F.6/7 Chemistry Practical: Acid-base titration
Object:
(a) To standardize an unknown sodium hydroxide solution by a standard solution of potassium hydrogenphthalate and
(b) To determine the molarity of an unknown hydrochloric acid solution.
Group size: Individual
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Theory:
(The following questions should be useful for you to write this section.)
(a) / A bottle of sodium hydroxide solution should be standardized before use in volumetric analysis. Explain.(b) / Potassium hydrogen phthalate is a good primary standard. Give at least five reasons.
(c) / Give an equation for the standardization of sodium hydroxide solution by using standard solution of potassium hydrogen phthalate (Suppose the formula of potassium hydrogenphthalate is represented by KHA2).
(d) / Which pH indicator is suitable for the standardization? Explain.
(e) / How to prepare a standard solution of potassium hydrogen phthalate?
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Chemicals:
Potassium hydrogenphthalate, C6H5O4K, A.R. grade, 250 cm3 of 0.1 M NaOH solution,
Additional Materials:
Weighing bottle, pipette filler, balance, readable 0.001 g, burette, phenolphthalein
Safety Precaution
Sodium hydroxide is corrosive, potassium hydrogenphthalate is harmful
Procedure A
1. Transfer about 5 g of potassium hydrogenphthalate into a weighing bottle and weigh it to the nearest 0.001 g.
2. Put about 50 cm3 of water into a 250 cm3 beaker. Carefully transfer the bulk of the potassium hydrogenphthalate from the weighing bottle into the beaker.
3. Reweigh the bottle with any remaining potassium hydrogenphthalate to the nearest 0.001 g.
4. Stir, by using a clean glass rod, to dissolve the solid, adding more water if necessary. [Note: Do not place the glass rod on the bench which is not clean.]
5. Transfer the solution to the volumetric flask by using the glass rod. Rinse the beaker wall, making sure all liquid go into the volumetric flask. Add the rinse to the flask by using the glass rod.
6. Add de-ionized water into the volumetric flask from the washing bottle until the level reach the stem of the flask. Insert the stopper and shake to mix the contents.
7. Add de-ionized water into the flask from the washing bottle until the level is within about 0.5 cm below the graduation line. Using a dropper, add enough water to bring the bottom of the meniscus to the mark. Insert the stopper. Turn the flask upside down and wait for the rise of air bubble and then turn upside. Repeat this mixing for at least ten times to ensure complete mixing.
Data and Results
Mass of bottle and contents before transfer, m1 /g / =Mass of bottle and contents after transfer, m2 /g / =
Mass of potassium hydrogenphthalate, m /g / =
Procedure B: Standardization of sodium hydroxide solution
1. Rinse a clean burette by using minimum amount (about 20 cm3) of the sodium hydroxide solution. Fill up the burette with the sodium hydroxide solution. Do not forget to rinse and fill the tip of the burette. Record the initial burette reading.
2. Rinse a clean pipette with the standard potassium hydrogenphthalate solution. Use this pipette transfer 25.0 cm3 of the potassium hydrogenphthalate solution to a clean 250 cm3 conical flask.
3. Add 4-6 drops of phenolphthalein indicator to the conical flask.
4. Run sodium hydroxide solution from the burette into the flask, with swirling, until the solution just turns pink. You should add the alkali drop by drop when you think the colour is about to change. Record the final burette reading.
5. Refill the burette with the sodium hydroxide solution.
6. Repeat step 2,3,4 and 5. Try to get two concordant readings. However, you are limited to 4 runs.
7. Empty the burette and wash it carefully immediately after the titration by the use of tap water, then followed by de-ionized water.
Procedure C: Determination of concentration of hydrochloric acid by using standardized
sodium hydroxide solution
1. Receive a sample of hydrochloric acid (about 125 cm3) from your teacher. Cover the beaker.
2. Rinse a clean pipette with a minimum amount of the hydrochloric acid. Transfer 25 cm3 of the acid to a clean conical flask by using the pipette. Add 5-7 drops of litmus to the acid.
3. Titrate the acid with the standardized sodium hydroxide solution. When the end point is close, add the titrant drop by drop.
4. Repeat the above steps and try to get two concordant results. However, you are limited to 4 runs.
Data and Results
Concentration of potassium hydrogenphthalate = ______mol dm-3
Colour change of the indicator:
Sample number of sodium hydroxide = _____
Table B:
Standardization of solution of sodium hydroxide
Run / Trial / 1 / 2 / 3Final burette reading/cm3
Initial burette reading/cm3
Volume of NaOH used/cm3
Average titrant: ______
Determination of molarity of hydrochloric acid
Colour change of the indicator before and after the end point:
Sample number of hydrochloric acid =
Titration Data
Run / Trial / 1 / 2 / 3Final burette reading/cm3
Initial burette reading/cm3
Volume of HCl used/cm3
Average titrant: ______
Calculation and Conclusion
Calculate (a) the molarity of potassium hydrogenphthalate, (b) the molarity of the sodium hydroxide solution and (c) the molarity of hydrochloric acid
Discussion
1. List two reasons that 'end point' is only an approximate value of the equivalence point in a titration.
2. What effect would each of the errors described below have on the concentration of potassium hydrogenphthalate? Explain your answer.
(a) Several drops of the solution of potassium hydrogenphthalate were spilt in the transfer from the beaker to the volumetric flask.
(b) Not enough water was added to bring the volume up to the mark.
3. Estimate the errors (on the calculated value of the concentration of sodium hydroxide) for the following practices.
(a) The burette (for keeping sodium hydroxide solution) is rinsed with distilled water only.
(b) The pipette (for transferring potassium hydrogenphthalate solution) is rinsed with distilled water only.
(c) The conical flask contains some de-ionized water before the addition potassium hydrogenphthalate.
4. Other comments for this practical.
End
VA01_KHA2 /p.3