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Finding the concentration of an acid sample

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Introduction

Finding the concentration of an acid sample Concentration is the amount of a substance in a given volume1. There is different concentration in all solids, liquids and gases. And here lies the aim of my coursework. "to find the accurate concentration of a given sample of acid solution" The solution of acid I have been given is Sulphuric Acid (H2SO4 herein). We have been told that the concentration lies in between 0.05 and 0.15 mol dm-3. To find the concentration accurately I will titrate the solution using a known indicator with a known endpoint and Sodium Carbonate (Na2CO3 herein) using the following equation: H2SO4(aq) + Na2CO3(aq) � Na2SO4(aq) + H2O(l)+ CO2(g) Background Theory A titration is when and acid is run into a base or an alkali with an indicator added. This indicator will have a known end point. So once you have measured how much of the acid or base required to make the solution reach the endpoint. From there we can measure the concentration. So what I need to decide is which indicator I will use. There can be strong and weak acids, and the same for alkalis and bases. The Bronsted-Lowry theory2 states that acids are H+ donors and that bases are a H+ receiver. A strong acid is defined as having a strong tendency to donate a H+. H2SO4 is a well known acid with a strong base. While the sodium Carbonate is the opposite as it is relatively weak. So now that I know the strength of my reactants I am able to choose an indicator. ...read more.

Middle

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 Titration Initial Burette Reading Final Burette Reading Titre Rough 1 2 3 4 5? 6? 7? 8? 9? 10? Justification Table What I did... Justification ... begin adding the Anhydrous Sodium Carbonate with a spatula By adding directly into the beaker this reduces a major transference error as if I used a weighing boat it would be another step that would cause inaccuracies ... press "tare" This means that the weight of the beaker will not be factored in when the weight is calculated ...using electric balance Is more accurate and has the added bonus of the tare function, this means that I am able to easily weight the carbonate in the beaker as its weight will not be factored in ...Stir with glass rod To ensure that the carbonate dissolves, as if it wasn't it would require more acid to reach the endpoint as its surface are to volume ratio is decreased ...rinse rod As some of the carbonate may still be on it and not fully dissolved using distilled water as the full solution is not yet made up and the rest of the 250cm3 will be filled with the water so there will be no adverse effect of rinsing with distilled as it will be going into the solution anyway ...step 6 there may be some small deposits that have clung to the funnel, also as distilled water is going to go it we rinse with the distilled (as above) ...read more.

Conclusion

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. 1 Definition Chemical Ideas - Second Edition - Heinemann - pg 12-14 2 The Bronsted-Lowry theory - http://dl.clackamas.edu/ch105-04/bronsted.htm 3 Extract from pH curves - http://www.chemguide.co.uk/physical/acidbaseeqia/phcurves.html 4 Graph from same page as pH curves - see above link 5 Graph from - http://www.chemguide.co.uk/physical/acidbaseeqia/indicators.html#top 6 From AS level Salter Chemistry booklet - Elements of life - PR2/3 7 From CLEAPSS 1998 Hazcards ?? ?? ?? ?? B 12MW Chemistry: Acids and Bases February 2008 Barrow Hall College 1 ...read more.

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