2015 - expt 2a acid base titration

8/18/2019 2015 - Expt 2A Acid Base Titration

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H2 Chemistry Practical BMC International College 2015

Name : ______________________________________________ Date : ________________________

Theory Ethanedioic acid or oxalic acid is an organic acid, thus it is reacted with a strong base

will produce salt solution that is of pH greater than 7. Hence need to choose anindicator that could change colour above pH 7 to determine accurately. The choice ofindicators used is thymol blue or thymolphthalein.

pH of thymol blue indicator at end point is between 8.0 (yellow) to 9.2 (blue). Hence thecolour of thymol blue indicator at end point is pale green or almost colourless if thesolution too diluted.

pH of thymolphthalein indicator at end point is between 8.8 (colourless) to 10.5 (blue).

The end point colour is pale blue (from acidic to alkaline) or colourless (from alkaline toacidic)

Sodium hydroxide reacts with ethanedioic acid as follows:2NaOH + (COOH)2 → (COONa)2 + 2H2O

From the equation, 2 2number of moles of (COOH) .2H O 1

number of moles of NaOH 2=

BMC INTERNATIONAL COLLEGEH2 CHEMISTRY PRACTICAL

EXPERIMENT: 2TITLE: STANDARDISATION OF SODIUM HYDROXIDE SOLUTION WITHSTANDARD ETHANEDIOIC ACID SOLUTION




Chemicals 1 @ thymol blue or thymolphthalein indicator 1 @ 250 ml beaker labelled as FA 1 contains sodium hydroxide NaOH.

1 @ test tube with stopper labelled FA 2 contains solid ethanedioic acid crystal,(COOH)2.2H2O

Procedures Refer to the Appendix which describe the process with illustrations.

a) Weigh accurately about 0.75 g of FA2 in a weighing bottle and record thereadings below. Use the TARE button.

b) Dissolves the FA2 sample in 50 cm3 of distilled water in a 100 cm

3 beaker.

Transfer the solution and washings into a 250 cm3 graduated flask. Make up tothe mark with distilled water. Shake well and label the solution FA3.

c) Place FA 1 into the burette. .

d) Pipette 25.0 cm3 of FA3 into a conical flask.

e) Add about two drops of the indicator assigned on the lab bench to FA 3 in theconical flask.

f) Perform titration of FA 3 against FA 1 until close to the end-point of titration.

g) Prepare the titration table and record your observations and readings..

h) Repeat the titration until concordant results are obtained.




Calculationsa) Calculate the concentration (in mol dm

– 3) of ethanedioic acid crystals in FA3.

[Date: H, 1.0; C, 12.0; O, 16.0]

b) Calculate the number of moles of ethanedioic acid in 25.0 cm3 of FA3.




d) State whether each of the following would lead to a greater/smaller or similar value to your answer in (c)

1. The burette was rinsed with deionised / distilled water and then filled up immediately with FA 1 fortitration.

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2. The conical flask used for the titration was rinsed with FA 3 followed by the addition of 25.0 cm3 of

FA 3 from the pipette for titration with FA 1

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3. The pipette was rinsed with deionised / distilled water and then used to transfer 25.0 cm3 of FA 3 into the conical flask for titration with FA 1

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4. The small beaker containing FA 2 dissolved in deionised / distilled water was not rinsed at all duringthe transfer of the solution into the graduated flask.

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ExtensionExercise

Ethanedioic acid is one of the common reagents used for both acid base titration andredox titration as it can be oxidised by potassium manganate.

Ethanedioic acid is a dibasic acid which is part of polybasic acid family. Acid basetitration method often employed to find the basicity of the acid through standardisedconcentration of base.

The following are some exercises for further practise.

Exercise 1Solution S contains a 0.0350 mol dm

–3 polybasic acid with the formula Hn A.

Solution T contains 0.12 mol dm –3 aqueous sodium hydroxide.

In a titration experiment, 25.0 cm3 of solution T required 28.6 cm

3 of solution S for complete neutralisation.

(a) Calculate the value of n in the acid Hn A from the information given.

(b) Hence, write the chemical equation, including state symbols, for this neutralisation reaction.




Exercise 2 Lemon juice contains citric acid, which is a tribasic organic acid.

A solution containing 3.84 g of citric acid required 40.0 cm3 of a 1.50 mol dm

–3 sodium hydroxide solution for

neutralisation.

(a) Calculate the relative molecular mass of citric acid from the above information.

(b) The molecular formula of citric acid is shown below. Predict the group represented by R.[Ar: C = 12; O = 16; H = 1]

CH2COOH|

R ─ C ─ COOH|CH2COOH




Exercise 3Solution P contains 1.55 g of the acid H3XO3 in 1 dm

3 of solution.

Solution Q contains 1.70 g of hydroxide (OH –

) ions in 1 dm3 of solution.

In a titration experiment, it was found that 25.0 cm3 of P reacted with 12.5 cm

3 of Q.

a) Calculate the concentration in mol dm –3

of OH –

ions in solution Q

b) Calculate the number of moles of OH – ions used in the titration.

c) Calculate the mass of H3XO3 in 25.0 cm3

of solution P

d) (i) Assuming that 1 mole of H3XO3 reacts with 1 mole of OH – ions work out the relative molecular

mass of the acid.

(ii) Assuming that 1 mole of H3XO3 reacts with 2 moles of OH – ions work out the relative molecular

mass of the acid.

e) From your answer in (d), deduce the most likely element represented by the letter X in the formula of

H3XO3.

2015 - expt 2a acid base titration

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