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Determining the concentration of acid in a given solution

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Introduction

Determining the concentration of acid in a given solution Planning 4 I have been given a sample of sulfuric (VI) acid solution with a concentration between 0.05 and 0.15 mol dm-�. I am going to find out the accurate concentration of the sulfuric acid. To find out the concentration of the acid I will react it with a known volume and concentration of a base and see how much base was needed to neutralise the acid. The acid is a strong acid which means that I know all the H+ ions have been disassociated and are in the solution. The H+ ions will react with the OH- ions in the alkali which will neutralise the solution. I am provided with solid, hydrated sodium carbonate with the formula Na2CO3�10H2O.1 This is a readily available base and I can dilute it down to achieve the concentration I want to react with the acid. The formula of the reaction that will take place is H2SO4 (aq) + Na2CO3 (aq) --> Na2SO4 (aq) + CO2 (g) + H2O (l) So 1 mole of H2SO4 reacts with 1 mole of Na2CO3. A titration will give me the most reliable and accurate results with the available equipment. To do my titration I will need: A Burette 7 I will need a burette to add the sodium carbonate to the sulfuric acid solution. The burettes provide me with very accurate results of volume of solution added. The class set of burettes measure 50cm3. I want to do a titration of around 25cm3 - 35cm3 each time. This is because the larger the amount of solution I titrate, the smaller the percentage error. I don't want to plan for each titration to exceed 35cm3 as if each titre is slightly larger then I may run off the scale of the burette. If I had to refill my burette during a titre, then the percentage error would double as I would have to read the burette at the beginning and the end of the first amount of solution and again at the beginning and end of the second solution. ...read more.

Middle

measurements can be read clearly * Pour the sodium hydroxide solution in the top of the burette * Check the burette is straight from the front and side * Read the measurement on the burette to the nearest 0.05cm3 * Record the value on a piece of paper as the initial burette reading * Use conical flask and rinse it with distilled water * Use the 25cm3 glass pipette and pipette filler and suck up sulfuric acid solution, swirl it round and pour excess into the sink or plastic beaker. Do this twice to ensure the pipette is not contaminated with anything but sulfuric acid. * Pipette 25cm3 of the sulfuric acid solution and squeeze into the conical flask * Touch the last drop from the pipette on the surface of the liquid * Rinse down the edges using a distilled water bottle to ensure that all the acid particles are down in the base of the solution * Place the burette and stand back on the bench * Place the conical flask on the stand underneath the burette, with a white tile underneath so that the colour change is more easily visible * Place 2 drops of methyl red into the acid solution in the conical flask and rinse down the sides again to ensure all of the indicator is in the solution. * Adjust the burette so that the tip is inside the main body of the conical flask * Wrap your left had around the tap of the burette and hold the tap between your fingers * Hold the conical flask with your right hand * Turn the tap on so that the sodium carbonate solution pours into the conical flask * Mix up the solution in the flask by moving the conical flask in circular motions keeping the tip of the burette well inside the conical flask * For the rough titration slow down the flow of sodium carbonate solution when about 25cm3 has passed through the taps so that ...read more.

Conclusion

It is 100% guaranteed by the manufacturer to be +/- 0.06cm3. If I misused the pipette, the error would be significantly increased. I therefore made sure I used the pipette correctly and read the line accurately by placing paper behind the pipette and touched the last drop on the surface of the liquid when it was emptied. Volumetric flask 0.2cm3 (0.2 / 250.0) x 100 =0.08% The error is in the volumetric flask as the percentage error is the error there is if the flask is used correctly. It is guaranteed by the manufacturer to be accurate to 0.2cm-3. I therefore read the meniscus with paper behind so ensure it was correct to reduce any further errors that could occur. The percentage errors calculated above have a total percentage uncertainty of: (0.20 + 0.24 + 0.08) = 0.52% To work out the range of accuracy of my results I will calculate 0.52% of the concentration, 0.103 mol dm-3: ( 0.103 / 100 ) x 0.52 = 0.000536 mol dm-3 The total precision uncertainty for the concentration of sulfuric acid calculated in my analysis would therefore be +/- 0.000536 mol dm-3. If the concentration was inaccurate by + 0.000536 mol dm-3 this would results in me calculating a concentration of (0.000536 + 0.10285) = + 0.10338 mol dm-3. If this was rounded down to 3 significant figures it would also give me a result of 0.103 mol dm-3, so it wouldn't have any effect on my overall results, but if my measurements were accurate to 4 significant figures then this error would mean my concentration would be 0.1035 mol dm-3. If the concentration was inaccurate by -0.000536 mol dm-3 it would mean my overall concentration calculated would be (0.10285- 0.000536) = 0.10234 mol dm-3 If this was accurate to 3 significant figures then my concentration could possibly be 0.102 mol dm-3. This proves that the precision uncertainties of the equipment I used are great enough to have a possible impact on my results and mean that they are not completely accurate. ...read more.

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Response to the question

Response to the question is very clearly set out, this piece of coursework is an exemplary piece for the level I would expect from A level candidates. The introduction, main text and evaluation are all carefully considered and the only ...

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Response to the question

Response to the question is very clearly set out, this piece of coursework is an exemplary piece for the level I would expect from A level candidates. The introduction, main text and evaluation are all carefully considered and the only way the candidate could have improved the experiment and content would have been to consider properties of the experiment that show thinking outside of the A level syllabus.

Level of analysis

Planning is adequate and sets out the scientific background and intentions of calculation for the experiment well. The use of each piece of apparatus is explained well, with the percentage errors that might arise from each piece of equipment considered before the experiment is set out, also suggesting basic improvements that may be done to improve the test and implementing them, although I would have liked the candidate to have considered a wider range of improvement strategies. Method of the experiment is explained logically and clearly with the results set out correctly and consistently within a suitably designed table, although I'm not sure why the candidate highlighted some figures in bold. Considers improvements and things that may have gone wrong in the experiment to a very high level in the evaluation, and the fact the candidate considered improvements both before and after the experiment shows a higher level of thinking and consideration. Overall an examplary piece.

Quality of writing

Spelling, grammar and puncutation all clearly set out and demonstrated to a very accurate and high level.


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