AgNO3”.

9. Add 2 drops of HCl to the same well. Wait a few seconds and make and record your observations in Data Table 1 under “Addition of HCl”.

10. Compare the anion tests for the Unknown #103 to determine the identity of the unknown anion. Record your findings in Data Table 1in the “Anion” column next to “Unknown #103”

11. Repeat steps 7 – 10 for Unknown #104

12. Place the well plate aside along with the chemicals. It will be used in Exercise 2.

Exercise 2

1. Be sure to put on you goggles and safety gloves.

2. Put the burner fuel on the pie plate, take the lid off of the burner and light the wick.

3. Read all of the steps in this exercise before completing step 5.

4. Using a spatula, hold a pea size amount of Kl on the end.

5. With the KI on the end of the spatula, hold the end of the spatula in the flame. Keep the spatula in the flame until you see a reaction. This could take up to 2 minutes.

6. Continue to observe the flame until the flame changes. Record your observations in Data Table 2. When complete take the spatula out of the flame and allow it to cool.

7. Rinse the spatula once it has cooled. Be sure to remove any chemical residue. Repeat steps 5 – 6 for the remainder of the chemicals listed in the Data Table 2.

8. Repeat steps 5-6 for Unknown #105. Compare the color of the flame to the Data Table 2. Determine the identity and record your findings in Data Table 2.

9. Repeat steps 5-6 for Unknown #106 and unknown #107

10. Put out the flame out by carefully putting the cap on the flame.

11. Get the well plate used in Exercise 1, distilled water and an empty pipet.

12. Use the results in Data Table 2 to fill in the identification of the cation for Unknown #105, Unknown #106 and Unknown #107 in Data Table 3.

13. In an empty well, place a pea-sized amount of unknown #105.

14. Using the pipet, add 4 drops of distilled water onto the chemical.

15. Preform the anion confirmation test for Unknown #105. Use what you learned in Exercise 1. Record what you observe into Data Table 3.

16. Compare the observations for unknown #105 to the data in Data Table 1, determine the identity and record it in Data Table 3.

17. Combine the determined identity of the cation and anion for Unknown #105 to determine the complete chemical identity of Unknown #105. Record in Data Table 3.

18. Repeat Steps 13-17 for Unknown #106 and Unknown #107.

19. Clean up and return items back to the kit to be used again.

Observations

Exercise 1: Qualitative Anion Tests

Data Table 1. Anion Confirmation Tests.

Chemical

Anion

Addition of AgNO3

Addition of HCl

NaBr

Bromide

It became white and a little cloudy

Still cloudy and still white

Na2CO3

Carbonate

Cloudy and white around the outside and clear on the inside

Still cloudy, bubbles start to form, white

NaCl

Chloride

Cloudy and white at the bottom and clear on top

No change. Clear on top cloudy on bottom

NaI

Iodide

Dark in the middle, cloudy and lighter around the edged

Started to separate

Na2SO4

Sulfate

Clear

White and Cloudy

Na2S

Sulfide

Turned brown in the middle, began to separate

The brown becomes more separate from the clear

Unknown #103

CO3^2

Small particles

Bubbles form, white and cloudy

Unknown #104

S^2

Brown in middle clear around

Some brown disappeared

Exercise 2: Qualitative Cation Tests – Flame

Data Table 2. Cation Flame Tests.

Chemical

Cation

Flame Observations

KI

Potassium

Light Pink

CaCl2

Calcium

Red

LiOH

Lithium

Vibrant Pink

NaCl

Sodium

Orange

Cu(NO3)2

Copper

Teal

Unknown #105

Copper

Teal

Unknown #106

Potassium

Light Pink

Unknown #107

Calcium

Red

Data Table 3. Complete Chemical Identification.

Chemical

Cation

Anion Test Observations

Anion

Complete Chemical Name and Formula

Unknown #105

Copper

With AgNO3: No Change

CO3^2

Copper(lll) nitrate, Cu(NO3)2

Unknown #106

Potassium

With AgNO3: In the middle very dark and cloudy. Fades lighter white

l-

Potassium iodide, Kl

Unknown #107

Calcium

With AgNO3: Turn white cloudy

Cl-

Calcium Chloride,CaCl2

Questions

A. What was the identity of the anion in Unknown #103? Explain how you used the observations recorded in Data Table 1 to reach this conclusion.

The anion in Unknown #103 was carbonate. I compared my observations in Data Table 1 and both Unknown #103 and carbonate had the same reactions.

B. What was the identity of the anion in Unknown #104? Explain how you used the observations recorded in Data Table 1 to reach this conclusion.

The anion in Unknown #104 is sulfide. I compared my observations in Data Table 1 to the observation of Unknown #104 and saw that 104 matched with sulfide.

C. Write the chemical equation for ionic reaction between Na2S and AgNO3.

AgNO3 + Na2S ==> Ag2S + NaNO3

D. Write the chemical equation for ionic reaction between NaBr and AgNO3.

AGNO3 + NABR = NANO3 + AGBR

E. Use your results and observations in Data Table 1 to create a flow chart for identification of unknown anions, using the AgNO3 and HCl confirmation tests. Insert the flow chart below.

Solution containing: Br-, CO3^2-, Cl-, I-, SO4^2-, and S^2

{AgBr(cloudy), Ag2CO3(cloudy), AglCloudy),Agl(very dark), Ag2S-(brown)} {So4^2-}

AgBr(cloudy), AgCl(cloudy), Agl(very cloudy), Ag2S(brown)} {CO3^2-(decomposes to give

CO2 gas)}

F. What was the identity of the cation in Unknown #105? Explain how you used the observations recorded in Data Table 1 to reach this conclusion.

After comparing results I can conclude that Unknown #105 is copper. The color is how I determined my final answer.

G. Use your results and observations in Data Tables 1, 2, and 3, to create a flow chart for the identification of an unknown (both an unknown anion and cation), using the AgNO3 and HCl confirmation tests and the cation flame tests.

H. If you were asked to confirm the identity of an unknown chemical as lithium sulfide (Li2S), describe the tests that you would use to confirm the identity. Use your results in both Exercise 1 and Exercise 2 to describe the observations in both the cation and anion tests that would allow the unknown to be confirmed as lithium sulfide.

I would conduct the tests that were used in this lab. I would put the unknown chemical in a well and add HCl. I would then record my observations. I would then conduct the flame test. After analyzing my observations I should be able to positively identify the chemical.

Conclusion

It is always interesting to conduct tests with known chemicals and then conducting tests on unknown chemicals to find out what we are working with / testing. I enjoy working with chemicals and reactions that turn into colors rather then just get cloudy.