Determination of the Ka value of a weak acid using the half neutralisation point method David Martin City and Islington College Students Name: …………………………………………………………………..

Topic Chemistry Aims  This is a good little practical that reminds students of their titration skills and also allows a practical determination of a Ka value – good for BTEC unit 26 on the Applied Science course Level Level 3 Method This is a practical experiment (NO ANSWERS INCLUDED) PowerPoint slides ALL SLIDES ARE HANDOUTS (apart from slide 2, which will be hidden) – Go through the slides with the Students, and ask them to fill in the answers (group discussions around questions) Equipment needed: 0.1 moldm -3 sodium hydroxide solution 0.1 moldm -3 ethanoic acid Weak acid/strong base – phenolphthalein indicator Glassware to perform a titration pH meter – just one for the class Equipment  Laptop & Projector  Pens  Hand-Outs Duration >30 Minutes

Ka Determination of the Ka value of a weak acid using the half neutralisation point method Theory Ethanoic acid is a weak acid and dissociates only partially according to the equation below: CH 3 COOH (aq) CH 3 COO - (aq) + H + (aq) Write an expression for the Ka value for this acid in the space below: Ka = If the CH 3 COOH and CH 3 COO - concentration values were equal then they would cancel out. Write an new expression to show the value of Ka, in the space below. Ka = When performing a titration it is possible to reach this condition when exactly half the weak acid has been neutralised - hence the term “half neutralisation point”. If the pH is measured at this point then the [H + ] ion concentration can be found and hence the Ka value. If a minus sign is added to the pH value and then the 10 x function on the calculator is used we can convert pH into the [H + ] ion concentration – and then this is the Ka value for the acid.

Ka Method – part 1 Perform a titration as follows: Place the 0.1 mol dm -3 sodium hydroxide solution into a burette. Place 25cm 3 of 0.1 mol dm -3 ethanoic acid into a conical flask using a pipette. Select a suitable indicator – methyl orange or phenolphthalein? Carry out the titration and fill in the table below until two concordant results have been obtained. TitrationRough123 Final value Start value Titre value My accurate titration value is = Divide this value by two =

Ka Method – part 2 Pipette 25cm 3 of 0.1 mol dm -3 ethanoic acid into a conical flask. Add half your titration value of sodium hydroxide to the flask using the burette. Record the pH of your solution using a pH meter. Make sure the pH meter is calibrated. pH of solution = Calculation Calculate the value of the Ka of ethanoic acid. Add a minus sign Use the 10 x function on your calculator. Ka value = The theory value = 1.7 x10 -5 mol dm -3 How close was your value using this practical method? What errors were there in this practical? _______________________________________________________ _______________________________________________________ _______________________________________________________ _______________________________________________________

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