Doc Brown's Chemistry GCE AS A2 Level Chemistry

Advanced Level Chemistry Revision on Titrations

GCE A Level AS-A2 IB Acid-base and other non-redox volumetric titration quantitative calculation questions PART 2 Questions 21 onwards

In PART 2 you have to work out the method a bit more on your own! I have not broken the problems down into 'mini-steps' for you, but request you use your experience from solving the PART 1 Questions. I have also included some 'gas volume' and 'gravimetric analysis' questions to provide more variation of quantitative analysis questions and standardising hydrochloric acid calculations.

Relative atomic masses that may be needed: H = 1, C = 12, O = 16, Na = 23, S = 32, Cl = 35.5, Ca = 40, Cu = 63.5, for questions 21 onwards

I've tried to quote the data to the appropriate significant figures and associated 'trailing zeros'.

Q21 Analysis of limewater.  25.0 cm³ aliquots of a calcium hydroxide solution were titrated with 0.100 mol dm^-3 (0.100M) hydrochloric acid using phenolphthalein indicator. On average it took 10.50 cm³ of the acid to neutralise the limewater.

Calculate

(a) the molarity of the calcium hydroxide solution

(b) the concentration of calcium hydroxide in (i) g dm^-3 and (ii) g/100 cm³

Q22 If it took 20.55 cm³ of 0.100 M HCl to neutralise 25.0 cm³ of an NaOH solution, calculate the molarity of the alkali.

Q23 A standardised solution of sodium hydroxide had a concentration of 0.1025 mol dm^-3 (0.1025M). If 25.0 cm³ of a sulfuric acid solution required 17.65 cm³ of the NaOH to neutralise it, calculate the molarity of the acid.

Q24 Citric acid has the formula and is tribasic acid, forming the tri-sodium salt on complete neutralisation with sodium hydroxide.

A 25.0 cm³ sample of a concentrated citrus fruit cordial component (e.g. for lime juice), used in the food & drinks industry, was diluted to 250 cm³ in a graduated volumetric flask.

A 25.0 cm³ sample of this diluted solution, required, on average, 22.5 cm³ of a standard 0.100 molar sodium hydroxide solution using phenolphthalein indicator for the titration end-point.

Assuming all the acid in the cordial was citric acid, calculate the concentration of the acid in g/dm³ in the original solution.

Q25 4.28 g of a hydrated form of the salt sodium sulphate, Na₂SO₄.xH₂O, was heated at 180^oC until the remaining mass became constant at 1.89 g. Calculate x, the number of molecules of 'water of crystallisation'.

Q26 On reaction with dilute hydrochloric acid, 0.428 g of a group 2 metal (M) formed 75.0 cm³ of hydrogen gas at 298K/1 atm pressure.

If the molar volume of gas is 24.0 dm³ at 298K/1 atm, deduce the atomic mass of the metal and identify it from the periodic table.

Q27 To standardise a solution of hydrochloric acid, it was titrated against accurate masses of anhydrous sodium carbonate (which should be dried in an oven prior to use). e.g. 0.132 g of Na₂CO₃ required 24.8 cm³ of the hydrochloric acid for complete neutralisation, calculate the molarity of the hydrochloric acid.

Q28 Follows on from Q27. You can make up a solution of sodium carbonate from which you pipette 25.0 cm³ aliquots to obtain an accurate average titration value as an alternative to multiple weighings. 1.30g of anhydrous sodium carbonate was dissolved in 250 cm³ of de-ionised water in a volumetric flask. 25.0 cm³ aliquots were pipetted and titrated with a hydrochloric acid solution (in the burette) to be standardised.

(a) Calculate the molarity of the sodium carbonate solution.

(b) If the average titration was 24.35 cm³ of hydrochloric acid, calculate the molarity of the acid assuming the sodium carbonate is fully neutralised to sodium chloride.

Q29 The purity of anhydrous sodium carbonate can be determined by titration with standard hydrochloric solution. The following results were obtained on titrating accurately weighed amounts of sodium carbonate with 1.00 mol dm^-3 hydrochloric acid (1.00 M HCl) to complete neutralisation given by the equation below.

Na₂CO₃ + 2HCl ==> 2NaCl + H₂O + CO₂

Analysis Result	Mass of Na2CO3	titration
(i)	1.113 g	20.95 cm3
(ii)	1.092 g	20.55 cm3
(iii)	1.166 g	22.00 cm3

The titration values were recorded to the nearest 0.05 cm³ for a burette calibrated in 0.1 cm³ increments.

In each case calculate (a) the moles of HCl used in each titration, (b) the moles of Na₂CO₃ titrated based on the moles of HCl used, (c) the mass of Na₂CO₃ titrated based on (b), and finally, (d) from your (b) and (c) calculations, calculate the % purity of the anhydrous sodium carbonate, quoting what you think is the most accurate analytical result.

Q30 How to determine the water of crystallisation in hydrated sodium carbonate crystals ('washing soda') by titration with standard hydrochloric acid solution.

If 0.352g of hydrated sodium carbonate crystals, Na₂CO₃.xH₂O, was titrated with 0.100 mol dm^-3 (0.100M) standard hydrochloric solution. If the titration value was 24.65 cm³,

calculate the moles of Na₂CO₃ titrated and hence deduce x, the number of molecules of water of crystallisation in washing soda crystals.

Q31 Blue hydrated copper(II) sulfate crystals have the formula CuSO₄.xH₂O, where x is the number of molecules of 'water of crystallization'.

If 3.33g of copper sulphate crystals where heated at 150^oC until the residue of anhydrous copper sulphate remained constant at 2.12g.

(a) Calculate the % water of crystallisation in the blue copper sulphate.

(b) From your answer to (a) calculate the value of x in the formula of the blue crystals.

Q32 ...

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