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Find the exact concentration of an acid solution

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Introduction

PLANNING The task set is to find the exact concentration of an acid solution. It is believed to have a concentration of between 0.05 and 0.15 mol dm��. Introduction: Acids dissolve in water with dissociation and the formation of hydrogen ions. They are classed as strong or weak depending on the extent to which this dissociation occurs. Bases are substances that liberate hydroxide ions in water. They too are classed as weak or strong. The strength of an acid will depend upon its tendency to donate protons. Similarly, the strength of a base will depend upon its tendency to accept protons: the stronger this tendency, the stronger the acid or base. An acid with a strong tendency to lose a proton (a "strong acid") will have a conjugate base with a weak tendency to accept a proton (a "weak base"). An acid with a weak tendency to lose a proton (a "weak acid") will have a conjugate base with a strong tendency to accept a proton (a "strong base"). Titration is 'a technique used for finding the volume of one solution chemically equivalent to a given volume of another, usually by adding the first solution slowly until equivalence is reached. This can be detected by the addition of a small amount of an indicator material.' - The Cambridge Encyclopaedia It is this technique that will assist in finding the exact concentration of the acid in this experiment. ...read more.

Middle

The information necessary for these calculations are: * The balanced equation Na2CO3(aq) + H2SO4(aq) --> Na2SO4(aq) + H2O(l) + CO2(g) * The concentration C1 and the reacting volume V1 of Na2CO3(aq). * The concentration C2 and the reacting volume V2 of H2SO4(aq). From the titration results, the amount of Na2CO3 (in mol) can be calculated: Amount of Na2CO3 = C1 x = 0.100 x = 0.00247 mol Calculating the Concentration of H2SO4: From the equation, the amount of H2SO4 (in mol) can be determined: Na2CO3(aq) + H2SO4(aq) --> Na2SO4(aq) + H2O(l) + CO2(g) 1 mol 1 mol (balancing numbers) Therefore, 0.00247 mol Na2CO3 reacts with 0.00247 mol H2SO4 Amount of H2SO4 that reacted = 0.00247 mol The concentration (in mol dm��) of H2SO4 can be calculated by scaling to 1000cm�: 25cm� H2SO4(aq) contains 0.00247 mol H2SO4 1cm� H2SO4(aq) contains mol H2SO4 1dm� (1000cm�) H2SO4(aq) contains x 1000 = 0.099 mol H2SO4 (to 3 d.p.) I have thus calculated that the concentration of H2SO4 is 0.099 mol dm��. However, my scientific knowledge tells me that the actual concentration is more likely to be 0.1 and certain procedural errors have slightly altered the figures used in my calculations. EVALUATING EVIDENCE AND PROCEDURES I believe that my results were quite reliable although my first reading was not entirely concordant with the others. Although the difference was only 0.3-0.4 cm� I have not included this result in my calculations to find the average as the repeat titrations produced much more concurrent results and the average would be more reliable with the anomaly excluded. ...read more.

Conclusion

* Procedural Errors - I think that I was unclear as to the point of complete neutralisation. I obviously stopped the titration too early and I think this is why my results are a little under what was expected. Greatest Sources of Error: Making the solution of Na2CO3 homogeneous: even when the solution was shaken in the volumetric flask, the concentration of ions may have been different in the neck of the flask than in the bottom. This would have affected the results as the solution may have not had an accurate concentration. The end point: It was fairly difficult to tell the end point as after the solution turned yellow, it gradually became pink again. This was probably because carbon dioxide in the air made the solution more acidic. This source of error could be taken away by performing the experiment under carbon dioxide free conditions (if the flask were sealed, the results would be more accurate). In conclusion, I believe that effect of the faults found in the equipment and method is almost negligible when it comes to analysing the accuracy of the results. The greatest error was in my own inaccuracy in judging the endpoint when equivalence was reached. Were I more clear on this, I am confident that my results would have showed almost, if not exactly, 25cm� of the sodium carbonate solution used to titrate 25cm� of the sulphuric acid solution. An Investigation to Find the Accurate Concentration of an Acid Elizabeth Dodwell ...read more.

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