Experiment 3 Standardisation of hydrochloric acid with borax
Goal: This experiment is designed to enhance your skills in the precise use of volumetric glassware and the analytical balance. In addition, you will be introduced to the procedure for preparation of a solution of a primary standard and the importance of selecting the correct indicator for an acid-base titration.
Objectives: On completion of this lab you should be able to: Weigh accurately using an analytical balance and assess random errors in balance readings and masses weighed. Prepare a standard solution.
Select an appropriate indicator for an acid base titration
Determine the concentration of acid in an unknown solution using a primary standard Determine the error in calculated concentration.
Borax (Na2B4O7 .10H2O) can be used as a primary standard since it does not decompose under normal storage, it is readily obtained in a very pure form (99.999% purity), it reacts with a known stoichiometry and can be weighed and used directly.
In water, the salt dissociates:
Na2B4O7.10H2O → 2Na+ + B4O72- + 10H2O
and the anion is hydrolysed:
B4O72- + 7H2O → 4H3BO3 + 2OH-
The liberated hydroxide ions can be titrated with a strong acid and at the point of their neutralization, the solution in the flask will contain the very weak acid H3BO3, which will dissociate according to:
H3BO3 + H2O → H2BO3- + H3O+. Ka = 5.8 x 10-10.
Thus at the neutralization point (or equivalence point: the point when all the liberated OH- have been converted to H2O) the solution will be slightly acidic (pH ~ 4.8; the actual pH depending on volumes and amounts used). The indicator used to identify the strong base-strong acid equivalence point must therefore change colour over the pH range 4.8 ± 1.
During this experiment you will be working with a weak base and a strong acid. You must wear eye protection at all times. In the event that any reagent used in this investigation comes...
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