To determine the concentration of Limewater solution

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To determine the concentration of Limewater solution

Introduction

I have been provided with 100cm3 of Lime water or Ca(OH). It has been made so that it contains around 1g dm-3 of calcium hydroxide. Hydrochloric acid (HCl) is also available with a concentration of exactly 2.00 mol dm-3. The problem is that this is too concentrated, so the reaction or end point will be reached too quickly and will give me a poor result. This means that the HCl needs to be diluted which is what is going to be calculated later on. This investigation will plan an experiment which will enable me to determine the concentration of Limewater in g dm-3 with as much accuracy as possible. I have been allowed to use the available apparatus however other chemicals cannot be used. Information from “Assessed Practical 2” (AS sheet). 

The equation for this reaction is Ca(OH)2(aq) + 2HCl(aq)               CaCl(aq) + 2H2O(L). As you can see the Ca(OH) reacts with the HCl producing Calcium Chloride and 2H2O. However even though these are the products, they can be the reactants to as well since Ca(OH)2 is made up by water (H2O) and calcium chloride (CaCl). Also Ca(OH) is not very soluble. When is dissolves with HCl for example, only a little of the Ca(OH) will react as some of it will be left in the solution.

In this planning, I also need to decide what experiment I am going to use to determine the concentration of Ca(OH). I used preliminary work to decide that the best method is titration. Knowledge from “determination of the relative atomic mass of lithium” (preliminary work). I have decided to use this method because it is a very accurate way of finding out the concentrations of substances. Titration is when you have your solution in a flask and have HCl, for example, above it in a burette. You can then add very precise amount of the HCl to the substance in order to find the end point using an indicator. By titrating the two substances you can find out how many moles they both need to react with each other. I will calculate this later on.

Also it is very important to find out which indicator to use as certain chemicals require certain indicators. Preliminary work show how screened methyl orange should be used. However what is the difference methyl orange and screened methyl orange? The answer is that the purpose of a screened indicator is to produce a more noticeable colour change at the end point. These types of indicators are made up of either a mixture of two indicators, or a mixture of an indicator and an ‘inert dye’. Methyl orange changes colour at pH 3.2 from red to yellow/orange. Screened methyl orange is a mixture of methyl orange and ‘xylene cyanole’ (a redox indicator). It changes colour at a pH of 3.8 from green to violet or reverse. Knowledge from “http://www.sci-ctr.edu.sg/ScienceNet/cat_physical/cat_che01617.html” (web site).

(Variables)

In this experiment there are many variables which could be changed. However it is very important that I keep the correct variables constant through the practical or it will not be a fair test. For example one variable I will change will be the concentration of the HCl. However once it is at the desired concentration it will have to stay that way throughout the remainder. Other than that every other variable must stay constant! For example if I was to change the amount of Ca(OH) used, it would be an unfair test because of the amounts would be different. I must also keep the amount of indicator constant because if this is not done, the end point would be found at different times! I will also keep other variables such as temperature the same as a higher temperature will give the molecules more heat energy which will cause a faster reaction giving an unfair result. I will keep all the variables constant by leaving the chemicals to reach room temperature and to ensure that I measure the amounts of substances extremely carefully.

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Method 

(Dilution calculation)

                          Ca(OH)2(aq) + 2HCl(aq)               CaCl(aq) + 2H2O(L).

You must see that the limewater solution contains around 1g mol dm-3 of calcium hydroxide. We need to convert the grams into mol dm-3 because the concentration of the HCl is in mol dm-3 . This is done by finding the number of moles of Ca(OH). This can be found by using the formula triangle.

(Mr of Ca(OH)2 = Ca:40.1 + O:32 (16X2) + H:2 (1X2) = 74.1)

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