7/30/2015. by ph Titration TEST EXERCISE (105 points) Quiz 2 Tuesday August 4. SUSB013 Colorimetric Determination Aspirin

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1 // Quiz Tuesday August SUSB Colorimetric Determination Aspirin SUSB Colorimetric Iron in Multivitamins SUSB Complexometric Titration Calcium in Antacid? TITRATION OF SA AND ASA WITH NaOH vs VOLUME OF NaOH ADDED mg SA VOLUME OF ADDED NaOH IDENTIFICATION OF AN UNKNOWN WEAK ACID by Titration TEST EXERCISE ( points) [HA] = [A - ] Purpose: To determine the molar mass and pk a, and thereby, the identity of an unknown acid by titration Same technique as last exercise! Concepts:, pk a Nernst Equation Calibration and use of the meter Effective Molar Mass* Titration Curve Stoichiometry Equivalence Point You will be given a sample of an unknown weak acid You need to determine two properties of the acid through chemical analysis From a list of these properties for different weak acids you identify which is yours Techniques: Weighing by Difference Using a Meter Graphing Apparatus: Meter Buret Analytical Balance

2 // Which Chemical Analysis Method? SUSB- Titration of aspirin product (weak acid) SUSB- Identify an unknown weak acid by the same method A solution of your unknown acid is colorless Colorimetry?? mol acid (H+) = mol NaOH = vol OH M OH at equivalence point (from V graph) EMM = mass of sample / mol NaOH st Identifier pk a = at half equivalence point nd Identifier Compare these two values to those of known weak acids Review Titration Techniques 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 ( mol acid = V b M b ) STEP : 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 ml beaker. Titrate to phenolphthalein end-point - No METER

3 // e.g. STEP. mg of your acid requires. ml of the NaOH, Your Acid requires =. mg /. ml =. mg / ml NaOH STEP. Using that number, calculate the weight of your acid that would consume ± ml of NaOH. e.g. WEIGHT = ( ± ml ) x. mg / ml = ± mg OF ACID STEP. Weigh out an amount of your acid in the above range on the ANALYTICAL BALANCE BY DIFFERENCE!!! No spatulas, no intermediate containers STEP. Calculate volume of NaOH required to titrate actual sample. Suppose you weigh out. g =. mg That will require:.mg /. 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 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 TITRATION OF SA AND ASA WITH NaOH vs VOLUME OF NaOH ADDED mg SA VOLUME OF ADDED NaOH You have to make a Table in your Notebook Do this for the entire titration, to the endpoint, and about ml past the endpoint on Graph Paper Remember that the. ml is the net added volume, not the buret reading When your done... plot vs ml NaOH added

4 // How does vary with added NaOH for a weak acid? 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 Titration curve - ml of. M Weak Acid with. M NaOH pka =. What NOT TO DO! Add NaOH TOO FAST NEAR EQ PT x x x x x x x x 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! ` TOTAL Vol NaOH added Plan for Titration EQ ml add start - ml BEFORE EQ. PT () ml ml BEFORE ( - ) ml NOTE Jump in drop =. ml ml BEFORE ( ) ½ ml ml BEFORE () drops EQ PT ( JUMP IN!!! ) ml AFTER () drops ml AFTER ( ) ½ ml Bracketing EQ PT ml AFTER ( ) ml ml ml AFTER ( ) ml

5 // Buret Rdg SAMPLE TABLE FOR TITRATION Cumul Vol Vol Incr Buret Rdg Cum Vol ml... Incr... ml... drop......drop / ml / ml.... Start using - drop increments about ml before the equivalence point. YOU MUST NOT SHARE METERS DURING A TITRATION THE SECOND USER MUST WAIT UNTIL THE FIRST USER HAS FINISHED THE TITRATION Electrode must be in solution during titration.

6 // 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 ) 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 ] At the point where [ HA ][ = HA [ A] - ], [ H + ] = = K a K a = [ H + ] and [ A - pk] a = For a sufficiently weak acid, [ HA ] will be equal to [ A - ] when half of the acid has been neutralized. I.e., when the volume of NaOH added is ½ that required to completely neutralize the acid Titration Titration Curve curve of - weight ml of. of M Weak a Weak Acid with Acid. M with NaOH M NaOH solution pka =. What about the at the equivalence point? Half Equivalence Point Equivalence Point weight EMM = V NaOH *M NaOH AGAIN, HOW THE STRENGTH OF AN ACID AFFECTS ITS TITRATION CURVE pk a Equivalence Point / V NaOH / Volume of. M NaOH added (ml) V NaOH

7 // 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 ~ ml is, you have a moderately strong acid. If the at ~ ml is, you have a weak one. 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?. 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 ±. /. = ± % ERROR IN HALF TITRATION VOLUME & ± % ERROR IN pk a While primary purpose of part (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

8 // The Half Equivalence Point 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 % TITRATION OF SA AND ASA WITH NaOH vs VOLUME OF NaOH ADDED mg SA VOLUME OF ADDED NaOH A web page deals extensively with this approximation SAMPLE DATA SHEET Mass of vial + Acid. g Mass of vial - sample. 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. mmol. mmol Molar. Mass X. of Acid =.. mg/mmol Vol Half Equiv. ml. /. Half Equiv. pk a of Acid Assuming it is monoprotic. FROM CONTAINER FROM GRAPH So, from the graph, we have determined: Note on Unknown Acids COMPOUND Molar Mass =. pk a =. FORMULA MOLAR MASS pk a Acetic acid HC H O.. Propanoic acid HC H O.. Crotonic acid HC H O.. dl-lactic acid HC H O.. Chloroacetic acid HC₂ClH₂O.. We will NOT assign as unknowns polyprotic acids with two pk a s which are both less than. Glutaric Acid H C H O..,. Potassium dihydrogen phosphate KH PO..,. Astute students will edit the list of possible unknowns in accordance with the above! X

9 // REMINDERS ABOUT GRAPHING (SUPL-) Use rational number of boxes per unit (,,,,, ) 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. ml or. unit Sig Figs are determined by the precision with which you can, and do, read the graph