8/2/2017. IDENTIFICATION OF AN UNKNOWN WEAK ACID by ph Titration TEST EXERCISE (105 points) TEST EXERCISE 105 POINTS. PH Titration YOU MUST FINISH

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1 //1? IDENTIFICATION OF AN UNKNOWN WEAK ACID by Titration TEST EXERCISE ( points) [HA] = [A - ] 1 1 TITRATION OF SA AND ASA WITH NaOH vs VOLUME OF NaOH ADDED mg SA VOLUME OF ADDED NaOH TEST EXERCISE POINTS PH Titration YOU MUST FINISH 1 Purpose: To determine the molar mass and pk a, and thereby, the identity of an unknown acid by titration Same technique as aspirin titration! Concepts:, pk a Nernst Equation Calibration and use of the meter Effective Molar Mass* Titration Curve Stoichiometry Equivalence Point Techniques: Weighing by Difference Using a Meter Graphing Apparatus: You will be given a sample of an unknown weak acid You need to determine properties of the acid through by Titration From a list of these properties for different weak acids, you must identify which one is yours Which properties? Meter Buret Analytical Balance 1

2 //1 Which Chemical Analysis Method? A solution of your unknown acid is SUSB-1 Titration of aspirin product (weak acid) SUSB-1 Identify an unknown weak acid by the same method Colorimetry?? mol acid (H+) = mol NaOH = vol OH M OH at equivalence point (from vs Vol graph) Titration phenolphthalein Titration to determine what mass of sample consumes ml of titrant EMM = mg of Acid sample / mmol Acid 1 st Identifier pk a = at half equivalence point nd Identifier Followed by a Titration Compare these two values to a list of weak acids and their We re Titrating an Acid of an Unknown Molar Mass Titration It s IMPORTANT that we know : What mass of our Acid to Titrate for accurate results Where the End Point of the Titration is As usual, we seek to use about ( ± ) ml of NaOH to reach equivalence point Reminder: WHY? Optimizes buret precision, and produces an easily interpretable graph BUT Molar mass of the Acid is unknown Normally we would do : ml X M NaOH = mmol NaOH mmol NaOH = mmol Acid Review Titration Procedure mmol Acid X Molar Mass Acid = mg Acid to Titrate

3 //1 STEP 1: Determine how many mg of your acid will be required to use ~ ml of. M NaOH. So, everyone uses ~ mg of acid weighed out ACCURATELY into a 1 ml beaker. WEIGH BY DIFFERENCE!!!! Titrate to phenolphthalein end-point An Example STEP 1.1 mg of your acid requires 1. ml of the NaOH, Your Acid requires =.1 mg / 1. ml = 1.1 mg / ml NaOH STEP. Using that number, calculate the weight of your acid that would consume ± ml of NaOH. e.g. WEIGHT = ( ± ml ) x 1.1 mg / ml = ± mg OF ACID ( - 1 mg ) STEP. Weigh out an amount of your acid in the above range on the ANALYTICAL BALANCE BY DIFFERENCE!!! No spatulas, no intermediate containers STEP. Target Mass mg - 1 mg Find volume of NaOH required to titrate actual sample. Suppose you weigh out.1 g = 1. mg This will require: 1.mg / 1.1 mg/ml = ml of NaOH The expected end point STEP. Check the METER with the =. BUFFER. If differs from. by more than. units, ask TA to recalibrate METER 1

4 //1 Electrode must be in solution during titration. Beaker STEP. Titrate using the METER Be sure to: Stir solution (glass rod) while titrating Get several points along the steepest rise of the curve Add NaOH slowly well before the. ml of added* NaOH calculated in Step Follow titration WELL PAST Equivalence Point at least ml i.e to ~ ml added NaOH in this case Record the concentration of NaOH 1 1 TITRATION OF SA AND ASA WITH NaOH vs VOLUME OF NaOH ADDED mg SA VOLUME OF ADDED NaOH Remember that the. ml is the net added volume, not the buret reading 1 You have to make a Table in your Notebook Record for the entire titration, to the endpoint, and about ml past the endpoint on Graph Paper BY HAND!! When your done... plot vs ml NaOH added 1 How does vary with added NaOH for a weak acid? 1 1 Titration curve - ml of. M Weak Acid with. M NaOH pka =. Very sharp rise at the Equivalence (end) Point!!! Volume of. M added (ml) ml of NaOH Added 1

5 //1 Titration curve - ml of. M Weak Acid with. M NaOH 1 1 pka =. What NOT TO DO! Add NaOH TOO FAST NEAR EQ PT x x x x x x x x 1 x x x x Volume of. M NaOH added (ml) x x x x x x x x x x x Missed the Equivalence Point START OVER! ` 1 NOTE Jump in... 1 drop =. ml 1 Plan for Titration EQ ml TOTAL Vol NaOH added start - ml BEFORE EQ. PT (-1.) add ml ml BEFORE (1. 1.) 1 ml ml BEFORE (1..) ½ ml ml BEFORE (.) 1 drops EQ PT ( ml) ( JUMP IN!!! ) ml AFTER (.) 1 drops ml AFTER (..) ½ ml ml AFTER (..) 1 ml ml ml AFTER (..) ml Bracketing EQ PT 1 Buret Rdg SAMPLE TABLE FOR TITRATION Cumul Vol Vol Incr Buret Rdg Cum Vol Incr...1 ml... 1/...1 ml... 1 drop drop / ml drop... 1/ ml.... Start using 1- drop increments about ml. ml before the equivalence point.

6 //1 YOU MUST NOT SHARE METERS DURING A TITRATION THE SECOND USER MUST WAIT UNTIL THE FIRST USER HAS FINISHED THE TITRATION 1 If you think it is necessary to repeat the titration, you may do this only after everyone has finished one titration. You may repeat normal titrations. You obviously must work alone, but meters are shared by two or more students Titration curves permit the determination of two characteristics of the unknown acid 1) END POINT determines the EFFECTIVE MOLAR MASS (EMM) ) at HALF EQUIVALENCE POINT determines pk a The Half Titration Point HA H + + A - K a = [ H + ] [ A - ] [ HA ] In the form: At the point where [ HA ][ [ = HA A [ - A] ] - ], [ H = + pk ] = = K a a + log K a = [ H + ] and [ A HA pk - ] a ] = Henderson-Hasselbalch equation. For a sufficiently weak acid, [ HA ] will be equal to [ A - ] when half of the acid has been neutralized. Titration Titration Curve curve of - mass ml of. of M Weak a Weak Acid with Acid. M with NaOHM NaOH 1 pka =. Half Equivalence Point solution Equivalence Point mass EMM = V NaOH *M NaOH I.e., when the volume of NaOH added is ½ that required to completely neutralize the acid 1/ V NaOH / Volume of. M NaOH added (ml) V NaOH

7 //1 HOW THE STRENGTH OF AN ACID AFFECTS ITS TITRATION CURVE Equivalence Point You will have a pretty good idea about the strength (pk a ) of your acid from the early part of the titration. E.g., if your calculated end point is ~ ml of added NaOH and the at ~1 ml is, you have a moderately strong acid. If the at ~1 ml is, you have a weak one. pk a The pk a values in this table should be viewed as approximate. Measured values may differ from the tabulated values by ±. units or more. WHAT CAN CAUSE ERRORS IN THIS EXERCISE? 1. SAMPLE WEIGHT (~ mg) Weigh BY DIFFERENCE!!!!!!!!!!! Suppose you weigh on watch glass and lose mg of sample in transfer - / = -% ERROR. TITRATION (~ ml) Miss End Point by ±. ml ( drops) ±. / = ± % ERROR IN EMM ±. / 1. = ± % ERROR IN HALF TITRATION VOLUME & ± 1% ERROR IN pk a

8 //1 While primary purpose of part 1 (titer determination) is to determine appropriate weight of sample to use for the titration, first titration also provides a good estimate of acid EMM. Can use it to verify EMM obtained from titration. If EMM s from regular titration and titration differ by % or more, you have either made a weighing error, or, not plotted vs the appropriate values for the delivered volume. YOU ARE URGED TO USE THE SAME BALANCE FOR ALL MASS MEASUREMENTS 1 The Half Equivalence Point1 How good is our approximation that at half-equivalence point = pk a? For the concentrations used in this exercise and the pk a s of the possible unknown acids, the error in the approximation = pk a at ½ of the volume of base delivered at the equivalence point is less than % For pk a s >, it is less than 1% TITRATION OF SA AND ASA WITH NaOH vs VOLUME OF NaOH ADDED mg SA VOLUME OF ADDED NaOH SAMPLE DATA SHEET Mass of vial + Acid 1. g Mass of vial - sample 1.1 g Mass of Acid Sample (g). g Mass of Acid Sample (mg). mg Vol of Equiv Pt. ml Concentration of NaOH. M mmol of NaOH mmol mmol of Acid 1.1 mmol 1.1 mmol Molar Mass of Acid. mg/mmol Vol Half Equiv 1. Half Equiv. pk a of Acid. So, from the graph, we have determined: Molar Mass =. pk a =. COMPOUND FORMULA MOLAR MASS pk a Acetic acid HC H O.. Propanoic acid HC H O.1. Crotonic acid HC H O.1. dl-lactic acid HC H O.1. Chloroacetic acid HC₂ClH₂O.. 1

9 //1 Note on Unknown Acids We will NOT assign as unknowns polyprotic acids with two pk a s which are both less than. Glutaric Acid H C H O.1.,. Potassium dihydrogen phosphate KH PO.1., 1. X REMINDERS ABOUT GRAPHING (SUPL-) Use rational number of boxes per unit (1,,,,, ) for and volume Arrange graph so that maximum total area of graph paper is utilized Draw a smooth curve through the experimental points Label equivalence (end) point and half titration point clearly Interpolate values of these points with precision (significant figures) consistent with your plot You should be able to read the ordinate and abscissa to at least the nearest.1 ml or.1 unit Sig Figs are determined by the precision with which you can, and do, read the graph