The aim of this experiment is to find out the concentration of a solution of acid rain. It is to be assumed that the concentration is approximately 0.1 moles dmˉ . Using a standard solution

Concentration= n x 1000 / Volume,

so Mass / RMM = n,

2.66 / 106 = 0.025 moles

0.025 x 1000 / 250 = concentration = 0.100 moles.

20. Label the standard flask with your initials, the date, the concentration and name of the solution.

METHODOLOGY 2:-

How to perform the titration, between sodium carbonate and sulphuric acid:-

1. Create a table for collecting your results:- such as the one shown below:-

2. Rinse the apparatus with the appropriate solutions, this ensures that there are no substances left over in the apparatus from previous experiments which would intrude with the overall results:

• Burette – with distilled water and then the sulphuric acid, ensuring no bubbles are trapped near the tap or the jet after adding the sulphuric acid.
• Dropping pipette – with distilled water and then the sodium carbonate.
• Conical flask- with only distilled water.
• Beakers- with distilled water and then the appropriate solutions that will go in them.

3. Collect the solutions of sodium carbonate and sulphuric acid, in clean dry labelled beakers.
4. Set up the apparatus as shown in the diagram.
5. Dispense the sulphuric acid into the burette to a point below the highest graduation.
6. Using a funnel to aid the operation and carry out the procedure below head height, at  eye level.
7. Remove the funnel from the apparatus so that no further drops of solution are added from now on.
8.  Open the tap and allow a small amount of solution to run to waste in order to fill the burette jet, because the burette readings include the jet.
9. Using a pipette and pipette filler, transfer 25cm³ of sodium carbonate into a conical flask, again using the white tile to give as a distinctive background to ensure that the bottom of the meniscus sits exactly on the line.
10. Touch the pipette onto the glass of the flask at about 45º in order to transfer exactly 25cm³.
11. Add 3 drops of your chosen indicator to the conical flask and gently swirl to mix thoroughly.

• The indicator chosen was screened methyl orange, because it gave the most obvious colour change, for the range that I needed. It turns from purple to blue/grey to green.
• Use only 3 drops of this indicator because the indicator is slightly acidic and therefore will effect the titration, so too much would show the sulphuric acid to be weaker than it really is. If only 2 drops were used it would be too difficult to notice the exact colour change.

12. Take the initial burette reading, from the base of the meniscus (white tile behind)
13. Place the conical flask beneath the burette on a white tile, so the colour change will be easily noticeable when it occurs.
14. Open the tap and run the acid from the burette into the alkali, swirling the flask constantly, to ensure that the acid and base is reacting evenly.
15. When the titration is almost complete (you will know this because where the acid hits the base a colour change is noticeable) slow the titration right down, and close the tap of the burette, only letting a few drops through at a time, with consistent swirling. This will ensure that you do not over shoot the end point of the titration, one should stop when they see it either just turn purple, or go blue/grey..
16. When the indicator changes colour close the tap and take a final reading (white tile behind)
17. Then calculate the titre (the amount of acid dispensed); this is your range finder titration result.

18. Wash out the conical flask thoroughly with tap water fist, and then distilled water to ensure there is no residue left from the range finding titration.
19. Ensure that there is sufficient acid in the burette for another titration.
20. Repeat from step 5-16
21. Keep on repeating the titration procedure until three or more results are in agreement to within 0.1cm³.
22. Calculate an average titre, using all results within 0.2cm³ of each other; indicating any anomalies.
23. Rinse and tidy away all apparatus from the bench.

LIST OF DEFINITIONS AND BACKGROUND INFORMATION:-

“CONCENTRATION:- a measure of the amount of a substance- usually in a given volume of solvent.”(1)

“TITRATION:- a technique used to find the concentration of one compound in a solution by determining how much will react with a  known amount of another compound in solution.” (1)

“NEUTRAL, CHEMISTRY:- a solution that  is neither acidic nor alkaline- it has a ph of 7 because it is equal numbers of H+ and OH- ions.” (1)

“MOLAR SOLUTION:- a solution containing 1 mole of substance dissolved in 1 litre of solution”(1)

“MOLE:- 1 mole of any substance contains AVOGADRO’S NUMBER (6 x 10²³) of particles of that substance” (1)

“ACID RAIN:- rainfall that has absorbed acidic gases. Its ph is normally less than 5.3.” (1)

“Acid Rain

How acid rain is formed in industrial areas and distributed over long distances, where it can kill trees and damage buildings and statues.

Acidic precipitation thought to be caused mainly by the release into the atmosphere of sulphur dioxide (SO2) and oxides of nitrogen (NOx), which dissolve in pure rainwater making it acidic. Sulphur dioxide is formed by the burning of fossil fuels, such as coal, that contain high quantities of sulphur; nitrogen oxides are produced by various industrial activities and are present in car exhaust fumes.

Acidity is measured on the  scale, where the value of 0 represents liquids and solids that are completely acidic and 14 represents those that are highly alkaline. Distilled water is neutral and has a pH of 7. Normal rain has a value of 5.6. It is slightly acidic due to the presence of carbonic acid formed by the mixture of CO2 and rainwater. Acid rain has values of 5.6 or less on the pH scale.

Acid deposition occurs not only as wet precipitation (mist, snow, or rain), but also comes out of the atmosphere as dry particles (dry deposition) or is absorbed directly by lakes, plants, and masonry as gases. Acidic gases can travel over 500 km/310 mi a day, so acid rain can be considered an example of transboundary (international) pollution.

Acid rain is linked with damage to and the death of forests and lake organisms in Scandinavia, Europe, and eastern North America. It is increasingly common in countries such as China and India that are industrializing rapidly. It also results in damage to buildings and statues.” (2)

“AVOGADRO was an Italian chemist. In 1811 he stated that equal volumes of gases contain equal numbers of particles.” (3,I)

“INDICATOR:- a chemical that shows by its colour whether a substance is acidic or alkaline” (3,II)”

(1)(GCSE Science, Essential word dictionary, Moore et al, Philip Allan, 2003.)

(2)(http://images.google.com/imgres?imgurl=http://www.tiscali.co.uk/reference/encyclopaedia/hutchinson/images/)

(3)(3rd edition Chemistry for higher tier, RoseMarie Gallagher, Paul Ingram, 2001--------(3,I) = pg 120, (3,II) = pg 254. )

PREDICTIONS:-

sodium carbonate + sulphuric acid → sodium sulphate + carbon dioxide + water

Na2CO3  (aq)     +  H2SO4 (aq) → Na2SO4 (aq)   +    CO2 (g)      + H2O (l)

The acid rain I used had an approximate concentration of roughly 0.1 molar, and because the ratio is one to one, it is reasonable to make the sodium carbonate 0.1 molar- because of this it is assumed that I will need the same volumes. Therefore to use 25cm³ of both solutions are reasonable volumes.  

RATIO   =           1        :       1

                    Na2CO3 (aq) + H2SO4 (aq)

(2x23)+(12)+(3x16)= 106g  in 1000cm³ of H2O(l) = 1 molar solution

Therefore there is 10.6g in 1000cm³ of H2O(l) = 0.1 moles, this is because 1/10 = 0.1 so 106/10 = 10.6 this shows that there is 10.6g in a 0.1 molar solution of sodium carbonate.

But for this titration I will only need up to 250cm³ of the 0.1 molar solution of sodium carbonate, so 1000/4 = 250, so then 10.6/4 = 2.65g. I will only need 2.65g of sodium carbonate.

Concentration= n x 1000 / Volume,

so Mass / RMM = n,

2.66 / 106 = 0.025 moles

0.025 x 1000 / 250 = concentration = 0.100 moles.

Showing that there is a ratio of one to one, means that I would expect the acid to be of a 0.1 Molar solution!!

Analysing

RESULTS:-

Calculating the exact concentration of the sodium carbonate solution:-

Concentration= n x 1000 / Volume,

so Mass / RMM = n,

2.66 / 106 = 0.025 moles

0.025 x 1000 / 250 = concentration = 0.10 moles.

Calculating the exact concentration of the acid:-

We use range finding results to work out roughly where about  

24.95+24.95+24.95+24.95+25.05 = 124.85/5 = 24.97 Average Titre

Sodium Carbonate + Sulphuric acid → Sodium Sulphate + Carbon Dioxide + Water

Na2CO3  (aq)     +  H2SO4 (aq) → Na2SO4 (aq)   +    CO2 (g)      + H2O (l)

RATIO   =           1        :       1

                    Na2CO3 (aq) + H2SO4 (aq)

The titration is completed when the acid neutralises/reacts completely with the sodium carbonate.

The number of moles, of carbonate reacting:

 n(moles) = M(moles dmˉ ³) x V (cm³)/1000

so n = 0.1 x 25/1000 = 2.5 x 10ˉ ³ moles of sodium carbonate used and because it has a one to one ratio with the acid, the moles o acid used will be the same:-

 Concentration of acid :-

moles of acid = 2.5 x 10ˉ ³

Therefore  M(moles dmˉ ³)  = n x 1000/ V(cm³)

So M (moles dmˉ ³)  = 2.5 x 10ˉ ³ x 1000/ 24.95(average titre)

M (moles dmˉ ³)  = 0.1001 M

Or to two d.p. = 0.10 moles dmˉ ³

This shows that the concentration of the sulphuric acid used was:- 

0.10 moles dmˉ ³. 

ANOMALIES:-

I HAD NO ANOMALOUS RESULTS! But if I were to have had any, I would have excluded them from my average titration value.

RANGE FINDER:-

The range finder is used as a trial run, for the experiment. In the process of doing the range finder, one is looking for a rough estimate for the titration. Also where about to stop on the burette before the titration has ended, and the neutralisation has occurred.

Upon doing this one can keep the flask with the neutralised product in it. Then compare it with the next titrations.

EVALUATION:-

PRECISION ERRORS:-

This is an error which could not be avoided in  the duration of the procedure.

% error = accuracy / your value x 100

Burette measures:- V ± 0.05cm³ so the % error was (0.05cm³ /24.97cm³) x 100 = 0.2% = the precision error.

Standard flask:- 250 ± 0.3 cm³ so the % error was (0.3/250) x 100 = 0.12% error

Digital balance :- Mass ± 0.01g so the % error was (0.02/2.66) x 100 = 0.75% error

Pipette measures:- 25 ± 0.06cm³ so the accuracy = (0.06/25) x 100 = 0.24% error

Overall error = 0.2 + 0.12 + 0.75 + 0.24 = 1.31% overall error.

PROCEDURAL ERRORS:-

Things that I did to help ensure as few errors as possible:-

1. (This is a large impact error)I collected a sample jar of anhydrous sodium carbonate from the warming cabinet, this ensured that the anhydrous sodium carbonate had not become hydrated, because it is hydroscopic, if this were so, much of the mass weighed out would have been water, not the sodium carbonate, this would have resulted in me gaining fewer moles from the same mass, and I would have ended up with inaccurate results.

(This is a large impact error)This method is used to ensure what exact mass of solute is used to make up the 250cm³ solution of sodium carbonate.

3. (This is a large impact error)I rinsed out a clean 250 cm³ standard (graduated) flask thoroughly with distilled water, to ensure that there was no residue left in the bottom of the flask, from previous experiments, which would have intruded with the solution that was made.
4. I stirred the solution with the glass rod to ensure all the solute had dissolved.
5. (This is a large impact error)I rinsed the beaker thoroughly through with distilled water into the standard flask, doing the same with the glass rod and finally the funnel, this ensured that all the solute had been transferred into the flask.
6. I used a white tile as a distinctive background and used a dropping pipette for the last few drops of water, which ensured that the bottom of the meniscus sat exactly on the line.
7. I tapped the flask to remove any trapped air bubbles and checked the level of the meniscus, if the level had dropped, I would have adjusted accordingly.
8. (This is a large impact error)I inserted the stopper, and invert the flask containing the solution of sodium carbonate a few times to ensure all the contents of the standard solution had mixed, doing this ensured that the entire solution had homogenised.

9. (This is a large impact error)I then rinsed the apparatus with the appropriate solutions; this ensured that there were no substances left over in the apparatus from previous experiments which would intrude with the overall results:

• Burette – with distilled water and then the sulphuric acid, ensuring no bubbles are trapped near the tap or the jet after adding the sulphuric acid, this is because the jet is included in the measurement of the burette.
• Dropping pipette – with distilled water and then the sodium carbonate.
• Conical flask- with only distilled water.
• Beakers- with distilled water and then the appropriate solutions that went in them.

10. I carried out the procedure at eye level, again with the white tile behind the meniscus. To ensure I was taking the reading horizontally to the meniscus.
11. I removed the funnel from the apparatus so that no further drops of solution were added to the solution from now on. This ensued that my readings of the meniscus were accurate.
12. I touched the pipette onto the side of the flask at about 45º in order to transfer exactly 25cm³.
13. I only added 3 drops of screened methyl orange to the conical flask and gently swirled it to mix the solution thoroughly.

• I used only 3 drops of this indicator because the indicator is slightly acidic and therefore would have effected the titration, so too much would show the sulphuric acid to be weaker than it really was. If I only added 2 drops it would have been too difficult to notice the exact colour change.

14. I placed the conical flask beneath the burette on a white tile, so as the colour change was easily noticeable when it occurred.
15. (This is a large impact error)When the titration was almost completed I slowed the titration right down, and closed the tap of the burette, only letting a few drops through at a time, with consistent swirling. This ensured that I did not over shoot the end point of the titration, if I did over-shoot  the end point then this would have implied, in my results, that the acid would have been weaker than it really was. If I under-shot the end point, the acid would have appeared stronger than it really was, in my results.  
16. I then repeated the experiment until I gained 3 or more concordant results that were within 0.2cm³ of each other, I then took an average of these results, to ensure consistency.

However there were still errors that could not be helped, but these were small impact errors.

1. The pipette filler was difficult to use and get the meniscus exactly on the line
2. The meniscuses were hard to judge, and every one judges them differently.
3. The fact that the screened methyl orange was slightly acidic could not be helped.