# Determine the unknown concentration of three acid solutions using a standard known concentration of alkali.

## Aim

The aim of this experiment is to determine the unknown concentration of three acid solutions using a standard known concentration of alkali (which is also known as ‘base’). The three acids that will be utilised in this practical are hydrochloric acid, ethanoic acid and sulphuric acid. The concentration of the acid will be determined by titrating the acid against the base.  In this scenario sodium hydroxide will be titrated against the three acids named above.

Theory

In water, sodium hydroxide is a strong base. It separates into sodium and hydroxide ions. The most common strong bases are the water-soluble hydroxides that belong to sodium, potassium & lithium; all three can be referred to as ionic solids. A solution containing 0.5mol of dissolved NaOH per litre of solution will also contain 0.5mol dm-3 of OH- ions.   This is due to the ratio being on a 1:1 basis.  Please refer to the example below.

NaOH                                                        Na+         +         OH-

When bases react with acids the reaction that takes place is known as neutralisation.   In a 1:1 ratio of acid if there are 0.1 mols of hydrogen ions (H+) present in solution (note H3 O+ in water) then you will need an equal amount of basic OH- ions present to neutralise them (also 0.1mols).

H+        +        H2O                                        H3O+

H3O+         = OH-   = neutral = end point of titration

So in simple terms the reaction that takes between the acid and base is always a neutralisations reaction. A base is titrated with an acid until all the acid ions are neutralised by the base. This is called the end-point and the solution is now neutral. The end point of the titration can be concluded by using solutions as indicators. The indicators follow the progress of the reaction and at the end-point change colour.   Once the colour has changed, we are then aware to the fact that we have reached the end of our experiment.  The next step would be to note how much of the base was added to the acid, as this can determine the concentration of the acid involved, if the initial concentration of the base is noted at first.

Some acids and bases are dibasic. The meaning of dibasic is that they either gain or release two protons per molecules rather than one.

1. Containing two replaceable hydrogen atoms.
2. Of or relating to salts or acids forming salts with two atoms of a univalent metal.

(Lexico Publishing Group, LLC)

An example of the above is Barium hydroxide which is a base in a form of solution;

Ba(OH)2         +        H2O                                Ba2        +        2OH-

If one has been instructed to calculate the pH of a solution, one must therefore base it upon OH concentration, which is twice ...