8/17/2019 Unit 2 Mod 2 Back Titration

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UNIT 2 MODULE 2 SKILLS TESTED: NONE

TO DETERMINE THE PERCENTAGE BY MASS OF CALCIUM CARBONATE IN AN EGG SHELL

 

INTRODUCTION

 This practical is to determine the percentage by mass of calcium carbonate in an egg shell but could infact be used to determine the amount of calcium carbonate in other samples such as limestone rock or seashells.In addition to the stated aim of determining the percentage of calcium carbonate in the shell you are alsointroduced to the technique of ‘back titration’. Calcium carbonate reacts with hydrochloric acid but it isdicult to determine exactly when all the solid calcium carbonate has reacted to one drop of standardhydrochloric acid solution if it is titrated directly. In this experiment a known amount of excess acid isadded to the sample to ensure that all the calcium carbonate has reacted. The excess acid is then dilutedand made up to a known olume. !liquots of this diluted excess acid solution are then titrated with astandard solution of sodium hydroxide. PROCEDURE

"rind the shell into small pieces #may hae been done preiously$ and weigh accurately about %.& g of theshell into a conical 'ask . !dd (%.% cm) of *.%% mol dm+) hydrochloric acid solution. !dd the acid slowlyand swirl the 'ask to preent any small amount of liquid escaping from the 'ask with the carbon dioxidethat is produced #some unpleasant odours may be emitted at this stage$.. ,nce the reaction hascompletely -nished #the mixture may contain some proteinaceous material$ add about (% cm) of distilledwater and transfer all the contents of the 'ask to a (% cm) olumetric 'ask. /se more distilled water toensure all the contents hae been transferred and to make the -nal olume to exactly (% cm). 0ipette ormeasure out *%.% cm) aliquots of this solution and titrate them with standard %.*%% mol dm+) sodiumhydroxide solution using phenolphthalein as an indicator. 1epeat until consistent alues hae beenobtained.

WRITE A FULL LAB REPORT.

PRESENT YOUR RESULTS IN A SUITABLE TABLE.

TREATMENT OF RESULTS

*. Calculate the total 2 of moles of 3Cl added to dissole the egg shell.(. /sing your aerage accurate titre olume4 calculate th 2 of mol of 5a,3 present in the mixture that wasused in the titration.

). 3ence determine the 2 mol of 3Cl in the reaction mixture that remained after the reaction with the acidand egg shell was complete.6. Calculate the 2 of mol of 3Cl in excess in the original olumetric 'ask and thus the 2 of mol of 3Cl thatreacted.. 7rite a balanced chemical equation between calcium carbonate and hydrochloric acid and thusdetermine the 2 mol of calcium carbonate present in the mass of egg shell used.&. 8etermine the 9 of calcium carbonate present in the mass of eggs shell used.:. "ie another example of the use of back titration for analysis.