mol/dm³ = moles / Volume
= 0.0005 / (25/1000)
= 0.02 mol/dm³
Now that I know the Acid will be approximately 0.02mol/dm³, I can say I will need approximately 25cm³ of acid to neutralise 25cm³ of limewater solution. The dilution factor of the acid is (concentration I began with/ concentration I am going to use) 2/ 0.02 = 100, this means the dilution factor by which I have diluted the acid is 100. I will need to make 250 cm³ of 0.02mol/dm³ hydrochloric acid. The amount of 2mol/dm³ I will need to add is 250/100 = 2.5cm³. However I cannot measure 2.5cm³ accurately. The smallest amount I can measure accurately is 5cm³; therefore I will need to double the amount of HCl I will make. So I will make 500cm³ of hydrochloric acid in a volumetric flask.
Method of Dilution
Apparatus
- 2Mol HCl
- 500cm³ volumetric flask
- 5cm³ pipette
- Pipette filler
- 250cm³ beaker
- Funnel
- Water
- Deionised water
- I will need to wear a lab coat and gloves
To begin with gather the required apparatus and collect the 2mol/dm³ HCl. You will need to wash out the 5cm³ pipette with de-ionised water by by using the pipette filler. You will need to place the pipette filler on the pipette and squeeze the air out with the bottom valve. And then rinse out the water. Repeat this process using the HCl. Then measure out 5cm³ of the HCl. Take off the pipette filler and use your finger to slowly release the HCl till the bottom of the meniscus is at the 5cm³ line.
Wash out the beaker with de-ionised water and with HCl Acid. Place the 5cm³ of HCl in the beaker, touch the bottom of the pipette onto the HCl in the beaker so that the last drop is removed from the pipette. Use a funnel to then pour the HCl into the volumetric flask, fill the beaker with de-ionised water and pour this into the volumetric flask, repeat this process 3 times, this will ensure as much of the HCl from the beaker is added to the flask. Then add de-ionised water to make up the rest of the 500cm³. When the flask is nearly full use a pipette so you can reach the 500cm³ mark more accurately. Place the lid on the flask and, with one hand on the top and one on the bottom, turn the flask upside down. And turn it back upright again, allowing the air bubbles to travel to either end. Repeat this last procedure 3 times.
Method of Titration
Apparatus
- 0.02M HCl
- 250cm³ of Ca(OH)2
- Beaker x2
- A conical flask
- White tile
- A tray
- Methyl orange
- Non-permanent marker
- A 5cm³ pipette
- A 25cm³ pipette
- A pipette filler
- A burette
- Funnel
- A clamp and a clamp stand
- In addition to these, goggles, a lab coat and gloves will be needed.
Firstly collect all the apparatus needed. Place the calcium hydroxide in a beaker. Next, label the beaker containing calcium hydroxide as Ca(OH)2 and the volumetric flask as HCl, using the non-permanent marker. Wash out the beakers, conical flask, burette and pipettes with deionised water and then with the substance that the apparatus is going to be used with.
To Wash the burette, take it and firstly wash it with de-ionised water, to do this, fill it half way with de-ionised water and open the burette tap so that some of the water runs through. Then close the burette tap and turn the burette upside down so that the top is facing the sink. Slowly let the water drain out whilst twisting the burette so that it is washed from all sides. Then turn it back the right way and open the burette tap, releasing all the water from the burette. Next repeat the process using HCl to get rid of the deionised water.
We now need to wash out the pipette. To do this you need the pipette filler. Place the pipette filler on top of the pipette and squeeze out the air in the filler by pressing the valve at the top of the filler, then to suck the limewater into the pipette you need to press the valve at the bottom of the filler. Fill it to approxamatly half way and repeat the process of cleaning used with the burette. Use deionised water first and then the substance going to be used, which is limewater solution. Finally the conical flask in which you will put 25cm³ of Ca(OH)2 needs to be washed, but with deionised water only.
You now need to set up the burette. Fix it to the clamp so that the 0 mark is roughly at eye-level. If it is too high, then you need to place it on a stool to fill the burette. You now need to fill the burette with HCl, using a funnel, to about twice the amount you need. Which, in this case, is approximately 50cm3. Place the remaining HCl on a white tray out of your way. Remember to remove the funnel before you take the initial reading. Using the 25cm³ bulb pipette measure out 25cm³ of Ca(OH)2 and place it in the conical flask. You will need to use the pipette filler to do this, using the same method as described in washing the pipette. In order to get the last drop touch the end of the pipette on the surface. You now need to add 2-3 drops of methyl orange indicator (or any other indicator you are using for convenience) into the calcium hydroxide and the indicator will turn yellow/orange indicating it is an alkali. Put the conical flask on a white tile to help visability. Then measures the volume of liquid in the burette by placing a piece of paper behind the burette and reading the base of the meniscus and note it down.
Firstly you will need to carry out a trial run. For the trial run add the HCl to the limewater fairly quickly, until the solution turns peach colour. As soon as it does turn the tap off and take a final meniscus reading again. Note down the difference between the initial and final reading.
You will now need to carry out your first real test. To do this first empty the contents of your conical and wash it with de-ionised water. Then measure out another 25cm³ of Ca(OH)2 and pour it into the conical flask, and refill the burette. Again take a reading of the meniscus in the burette but from the last reading we know approximately how much acid is needed. This time keep and eye on the reading and add approximately 5cm³ less. Once that is done add the acid in drop by drop so that the end point (when it turns peach) can be reached with more accuracy. This way the results will be more accurate as the end point will be closer to the exact end point. Once the end point is reached take the meniscus reading again and note down the difference between the your initial and final. Repeat this until you achieve concordant results (they are within 0.1cm³ of each other). Then find an average of your concordant results, this now tells you how much acid is needed to neutralise the base.
Safety
In this experiment you will be working with corrosive chemicals so there is a need for certain precautions. You must wear goggles, a lab coat and gloves. In addition to this ensure that all bags and coats are stored out of the way and stools are placed under the desks to avoid accidents.
There are many hazards involved with using calcium hydroxide and Hydrochloric Acid, as they are both carosive chemicals and if they are to come into contact with your eyes they can cause a lot of damage, hence the need to wear safety goggles when carrying out the experiment. Both chemicals are also dangerous if swallowed, inhaled or if they come into contact with your skin. In order to avoid these dangers as much as possible a lab coat and gloves must be worn. When not in use the chemicals should be stored in a sealed container in a fume cupboard. When the top is taken off the either of the bottles the ventilator must be turned on. When in use insure all doors and windows are open to provide maximum ventilation to the room to avoid inhalation of vapours.
If Calcium Hydroxide or Hydrochloric Acid were to come into contact with skin, wash the effected area under cold water for five minutes and if irritation is caused seek medical help. If inhaled then immediately leave the room and get as much fresh air as possible. Finally if either chemical comes in contact with eyes then wash eyes under cold water for fifteen minutes and seek medical advice. If spilt on the desk then wipe the chemical into the sink using a damp paper towel and then mop up using a dry paper towel.
The indicator being used, if it is methyl orange, also has its dangers so it should be stored in a sealed container in a cool environment. When using the methyl orange ensure that all doors and windows are open to provide good ventilation in the room. As with the other two chemicals when handling the methyl orange gloves, lab coats and goggles should be worn.
If methyl orange were to come into contact with either the eye or the skin, or if it were to be inhaled take the same procedures as with the Hydrochloric Acid or the Calcium Hyroxide.
Error Analysis
An error analysis is done on all the equipment that is used. This shows the percentage error in a reading. The higher the percentage error the less accurate the results.
The burette has an error analysis of ±0.05cm³. So at the start reading the error is ±0.05cm³ and at the final reading the error analysis is ±0.05cm³. Therefore the titre end point has an error analysis of 0.1cm³. To work out the percentage error you must divide the error analysis by the the reading taken, for example if the titre end volume is 25cm³:
Percentage error = (0.05/25) x 100 = 0.4%
% Error of the 5cm³ pipette
The pipette has an error of 0.1cm³. Therefore the percentage error is:
Percentage error = (0.05/5) x 100 = 1%
% Error of 25cm³ pipette
The pipette has an error of 0.05cm³ so the percentage error is:
Percentage error = (0.05/25) x 100 = 0.2%
% Error of 500cm³ volumetric flask
The volumetric flask has an error of 0.2cm³ so the percentage error is:
Percentage error = (0.2/500) x 100 = 0.04%
This is the reason for the equipment stated for experiment, as it has a lower % Error than a simple measuring cylinder. Also this is the reason for diluting the acid instead of using 2mol/dm³. The original acid would have given a much higher percentage error because the
The total error of the equipment used in the experiment is experiment is ±1.64%.
From the error calculations we can determine why it is better to use diluted HCl rather than the 2mol/dm³ HCl.
If the full concentration is used and the titre used is 20cm³ then the amount of HCl required is:
20/100 = 0.2cm³ therefore the percentage error is:
Percentage error = (0.1/0.2) x 100 = 50%
Bibliography
Essential AS Chemistry for OCR by Ted Lister and Janet Renshaw. (Pages 16 – 21) Published by Nelson Thornes
Timstar – Safety Data Sheet CD Rom
http://209.85.165.104/search?q=cache:BWjTd56F_BQJ:www.alienwolf.btinternet.co.uk/files/ChemAS.doc+to+determine+the+concentration+of+a+limewater+solution&hl=en&ct=clnk&cd=10&gl=uk
