Tut 3 - Volumetric Analysis

1. A 250cm3 volumetric flask contains exactly 100cm3 of a 0,05mol.dm-3 sulphuric acid solution as shown below.

1.1. Calculate the number of moles of hydronium ions in the flask assuming complete dissociation of the acid.
1.2.Then, ten sodium hydroxide pellets, each of mass 0,1g are dropped into the flask. After the pellets have dissolved completely, the flask is topped to the 250cm3 mark with water. The flask is then stoppered, and inverted a few times so that the resulting solution is of uniform concentration throughout at 250C. Calculate the pH of the resulting solution. Work to three decimal places.

2. You have a 4g impure sample of NaOH. It took 2g of this sample of NaOH to exactly neutralise a standard solution of HCl of concentration 0,2M and volume 0,24dm3.
2.1. Determine the percentage purity of the sample.
2.2. Calculate the mass of impurity in the original sample.

3. If HCl is dissolved in water, the water will act as :
3. B

4. Explain why a 0,1 mol.dm-3 KOH solution is a better conductor of electricity than
a 0,1 mol.dm-3 ammonia (NH3) solution.
4. KOH is a strong base and dissociates fully in water producing a large number of ions for conduction (good electrolyte)
NH3 is a weak base, producing a small quantity of ions in solution, hence poor conductor (weak electrolyte).

5 Two grams of solid calcium carbonate are partly dissolved in 20cm3 of water in a beaker. An HCl solution of concentration 0,5M and volume 100cm3 was added to the beaker.

5.1. Is HCl(aq) a strong or a weak electrolyte? Reason.
5.1. strong acid
ionises fully in water
5.2.1. Calculate the pH of the HCl solution before it was mixed with the carbonate solution.
5.2.2. Explain why the pH of the carbonate solution cannot be found by this method.
The calcium carbonate is a weak base, and is undergoes partial ionisation in water. Hence it is not known how many mols of OH- would be produced.
5.3. Calculate the number of sodium hydroxide pellets, each of mass 0,2g that must be added to the above solution so that the final solution in the beaker has a pH of 7.

6. A solution of HCl of pH 1,2 and volume 0,2dm3 is mixed with a solution of NaOH of that has a pH 13,2 and volume 0,2dm3.
6.1. Calculate the mols of H+.
6.2. Calculate the mols of OH-.
6.3. Calculate the concentration of the excess mols.
6.4. Calculate the pH of this resultant solution.

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