To Determine the Concenteration of a Limewater Solution

To Determine the Concentration of a Limewater Solution

The aim of this investigation is to determine the concentration of a limewater solution using hydrochloric acid with a known concentration of 2.00mol dm-3.

In this investigation, I will be using titration to determine the concentration of the limewater solution. “Titration is a way of measuring quantities of reactants, and can be very useful in determining an unknown concentration.” But before I can undergo the titration, I will need to dilute the hydrochloric acid. This is because the limewater has an approximate concentration of 1g dm-3, whereas the hydrochloric acid has a concentration of           2.00mol dm-3. This is important because if the hydrochloric acid has a higher concentration than the limewater, then not all of the solution will neutralise fully, which will not give us accurate or reliable results.

Before the titration, I will also have to work out which indicator should be used in this experiment. The indicator is essential as it shows the end-point where the entire base has reacted to give a neutral solution. As both hydrochloric acid and calcium hydroxide are strong bases , I have a choice of using either methyl orange or phenolphthalein as my indicators. I would be using phenolphthalein as my indicator, because the phenolphthalein will change colour just before the equivalence point and therefore, too much acid will not be added to the solution. This means it will be much more accurate than something like Methyl Orange, which would change after the equivalence point and after too acid much has been added.

To carry out the titration I also need to know the balanced chemical equation for the reaction.i

Balanced Equation for the Reaction

Ca(OH)2 (aq) + 2HCl (aq)                 CaCl2 (aq) + 2H2O (l)

1 mole of calcium hydroxide reacts with 2 moles of hydrochloric acid for a neutral reaction.

I need to work out the more diluted concentration of hydrochloric acid so the reaction is neutralised completely.

Concentration of Limewater

Approx.Conc = 1g dm-3

Mr (Ca(OH)2) =40 + 2(16 + 1) = 74

Moles = Mass ÷ Mr

Moles = 1/74

Approx.Conc = 1/74 mol dm-3 or 0.0135 mol dm-3

Moles of Limewater

No. of moles of Ca(OH)2 used in 250cm3 of limewater = concentration × volumes in dm3

= 0.0135 × 0.25

Moles   = 0.003375 approx.

Moles of Hydrochloric Acid

Ratio of Ca(OH)2 : HCl = 1 : 2

No. of moles of HCl needed for neutralisation = 0.003375 × 2

= 0.00675 approx.

Concentration of Hydrochloric acid

So we can see from the balanced equation that twice as much acid is used than alkali, I need to use acid with twice the molar to that of alkali to result in equal volumes being used.

Concentration of HCl required = 0.0135 × 2

= 0.027 mols dm-3

So we need to get ...