The reaction I’ve been discussing above is a strong acid with a weak alkali. On the next page there is a diagram of the graph demonstrating this type of reaction.
Indicators
There are many indicators that could be used in titrations, but as every indicator had different ranges we need to determine a range of indicators that will be suitable for the experiment. Several possible indicators should be listed, as at this stage we are unsure as to which indicators will be available when we carry out the experiment. Any indicators we use will have to be suitable for a strong acid and a weak base titration, this can be decided by checking what values the straight line falls upon, any indicators within this range will be acceptable. This means that any indicators, which work between the pH values 2-9, will be applicable. The following list shows the indicators I can use.
pH range
Methyl yellow 2.9 - 4.0
Methyl orange 3.2 – 4.4
Bromopheral blue 2.8 – 4.6
Bromocresal green 3.8- 5.4
Methyl red 4.2 – 6.3
Apparatus
250cm Limewater Indicator (Methyl Orange)
2.00 mol dm hydrochloric acid Clamp (cork)
Funnel Board
Burette Conical flask
Pipette Filler Tile
Top hand Balance Volumetric Flask
Method
- As the HCl is to concentrated, the first step is to dilute the solution to 0.02 mol dm . To do this 1cm HCl needs to be accurately measured using a pipette, to this 99cm of water needs to be added. The most accurate way of obtaining exactly 99cm of water is to weigh it on a top hand balance as 1cm of water weighs 1g.
- The burette should be placed securely in the stand, about 10cm above the table. Under the stand and burette a board should be used (to protect surfaces in the event of spillage).
- To ensure that previous experiments where the burette has been used won’t effect the results, a funnel must be used so the burette can be rinsed out with HCl. Rotate the funnel during this process to allow the HCl to reach all around.
- Fill the burette with 50cm of HCl. In your results table the initial reading needs to be recorded to 0.05.
- 10cm of limewater needs to be put into a clean conical flask. This can be achieved most accurately using a pipette filler. To the limewater three drops of methyl orange need to be added.
- Put the conical flask containing the limewater directly under the tip of the burette. Beneath the conical flask a tile also needs to be placed, so that any colour change is more noticeable.
- Carefully allow 1cm of the diluted HCl to run into the conical flask at a time, after each cm close the tip and swirl the conical flask around. When the solution changes colour record the volume that this occurs at in your results table.
- Dispose of the contents of the conical flask. Measure out another 10cm of limewater and place in the conical flask along with three more drops of methyl orange.
- Continually allow HCl to run into the conical flask to a volume that’s within 3cm of the volume used before, then add 0.05 at a time until the solution changes colour. Record the initial and final volumes in your results table.
- Repeat this experiment until you have three volumes that are within 0.1cm of each other.
Safety
During the experiment it’s very important to follow safety procedures, so that I don’t put others or myself in danger. Some of the basic safety measures involve wearing a laboratory coat to protect clothes and skin, also long hair should be tied back. It’s essential to wear goggles so that the eyes are protected against any of the chemicals you’re using. Wearing gloves will protect hands, as the hydrochloric acid we’re working with is highly concentrated. As many chemicals are corrosive special care should be taken while handling them.
Results
To work out the average titre all three titrations needed to be added together and then divided by three.
17.5 + 17.50 + 17.40 = 17.45
This means that on average 10cm of Ca(OH) required 17.45 cm of hydrochloric acid.
By using my results and using the balanced equation, which is shown on the next page, it’s possible to work out the concentration of the limewater solution in g dm .
Ca(OH) (aq) + 2HCl (aq) CaCl (aq) + 2H O (l)
The equation that I will use to work this out is:
CaVa = x
CbVb y
C = Concentration V = Volume a = calcium hydroxide b = hydrochloric acid
x = number of moles of a y = number of moles of b
The concentration of solution ‘a’ is the unknown, so using algebra I need to move it to one side of the equation on it’s own.
Firstly the x and y need to swap places, to enable them to be part of the equation.
Ca Va y = Cb Vb x
The letters now have to be replaced with the appropriate values.
Ca x 10 x 2 = 0.02 x 17.45
The final stage involves arranging the equation so that Ca is on one side of the equals sign alone.
Ca = 0.02 x 17.45
(10 x 2)
=
0.017 mol dm
To work out the concentration in g dm the value I’ve just worked out needs to be multiplied by the mass of one mole of calcium hydroxide:
74.08 x 0.017 = 1.26 g
From the series of calculations I’ve just carried out I can conclude that my results are fairly accurate as we were told before the experiment the limewater solution contained approximately 1g of calcium hydroxide, and I found the concentration was 1.26 g.
Evaluation
Overall I feel the practical went fairly well, I continually tried to be precise in all areas where it was required of me. The actual titration results I achieved must be quite reliable for all three results where within 0.1 decimal places of each other. Whenever I had to measure an amount of substance, I did it to the best of my ability and used appropriate equipment. An important aspect to consider when discussing how reliable the results are, is the approximate accuracies of the equipment. The burette, pipette and volumetric flask all share the approximate accuracy of 0.05cm . So any error that may have occurred during the experiment isn’t likely to be connected to the equipment, although it still needs to be considered.
The limitation which, has the biggest effect on the results I think, is human error. Each time we complete a titration we have to judge the colour of the solution and whether it’s reacted. It’s possible for our perceptions of this colour to change, and we might also be influenced by the result we achieved last and feel it’s changed so that our results are similar. One way I’ve already tried to counteract this problem is to dilute the strong concentration of acid, so that the colour change is more gradual and easier to judge. Another method of achieving this is to use a computer with a camera, after the first titration a picture could be taken so that during the next titration you can compare, which should leave you with near identical colours.
Another way to improve this experiment includes repeating the titrations more than three times so that an average of more results could be worked out. The consistency of my results supports the claim that they are reliable. While there are some improvements to be made, within the confinements of school supplies I think I carried out the assessment appropriately.
