Chemicals –
-
250cm3 of limewater containing approx. 1gdm-3 of calcium hydroxide
-
2.00 moldm-3 of hydrochloric acid
- Distilled water e.g. Methyl orange
Ca(OH)2(aq) + 2HCl(l) CaCl2(aq) + 2H2O(l)
Firstly I will need to dilute the concentration of the HCl, because it is too concentrated. I will do this by using the pipette and the filler. I will fill a 2.5cm3 pipette full of hydrochloric acid, and place this in the standardised volumetric flask and fill to the mark with 247.5cm3 of distilled water. This makes the concentration 0.02moldm-3, and I will use this volume instead of 0.2moldm-3 or 2.0moldm-3 because it uses larger volumes in the titration giving less error. To find out the concentration in gdm-3 I will need to work out the concentration of the limewater in moldm-3 and multiply this by the R.F.M of limewater, which is (40 + 32 + 2) 74 amu. I will now need to titrate the hydrochloric acid against the limewater. I will fill the burette with HCl using the burette funnel (not necessarily to the top) but remember not to fill the burette above eye level, and remember to take the funnel out after it has been filled. Get a conical flask and fill it with 25cm3 of limewater using the pipette and pipette filler. Place roughly five drops of the methyl orange indicator into the conical flask. Methyl orange is used because it would produce a rapid colour change from orange to pink due to the nature of the strong acid and weak base. Clamp the burette above the conical flask, which should be on top of the white tile, so that there is an obvious colour change recognition. Firstly do a rough titration followed by three more accurate titrations, where you drip the HCL into the limewater when close to neutralisation. Record your results and do an average of the accurate three titration’s and NOT the rough titration. you must carry out the experiment until concurrent results are obtained. For fast but accurate titrations, once you have a rough idea of how much HCl is needed, on the next go, you can release the HCl to free flow into the conical flask until you are near the total volume it took to neutralise it. You must constantly shake the conical flask throughout the titration because the methyl orange may turn pink, but it may not be permanent, and so you must keep titrating it until the first permanent pink colour change has been reached. Once you have recorded the volume, you are ready to start the calculations…
This shows how the apparatus should be set up.
Calculations:
Once you have found the average volume needed to neutralise the limewater, use the formula: Moles = Concentration x Volume to work out the number of moles present. Using the mole ratio of 1 : 2, divide this moles calculation by 2, to get the number of moles present in the limewater. Because you now know the moles and the volume of limewater used, you can work out the concentration of the limewater using the formula: Concentration = Moles / Volume. Now that you have the concentration, you can multiply this by ten (because we used one tenth of the limewater solution) and then by the R.F.M (74amu) to get the concentration in gdm-3.
Accuracy: The burette is +/- 0.1 cm-3. For example, if your volume you used to neutralise the limewater was 17.5cm-3, the error would be 17.5cm-3+/-0.1, giving an error of 0.57%, which is very accurate. When near neutralisation, you must drip the HCl into the conical flask so that you can obtain accurate results. The pipette used had an error margin of 0.6cm3.
Precision: You will need to repeat the titration roughly three times till you have concurrent results that are precise.
Safety:
- Never fill the burette above eye level.
- Always wear goggles and a lab coat.
- The burette must not be slanted at all in case of an imbalance.
- Never use a measuring cylinder or teat pipette to measure volumes as they are very inaccurate.
- If you find it difficult to read the burette, put a white piece of paper behind it when taking readings.
- When using a pipette filler, do not push the pipette end to hard into the filler because it easily liable to break.
Bibliography:
-
SEARCH + LIMEWATER
- Encarta encyclopaedia 2000 CD rom
- Chemistry 1 OCR
