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).
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.
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)
n = the number of moles.
n Mr M = the mass of the material or substance.
Mr = the relative molar mass
n = m = 1
= 0.01 (1.s.f)
Now looking at the ratio, we can see that the ratio of Ca(OH) to HCl is 1:2. This means that the number of moles (mol dm-3) of the HCl will double of that of the Ca(OH). So we need to times the number of moles of Ca(OH) by two to get the desired number of moles for HCl. We need to get this desired number of moles because the HCl is too concentrated.
Ca(OH)2(aq) + 2HCl(aq)
1 : 2
0.01 x 2 = 0.02
Now we have the number of moles for the HCl we need to figure out how much to dilute it by. To calculate this we need the original concentration of the HCl (2.00 mol dm-3) and divide it by the number of moles of HCl in mol dm-3.
2.00 / 0.02 = 100
Now we know that the water to HCl ratio is 100:1 or 100% to 1%. This will mean that one unit of HCl (1cm3) will need 99cm3 of distilled water added to it (it is 99 and not 100 because if you added the 1cm3 of HCl to 100cm3 of water, you will have 101cm3 in total). Now the acid has been diluted to 0.02 mol dm-3. If you are using the cylinder you will need 1cm3 of HCl and 99cm3 of distilled water. However if you are using a volumetric flask you will need to fill the flask with 2.5cm3 of HCl and then fill the rest of the flask up to 250cm3.
Firstly I will go over the apparatus needed to perform the experiment then experiment procedure step by step.
1. Pipette X2 (10ml)
2. Burette (50cm3)
3. Stand and Clamp
4. Volumetric Flask (250ml) (THIS METHOD REFFERS TO THIS)
5. Measuring Cylinder (25cm3)
6. White Tile
7. Beaker of diluted HCl (How much you need may vary / 0.02 mol dm-3)
8. Beaker of Calcium Hydroxide
9. Pipette filler
10. Distilled water (amounts may vary)
11. Normal Hydrochloric acid (2.00 mol dm-3)
12. Screened Methyl Orange (10 drops for each experiment)
13. Conical flask (250cm3)
14. Normal Beakers (X2)
* Measure out 2.5 cm3 of HCl (the 2.00 mol dm-3 acid) and pour it into the volumetric flask with pipette. For accuracy ensure none of the HCl touches the sides or some HCl will not be in which changes the concentration! Then simply add distilled water until the 250ml mark to dilute the acid to.
* Then get your pipette and rinse it with the Ca(OH) because it will make the results more accurate because previous liquids will be washed out. Then measure out (using the pipette filler) 10.0cm3 of your Ca(OH) and pour it into the conical flask making sure not to touch the sides of the flask. Add 5 – 10 drops of screened methyl orange and record how many drops you used.
* Get the clamp ready and place the tile on the clamp, then place the conical flask on the tile. Rinse your burette with your diluted HCl to replace any previous liquids which may intervene in the results. Now secure the burette to the clamp making sure the nozzle is facing in the conical flask. You should now have your apparatus set-up like this:
Burette filled with diluted HCl
Conical Flask Clamp
Ca(OH) and Indicator
* Now everything should be ready. From your volumetric flask, empty out the diluted HCl into a normal beaker. Using this beaker fill the burette up to the 0 mark with a funnel.
* Start titrating slowly until the solution has totally reacted. Then record the results noting how much HCl you used. Record the results in a suitable format and repeat the experiment at least three more time for more accuracy. Finally record the average (mean) titre.
* SAFTEY: this is extremely important as you are handling hydrochloric acid. Goggles must be worn at all times, hair must be tied back and bags must remain under the desk. If anything goes wrong, consult the teacher immediately. Always remember to work safely as HCl can be hazardous to your skin, the indicator may stain your clothing as well.
The pipette, burette and volumetric flask only have a percentage error of + or – 0.1%. This is a small error. That is why they are used in this experiment. Be sure to take precautions such as making sure that when you pour in the HCl or Ca(OH) you do not let it run down the sides because some will be left on the sides thus making an inaccuracy. As you can see above, the bold says10.0cm3 of your Ca(OH), when instead it should have been 25.0cm3. The reason for this is because there was not enough of the Ca(OH) to perform all the experiments. This is a limitation due to the fact that 25.0cm3 would have been a more accurate measure because 10.0cm3 may not be enough for a clear result.
Final burette reading cm3
Initial burette reading cm3
Volume used cm3
Mean titre cm3
This is the final part to the investigation. In this part of the investigation I aim to determine the final concentration of the limewater solution. I will go through my calculation step by step and explain what I am doing along the way.
Starting off, lets look at what we have. The equation for the reaction is:
Ca(OH)2(aq) + 2HCl(aq) CaCl(aq) + 2H2O(L).
For the Ca(OH) we are trying to find the concentration in mol g dm-3. However we have the volume (10.0cm3 is how much we used).
For the HCl we have the concentration (0.0200 mol dm-3) and we have the volume (37.2 cm3). The volume is calculated by using the average titre of the accurate results:
37.2 + 37.1 + 37.2 = 37.2
With this information we need to figure out the number of moles for the HCl. For this we use the formula triangle.
n = c v = 0.0200 x 37.2 = 0.744 = 0.000744 moles
Now that we have the number of moles for HCl, we can figure out the number of moles for Ca(OH). Due to the 1:2 Ca(OH) to HCl ratio, we simply divide the number of moles of HCl by two for the number of moles for Ca(OH).
0.000744 / 2 = 0.000372 moles.
Now for Ca(OH) we have the volume (10.0 cm3) and the number of moles (0.000372 moles). Again using the formula triangle and the figures I have now I can figure out the concentration of Ca(OH).
c = n = 0.000372 = 0.0372 mol dm3
v 0.01 (10.0 / 1000)
However this is not the end of it. We have the concentration in mol dm3 however we want it in mol g dm3. This is easily figured out. All we need to do is have our Mr of the Ca(OH) and times it by the moles we just figured out. The reason for this is to convert the units.
74.1 x 0.0372 = 2.77 mol g dm3 (2.75652).
So finally I can conclude the concentration of the limewater is 2.77 mol g dm3.
Using this method is very accurate; however there are still some errors. One of these errors includes the measuring apparatus. The pipette and burette supplied are very accurate however I noticed that they were both dirty on the inside and so would cause an error in my results. I state this error because dirt would influence the solutions. This dirt may have changed the concentration or it may have changed the amount of solution I thought was in there. This would happen because the dirt would take up space in the apparatus changing the amount of solution. A way to solve this is to simply wash the apparatus after use and to rinse the apparatus with the correct solution before usage.
Another limitation is human error. For example a source of error is in measuring the amounts of the substances or materials used. I should have made sure that I correctly measured out the solutions using the pipette or burette using the meniscus level. This means that I should have been looking at the measuring apparatus straight inline and not from an angle. A way to improve this problem is to simply ensure that you are very careful in measuring the results
Another limitation is in the accuracy of the experiment as a whole. There are a number of useful ways in which to make the experiment more accurate. First of all you can make sure that the measuring apparatus you use is at a high accuracy.
* “My own scientific knowledge” (Hassan Zaman). My own knowledge was extremely helpful as I based most of my work using this.
* “Knowledge from Chemistry 1” (Cambridge 2001). This was a little helpful as the book helped me to plan some of the experiment.
* “Preliminary work” (Determination of the relative atomic mass of lithium). This was very helpful as it help me plan the experiment and gave me the general idea on how to do the course work.
* “Assessed Practical 2” (AS sheet). This gave me the basis of my coursework, without it I would not know any of the figure, amounts or what the aim and plan of the experiment is.
* “Encarta Encyclopedia” (C-D rom 2000). This was rarely used in explaining what titration was even though I knew it anyway, was not very helpful.