# Investigation to determine the concentration of a sample of limewater

Investigation to determine the concentration of a sample of limewater

Introduction

You have been provided with a sample of 2.0M hydrochloric acid, chemical formula HCl. Hydrochloric acid is described as a strong acid, which means the H
+ ions and Cl- ions are fully dissociated into the solution. At any one time, virtually 100% H+ ions (protons) will have reacted with water to form hydroxonium ions, leaving Cl- ions. This particular sample is also quite concentrated.

You have also been provided with a sample of limewater. Limewater is a solution of calcium hydroxide and has the chemical formulas Ca(OH) 2.  It is formed when calcium oxide, chemical formula CaO (quicklime) is mixed or “slaked” into water and has a pH of about 9-10. Ca(OH) 2 is described as a strong base.

When an acid and alkali are mixed, the reaction forms a neutral solution of metal salt and water, which has a pH of 7. This particular reaction we are interested in is shown in the balanced chemical equation below. One mole of calcium hydroxide reacts with two moles of hydrochloric acid, to form one mole of calcium chloride and two moles of water.

Ca(OH)2(aq) + 2HCl(aq)            CaCl2(aq) + 2H20(l)

Base   +   Acid                          Metal Salt +  Water

Objective

The sample of limewater we have been given contains approximately 1g/ dm³ which is a concentration of approximately 0.135M. This is calculated by the equation below:

Mol = Mass = 1    = 0.0135M

Mr        74

Your objective is to determine the concentration as accurately as possible; the most appropriate method of completing this task is to carry out a titration.

An acid-base titration is a technique in chemistry by which you can determine the concentration of an unknown reagent using a standard solution of known concentration. It uses the neutralization reaction that occurs between acids and bases, and that we know how acids and bases will react providing we know their formula. The standard/known solution is referred to as the titrant. In this case the titrant will be the acid, which will be used to determine the exact concentration of the limewater, by delivering the acid to a known volume of limewater with a burette. The concentrations and volumes of the two reagents are related by the following equation, which is eventually how the unknown concentration will be calculated.

Volume of acid x Concentration of acid = Volume of base x Concentration of base

When the reaction is complete and the solution is neutral, it is referred to as the end point or equivalence point. The volume of titrant used to reach the end point is termed the titre. To determine this point precisely a chemical indicator needs to be used. Phenolphthalein is the most effective indicator because hydrochloric acid is a strong acid and calcium hydroxide is also a strong base. Phenolphthalein is colourless in acidic solutions and turns fuchsia in basic solutions. At the end point ...