Aim. To find the identity of X(OH)2 (a group II metal hydroxide) by determining its solubility from a titration with 0.05 mol dm-3 HCL
To find the identity of X(OH)2 (a group II metal hydroxide) by determining it’s solubility from a titration with 0.05 mol dm-3 HCL
Theory:1.
Titrations are the reaction between an acid solution with an alkali. In this reaction (called neutralization), the acid donates a proton (H+) to the alkali (base). When the two solutions are combined, the products made are salt and water.
For example:
2HCl(aq) + X(OH)2 (aq) XCl2 (aq) + 2H2O (l)
This shows the one of the products i.e. salt being XCl2 and water.
So titration therefore helps to find the concentration for a solution of unknown concentration. This involves the controlled addition of a standard solution of known.
Indicators are used to determine, at what stage has the solution reached the ‘equivalence point’(inflextion point). This means at which, does the number of moles base added equals the number of moles of acid present. i.e. pH 7
Titration of a strong Acid with a Strong Base:
As shown in the graph, the pH goes up slowly from the start of the tiration to near the equivalence point. i.e (the beginning of the graph).
At the equivalence point moles of acid equal mole of base, and the solution contains only water and salt from the cation of the base and the anion of the acid. i.e. the vertical part of the curve in the graph. At that point, a tiny amount of alkali casuses a sudden, big change in pH. i.e. neutralised.
Also shown in the graph are methyl orange and phenolpthalein. These two are both indicators that are often used for acid-base titrations. They each change colour at different pH ranges.
For a strong acid to strong alkali titration, either one of those indicators can be used.
However for a strong acid/weak alkali only methyl orange will be used due to pH changing rapidly across the range for methyl orange. That is from low to high pH i.e. red to yellow respectively pH (3.3 to 4.4), but not for phenolpthalein.
Weak acid/strong alkali, phenolpthalein is used, the pH changes rapidly in an alkali range. From high to low pH, that is from pink to colourless pH(10-8.3) respectively but not for methyl orange. However for a weak acid/ weak alkali titrations there’s no sharp pH change, so neither can work.
Therefore in this investigation, the titration will be between a 0.05 mol dm-3 of HCl with X(OH)2, using phenolphthalein.
Dependant Variable:
Is the volume of HCl to achieve a colour change that is from pink to colourless.
