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Back Titration to find the Concentration of Vinegar (Ethanoic Acid)

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Introduction

Back Titration to find the Concentration of Vinegar (Ethanoic Acid) Aim The aim of this investigation is to find the concentration of vinegar (ethanoic acid, CH3COOH(aq)) in the sample given. This is to be carried out with the following being provided: 0.1 mol dm-3 sulphuric acid (H2SO4(aq)), sodium hydroxide (NaOH(aq)) of unknown concentration, as well as microchemistry laboratory titration equipment. This will be a back-titration as we cannot find the concentration of ethanoic acid directly. The sulphuric acid will be titrated with sodium hydroxide, which will help to calculate the concentration of the base. This base will then be titrated with Ethanoic acid, from which the concentration can be calculated. Plan * Safety goggles and lab coat need to be put on. * The two acids and base will be poured into separately labelled beakers. * The titration equipment will be set up as follows: the well plate is placed on the white card, and the pipette clamp is fixed into position over the first well. * Using a 2cm3 micropipette, the base will be measured to 1cm3 and placed in a single well. * Three drops of bromothymol blue indicator solution will be added to the base. * The sulphuric acid is then carefully taken in the second 2cm3 micropipette and titrated with the base drop by drop, while being stirred. * This is done until traces of colour change occur, and it reaches to an equivalence point which is when the mixed solution is neutralised. ...read more.

Middle

This solution was continuously stirred especially when there was sign of slight colour change. Once the blue colour of the indicator within the solution significantly changed to green, the titration was ceased and the reading was taken and recorded. This was then carried out three more times in the other 3 wells with sodium hydroxide making sure readings are 0.02cm3 of each other. The second titration involved titrating the sodium hydroxide against the ethanoic acid. The 1cm3 micropipette was used to draw up the ethanoic acid and fill each of four new wells. The Bromothymol blue indicator was then added and stirred in. The sodium hydroxide was drawn up using the first 2cm3 micropipette and once attaching it to the clamp above a single well, it was carefully titrated drop by drop. This first was taken as the flush, which was an estimate for the other three to find the equivalence point. This in turn, saved time in the next three titrations where the sodium hydroxide was flushed through until it reached close to the flush point and then titrated drop by drop to be more accurate. Therefore this titration was repeated three times, to find an average in the three runs which were to be within 0.02cm3 accuracy. All readings were recorded within a table. Results Achieved Titration One: Sulphuric Acid against Sodium Hydroxide 2NaOH(aq) + H2SO4(aq) ...read more.

Conclusion

This meant that at the neutralisation stage the pH of the solution will be close to pH7, which is where the approximate pH range for colour change is for this indicator. The concentrations were not exact to the decimal place of 0.1mol dm-3, this could be due to a number of reasons. Human error plays quite a factor in these titrations carried out. The stirrers were used more then once during each titration, and working in such small amounts, some of the solution could have been transferred between wells. The micropipette should also be either attached to the clamp or held vertically to the well plate, else the meniscus will not be read correctly or at eye level to be accurate. The titrations should be watched very carefully too, as the colour change could be instant. In the above case there were subtle changes, therefore the judgement of the colour could differ from person to person. Safety Precautions taken All staff and students must be aware of the fire exits are and be able to locate the first aid kits. Safety goggles and lab coats must be worn in the laboratory at all times as part of a safety precaution. There must be no food or drink consumed, no smoking, running or throwing of equipment in the laboratory as it is prohibited. In case of any spillages or accidents with chemicals, wash with excess water. ?? ?? ?? ?? Chemistry Assignment One 1 ...read more.

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