Finding the concentration of an acid sample

As you can see the only one in range is the Methyl Orange

Sodium Carbonate

        Because I will need a 250cm3 sodium carbonate solution. I will have to make it my self. However I must first work out how much of the Na2CO3 I will need to dissolve into distilled water. As we have been told that the concentration of the Sulphuric acid is between 0.05 and 0.15 mol dm-3. That means it would make sense to use a molarity of solution that was in the middle of that. So I have decided to make my Na2CO3 solution 0.1 mol dm-3.

So now that I have the number of moles needed I can work out how many grams I need to dissolve.

By using the equation: Moles = Mass/Mr

If I put in the values I already know: 0.1 = ?g/106

Then I can rearrange it to get the mass in 1 dm3: 0.1 x 106 = 10.6g

However as 1 dm3 = 1000cm3 then to get the mass for 250cm3 all I have to do is divide by 4.

10.6/4 = Mass needed to make the solution for the titration = 2.65g

Now before I can begin the titration experiment I must risk assess the experiment so in case something was to go wrong I would know what to do and ensure that no harm comes to anyone.

Risk Assessment

Now that I have risk assessed the experiment I can begin it.

Apparatus

• Methyl orange
• Balance
• Anhydrous Sodium Carbonate
• 250cm3 volumetric flask
• 100cm3 beaker
• Burette
• Glass pipette
• Bung
• Conical Flask
• Glass rod
• Distilled water
• Sulphuric Acid solution
• Burette stand
• Pipette filler

Diagram

Method

1. Place 100cm3 beaker on electric balance and press “tare” then begin adding the Anhydrous Sodium Carbonate with a spatula
2. Weigh 2.65g of Anhydrous Sodium Carbonate using electric balance
3. Pour 25cm3 of distilled water into beaker. Stir with glass rod until it has dissolved. Adding more water if required.
4. Use Distilled Water to rinse rod
5. Transfer the mixed solution into a 250cm3 volumetric flask via a funnel
6. Rinse funnel with distilled water
7. Now add distilled water until the bottom of the meniscus is level with the line when looked at, at eye level
8. Set up the burette filling it with the acid solution and placing it in the stand.
9. Allow a few drops of acid to run through
10. Shake the solution in volumetric flask
11.  Using the pipette and pipette filler take 25cm3 of the solution and place in the conical flask
12. Place the conical flask under the burette on top of a white tile
13. Add 3 drops of Methyl Orange
14. Open the burette and constantly swirls the solution with the methyl orange indicator in
15. Note the end point
16. The first titrated solution should be put in a beaker
17. Repeat until you have 3 concordant titres (within 0.1)
18. However when doing the repeat results (after first titration) replace step 13 with Step 18 and 19
19. Allow enough acid to run thought so that the volume is at 3 cm3 more then your rough titre (first titre)
20. Now open the burette so that you are allowing only a drop at a time swirling constantly

When I have finished I will put my results into the table below

Results Table

Justification Table

Results

Finding the concentration

The first step is, by using he balance equation write down what we know and what we need to know.

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

Volume                            25.25cm3          25cm3

Concentration                      ?            0.1mol c

Moles                                   ?                   ?  

I know from prior work that moles = Concentration in mol dm-3 x Volume in dm-3

So I can work out the number of moles of Sodium Carbonate used in the titration. Using the formula n = c x v. The concentration of the Carbonate is 0.1mol dm-3 (c = 0.1). How ever the volume is in cm3 not dm-3. To convert the figure you must divide by 1000 as 1dm3 = 1000cm3. So v = 25/1000 = 0.025mol dm-3.

So using n = c x v, the number of moles equals 0.1 x 0.025 = 0.0025

We can now find the number of moles of Sulphuric Acid by using the ratio of sulphuric acid to the sodium carbonate form the equation:

H2SO4(aq)  : Na2CO3(aq)

1 : 1

0.0025 moles : 0.0025 moles

So to finally find the concentration of H2SO4 we rearrange the equation n = c x v to c = n / v. We know that n = 0.0025 and v = 25.25cm3 = 25.25/1000 = 0.02525dm-3:

Concentration = 0.0025 / 0.02525 = 0.0990099

= 0.1mol dm-3 (1 significant figure)

So we can answer he aim by sing that the concentration of the known acid is 0.1mol dm-3 (1 significant figure)

Evaluation

As I have now got my results to truly evaluate them and see weather they are truly reliable I have calculated the percentage errors of the equipment I used.

Percentage error of Volumetric Flask = (error x 100) / reading

% error = (0.02 x 100) / 25 = 0.8% (2 sf)

Percentage error of Burette = (error x 100) / reading

% error = (0.2 x 100) / 25.25 = 0.79% (2 sf)

Percentage error of pipette = (error x 100) / reading

% error = (0.06 x 100) / 25 = 0.24% (2 sf)

Percentage error of balance = (error x 100) / reading

% error = (0.00005 x 100) / 2.6489 = 0.0019% (2 sf)

So by adding the % errors together I get the % error for the entire experiment

% error for experiment = 0.8 + 0.79 + 0.24 + 0.0019 = 1.8319%

So the concentration of the acid could be any where between 0.1008mol dm-3 (4 d.p) and 0.0972mol dm-3 (4 d.p).

However these procedural errors are incredibly small and negligible anyway so i thought how else could by results be inaccurate.

There are some factors that were not taken into consideration were the fact the when the drops of methyl orange were added to the carbonate causing the concentration to decrease. So that less of the Acid would be needed to reach the end point.

Transference error is something that I can’t measure and is out of my control, although I tried to minimise this by rinsing things out there is no way that I can know weather or not small trace amounts are left in the apparatus. Although I could have used Class Apparatus but budgetary constraints would not allow   this.

Another likely source of errors is the judgement of the meniscus, as other people may disagree as to where it is in line. And as I do not have 20/20 vision I may have been impaired. As my glasses are not fully up to date this may be the cause of inaccuracies and should I do it again a laser node able to measure the meniscus more accurately should be used.

Also as I know that the concentration was supposed to be 0.1 moles, it is assumed that the scientific preparation technicians must have made the solution exactly. When I have sent the equipment they have used they have lower % error equipment.

The end point would also be another judgment that could have been a cuse of inaccuracies, however small. One way would again be to use a pH probe. So that I would know the exact moment of the endpoint.

One of the larger % errors was from the pipette, I would be for greater acuuracy to use a large automatic pipette. Here doing repeated results would be better as I only used the first three concordant results. Doing more and ensuring that there are 10 concordant results before working out the average titre.

One of the thing that I did not do was, to ensure that the carbonate solution I made was homogenised before I used it each time this could have lead to the sodium and the carbon oxide to split into two layer like oil and water. Which when poured out would not give accurate results. So to guard the results from this next time I will invert it before each withdrawal.

However all of this simply makes the result more accurate and as I already have the concentration with a small % error. And as my answer I extreamly close and is too 4 decimal places there is little need go any futher in depth.

 

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