Physical Sciences Break 1.0 – Physical Sciences Grade 10 ...



GRADE 12 PRESCRIBED EXPERIMENT 2: ACID-BASE REACTIONS

WORK SHEET

TOTAL MARK: 50

ACTIVITY

Titration of oxalic acid against sodium hydroxide to determine the concentration of the sodium hydroxide.

In this investigation you will prepare an acidic solution accurately and thus you will know its exact concentration. You will then react this acid with a base of an unknown concentration to determine the concentration of the base.

What you will need:

|Erlenmeyer flasks |Oxalic acid |

|Burettes |Sodium Hydroxide |

|Burette clamp |Phenolphthalein as indicator |

|Medicine dropper |Funnel |

|Retort stand |Beaker |

|White tile /paper |Spatula |

|Measuring cylinders |Glass rod |

|Mass meter |Pipette with sucker |

What to do:

1. Prepare a standard solution of oxalic acid which has a concentration of approximately 1mol.dm-3.

2. Now prepare a sodium hydroxide solution by dissolving approximately 2g of dry sodium hydroxide in 500ml of water.

3. Add two drops of the indicator solution.

4. Place the burette in the clamp.

5. Using the funnel, fill the burette to above zero mark with the acid solution.

6. Then, holding the beaker, with which you used to pour the acid, beneath the burette, gradually open the tap.

7. Allow the level of the base to come down to exactly zero (reading from the bottom of the meniscus).

8. Pipette using the sucker exactly 25ml of oxalic acid solution in a volumetric flask.

9. Add a few drops of phenolphthalein to the acid.

10. Hold the conical flask beneath the burette with your right hand and gradually open the tap with your left.

11. Swirl the conical flak continuously and watch it closely for the first sign of a colour change.

12. As you see that you are approaching the point of neutralization, close the tap slightly so that you are adding drop by drop.

13. When the colour changes completely the titration is finished.

14. Close the tap and read from the burette how much acid was used.

15. Repeat this procedure at least twice so that you have three readings for the volume of NaOH (of unknown concentration) required to neutralize exactly 25ml of oxalic acid (of known concentration).

16. Take an average of these three and use it to calculate the concentration of the NaOH.

17. Now calculate the concentration of the sodium hydroxide solution.

18. Make a neat labeled sketch to represent the apparatus

19. Now write a report using the format learnt in class.

Questions

1. What is the appropriate concentration of NAOH (2g in 500ml of water)

2. Calculate the theoretical concentration of NaOH from the actual mass of NaOH you measured.

3. How does your theoretical value for NaOH concentration (from the actual mass you measured) differ from the actual concentration you calculated (from the titration procedure)? Can you think of some reasons why your values may differ?

MEMORANDUM

1. Title

ACID- BASE TITRATION

2. AIM

To determine the concentration of Sodium Hydroxide (NaOH) using a standard solution of Oxalic acid (H2C204)

3. MATERIALS (APPARATUS & CHEMICALS)

|Apparatus: |Chemical |

|Erlenmeyer flasks | Oxalic acid |

|Burettes |Sodium Hydroxide |

|Medicine dropper |Phenopthalein as indicator |

|Retort stand | |

|White tile /paper | |

|Measuring cylinders | |

1. PRECAUTIONARY MEASURES

➢ Safety glasses must be worn

➢ The use of gloves is recommended

➢ Test run before doing actual titration experiment

➢ Contamination from dirty glassware

➢ Oxalic acid solution becoming lumpy / forming precipitate

➢ ‘Overshooting’ the mark

➢ Volumetric flask calibrated at a specific temperature (e.g. room temperature)

2. SAFETY AUDIT ON:

1. Oxalic acid

2. Sodium Hydroxide (4)

3. DATA COLLECTION

Experiment 1, 2 & 3

Concentration of oxalic acid: 1mol/dm3

Concentration of base (sodium hydroxide): unknown

|Experiment Number. |Volume of Acid(mℓ) |Volume of NaOH(mℓ) |

|1 |25 mℓ | |

|2 |25 mℓ | |

|3 |25 mℓ | |

The average of the results obtained: _____________ ml of NaOH solution. (2)

What is your observation/colour change at the end point? ________________________________________________________________________ (2)

4. CONCLUSION_________________________________________________________________________________________________________________________________________________________________________________________________ (1)

8. CALCULATIONS

(a) Write the balanced equation for the reaction

_________________________________________________________________________(3)

(b) Calculate the concentration of the unknown solution using the equation

nbCaVa = naCbVb

______________________________________________________________________________________________________________________________________________________

___________________________________________________________________________

______________________________________________________________________________________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

___________________________________________________________________________

__________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________________ (7)

TOTAL [50]

SAFETY AUDITS ON:

Oxalic acid √ √

|FIRE |Combustible. Gives off irritating or |NO open flames. |Powder, alcohol-resistant foam, water |

| |toxic fumes (or gases) in a fire. | |spray, carbon dioxide. |

|EXPLOSION | | |In case of fire: keep drums, etc., cool|

| | | |by spraying with water. |

|EXPOSURE | |AVOID ALL CONTACT! |IN ALL CASES CONSULT A DOCTOR! |

|Inhalation |Sore throat. Cough. Burning sensation. |Local exhaust or breathing protection. |Fresh air, rest. Half-upright position.|

| |Shortness of breath. Laboured | |Artificial respiration if indicated. |

| |breathing. Symptoms may be delayed (see| |Refer for medical attention. |

| |Notes). | | |

|Skin |Redness. Skin burns. Pain. Blisters. |Protective clothing. |First rinse with plenty of water, then |

| | | |remove contaminated clothes and rinse |

| | | |again. Refer for medical attention. |

|Eyes |Redness. Pain. Loss of vision. Severe |Face shield, or eye protection in |First rinse with plenty of water for |

| |deep burns. |combination with breathing protection |several minutes (remove contact lenses |

| | |if powder. |if easily possible), then take to a |

| | | |doctor. |

|Ingestion |Sore throat. Burning sensation. |Do not eat, drink, or smoke during |Rinse mouth. Rest. Refer for medical |

| |Abdominal pain. Vomiting. Drowsiness. |work. Wash hands before eating. |attention. |

| |Shock or collapse. Convulsions. | | |

|FIRE |Not combustible. Contact with moisture |  |In case of fire in the surroundings: |

| |or water may generate sufficient heat | |use appropriate extinguishing media. |

| |to ignite combustible substances. | | |

|EXPLOSION |  |  |  |

|EXPOSURE |  |AVOID ALL CONTACT! |IN ALL CASES CONSULT A DOCTOR! |

|Inhalation |Corrosive. Burning sensation. Sore |Local exhaust or breathing protection. |Fresh air, rest. Half-upright position.|

| |throat. Cough. Laboured breathing. | |Artificial respiration may be needed. |

| |Shortness of breath. Symptoms may be | |Refer for medical attention. |

| |delayed (see Notes). | | |

|Skin |Corrosive. Redness. Pain. Serious skin |Protective gloves. Protective clothing.|Remove contaminated clothes. Rinse skin|

| |burns. Blisters. | |with plenty of water or shower. Refer |

| | | |for medical attention. |

|Eyes |Corrosive. Redness. Pain. Blurred |Face shield or eye protection in |First rinse with plenty of water for |

| |vision. Severe deep burns. |combination with breathing protection |several minutes (remove contact lenses |

| | |if powder. |if easily possible), then take to a |

| | | |doctor. |

|Ingestion |Corrosive. Burning sensation. Abdominal|Do not eat, drink, or smoke during |Rinse mouth. Do NOT induce vomiting. |

| |pain. Shock or collapse. |work. |Give plenty of water to drink. Refer |

| | | |for medical attention. |

SODIUM HYDROXIDE (NaOH) √ √

APPROPRIATE SKETCH

[pic]

(6)

|Burette with oxalic acid |√ √ |

|Retort stand |√ |

|Erlenmeyer flask with NaOH |√ √ |

|White tile |√ |

6. DATA COLLECTION

Experiment 1,2 & 3

Concentration of Oxalic acid: 1mol/dm3

Concentration of base (NaOH): unknown

|No. |Volume of NaOH(mℓ) |Volume of Oxalic acid (ml) |

|1 |50 mℓ |43 mℓ √ |

| 2 |50 mℓ |44 mℓ √ |

|3 |50 mℓ |41 mℓ √ |

The average of the results obtained: 42 mℓ √

Colour of phenopthalein changes from pink to colourless

1. CONCLUSION:

50ml NaOH solution of 0.42 mol.dm-3 neutralises 42ml oxalic acid solution of 1 mol.dm-3 (1)

8. CALCULATIONS

a) Write the balanced equation for the reaction

2NaOH + H 2C 2O 4 √ → Na 2C 2O 4 √ + 2H 2O √

b) Calculate the concentration of the unknown solution using the equation

n bC aV a = n aC bV b

1x1x 0.042 = 2 X Cb x 0.05

C b = 0.42 mol/dm3

(7)

TOTAL [25]

-----------------------

b

urette

Retort stand

Erlenmeyer

flask

White tile

HOW SECTION 1.3 (CAPS) CAN FEATURE IN THE EXPERIMENT

Refer to CAPS page 5 d, bullet 1-5

➢ identify and solve problems and make decisions using critical and creative thinking;

➢ work effectively as individuals and with others as members of a team;

➢ organise and manage themselves and their activities responsibly and effectively;

➢ collect, analyse, organise and critically evaluate information;

➢ communicate effectively using visual, symbolic and/or language skills in various modes;

Suggestions on how to improve:

• You can let them write an equation before they perform the experiment

• Don’t ask questions like ‘is it a weak or strong acid: because there is 50% chance of guessing

• Pour the oxalic acid in the burette & NaOH in the Erlenmeyer flask because cleaning the NaOH from burette is difficult & burette might break

• You can let the learners prepare the standard solution

• Completion of a table could have been assessed, e.g. indicating units in the table

• Include everyday application of e.g. the oxalic acid (action of the antacid).

• List precautions just before the method is given

SUGGESTION: Don’t forget: structure the questions in such a way that, for marking, you use 20% rubric & 80% memorandum

NB: In a practical investigation: you use Scientific method. In an experiment you don’t use a scientific method coz u r verifying a hypothesis.

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