Last Semester: Titration of a weak acid (vinegar) with a strong base. NaOH. Equivalence (End) Point. mols NaOH = mols Acid.

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1 // VIRTUAL LAB & IDENTIFICATION OF AN UNKNOWN WEAK ACID RFS / Purpose: To conduct a titration, to use it to determine the Molar Mass and pk a of an unknown acid & from these two quantities, identify of the acid. Concepts:, pk a Molar Mass Titration Curve Stoichiometry Equivalence/End Point Strong vs Weak Acids Techniques: Weighing by Difference Titration Use of a Meter Calibration of Meter Graphing Handling the Glass Electrode Apparatus: Buret Analytical Balance Meter Glass Electrode Organization of the Lecture Weak Acids & Bases,, pk a Rate of Addition of Base Titration Volume of Base at End Point vs Volume NaOH Weak Acid Proper Base Increments Titration Set Up Special Instructions Half-Titration Point Graphing - General Henderson-Hasselbalch Equation Summary of Procedure Half-Titration Point - Revisited Special Note on Unknown Acids Titration Curves Weak Acids Sources of Error What do Titration Curves give us? Sample Data Sheet Mechanics of Titration An Example vs Regular Titrations Weak Acids, and pk a - Some Definitions HA H + + A - K a = [ H + ] [ A - ] = - log [ H + ] Acid ionization constant E.g., for CH COOH, K a =. X - & pk a =. pk a = - log K a For NH +, Weak Acid K a is small (i.e., K a << ) pk a is large (i.e., pk a > ) K a =. X - & pk a =. Monoprotic Acid an acid with only replaceable hydrogen atom, e.g., HCl, CH COOH Polyprotic Acid an acid with or more replaceable hydrogen atoms, e.g., H SO, HOOC-CH -COOH NaOH Equivalence (End) Point mols NaOH = mols Acid Indicator Phenolphthalein Last Semester: Titration of a weak acid (vinegar) with a strong base. Weak Acid vs Titration Added curve NaOH - ml of for. M a Weak Weak Acid with Monoprotic. M NaOH Acid pka =. Buffer Region End Point mols NaOH = V NaOH x M NaOH = mols Acid ml of Volume Mof NaOH. M NaOH added Added (ml)

2 vs UME OF NaOH ADDED // Meter NaOH This Exercise: Titration of a weak acid (unknown) with a strong base. Are there any points of interest in addition to the end point? Weak Acid Glass Electrode The Half Titration Point [ H + ] [ A - ] HA H + + A - K a = At the point where = [ A - ], [ H + ] = K a [ A - ] = pk a For sufficiently weak acids, 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 pk a vs Titration Added curve NaOH - ml of for. M a Weak Weak Acid with Monoprotic. M NaOH Acid pka =. Half-Equivalence Point V NaOH / End Point mols NaOH = V NaOH x M NaOH = mols Acid V NaOH ml of Volume Mof NaOH. M NaOH added Added (ml) An incidental observation. [ H + ] [ A - ] K a = Taking the logs of both sides of the equation gives us: [ A[ - [ A] A - ] - ] log KK[ a a H = + = ] log = log [ H log + [ ] H K + a ] log + log [ HA [ ] 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 weak acids, the error in the approximation is less than % for pk a s ~ TITRATION OF SA AND ASA WITH NaOH mg SA UME OF ADDED NaOH Which features of Titration Curves are primarily affected by the pk a of the Acid? Multiplying by - and rearranging we get: is less than % for pk a s > [ A - ] = pk a + log This is called the Henderson-Hasselbalch equation. * A web supplement discusses this approximation.

3 // Initial Behavior How Strength of an Acid affects its Titration Curve at Half Titration = pk a End Point In normal titrations (indicator only), we can determine (only) total number of available moles, n, of acid in a sample and from that: if we know the weight, we can compute the Molar Mass (=w/n) if we know the Molar Mass, the weight of acid (w = MM X n) E.g., if. ml OF. M NaOH is required to react with mg of a sample, the number of moles of acid in the sample is. ml X. mmol/ml =. mmol and Molar Mass = mg /. mmol = mg/mmol The Mechanics of Titration All the procedures of a normal (indicator) titration.. Rinse buret with base solution. Eliminate any bubbles in tip. Drain/fill to an arbitrary starting volume (not.). Stir/swirl solution constantly to insure complete mixing. Adjust amount of acid to require ± ml of base. Add base more slowly as you approach the end point - dropwise with stirring when very near end point. Record volumes to full precision (xx.xx) In a titration, in addition to the Molar Mass, we have the of the solution at each point up to, and past the end point. including the half-titration point at which: / = pk a pk a can distinguish acids with the same Molar Mass. In titration, also:. Calibrate meter. Rinse and insert glass electrode in solution being titrated - make sure tip of electrode is completely immersed. Leave glass electrode in solution for entire titration. Record volumes & to full precision at each cumulative volume. Monitor increments to control added volume increments Recommended Rate of addition of Base Buret The Glass Electrode is not a $ stirring rod The half-equivalence point determines the pk a. That point is in the middle of the Buffer region so changes slowly with added base. We can use large increments of base in that region The end point determines the Molar Mass, so we would like precision in the volume of base when it occurs. Meter Glass Electrode Buret Stand Glass Electrode Stirring Rod Use drop (. ml) increments starting about ml before the expected end point. How are you supposed to know the end point before titrating? From the results of a normal (phenolphthalein) titration on a weighed sample of your unknown

4 // Recommended Volume of Base at the End Point We seek to use about ( ) ml of NaOH to reach the end point WHY that volume? Optimizes buret precision & quantities of reagents and produces an easily interpretable graph BUT Molar mass of the acid is unknown. STEP : Determine how many mg of your acid will be required to use ml of NaOH. Everyone uses ~ mg of acid weighed out ACCURATELY into a ml beaker. Titrate to phenolphthalein end-point - No METER Suppose in the phenolphthalein titration:. g requires. ml of NaOH How much should I weigh to use ml? Between. g /. ml = X / ( ).. g X = (. /.) * ( ) =.. g I actually weigh out. g for the Titration What volume will that weight will actually require?. X. /. =. ml of NaOH End Point Start drop increments at.-. =. ml ADDED NaOH NaOH... Expected End point at. ml Added NaOH. +. =. ml Buret Reading Expected Half-Equivalence Point ½ X. ml +. =. ml Buret Reading NaOH... ml ml NaOH... ml ml NaOH... / ml ml NaOH.... ml / ml ml NaOH... ml

5 // BE SURE TO LEAVE ELECTRODE IN SOLUTION WHEN DOING TITRATION Improves reliability and saves time STIR THE CONTAINER into which you are titrating to insure good mixing. Otherwise, you measure of only a small, local part of the solution RINSE THE ELECTRODE WITH DISTILLED WATER and DRY IT CAREFULLY each time you immerse it into a NEW liquid Otherwise, electrode can still see the last solution with which it was in contact axis does not need to start at Graph Paper has X Want maximum precision on volume axis Determines Molar Mass You must plot the vs volume of NaOH added BY HAND not on a computer Titration Curve Volume of added NaOH Reminders about Graphing (SUPL-*) Use rational number of boxes per unit (,,,,, ) for and volume (not,,,,..) Arrange graph so that maximum total area of graph paper is utilized Draw a smooth curve through the experimental points Label End Point and Half Equivalence 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 * Available on the web at Summary of Procedure STEP. Preliminary Titration Allows calculation of weight required for ± ml of base STEP. Calculate weight of acid that would consume ml of base. 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. STEP. Check METER with =. BUFFER. If differs from buffer by more than. units, recalibrate METER STEP. Titrate using the METER Stir solution with stirring rod while titrating Get several points along the steepest rise of the curve Add NaOH slowly. ml before the volume of added NaOH calculated in Step Follow titration ml past the End Point STEP. Remove Glass Electrode from solution Rinse Electrode Replace it in its regular container Record the concentration of NaOH While primary purpose of Part (preliminary titration) is to determine appropriate weight of sample to use for the titration, The regular titration also provides an estimate of Molar Mass (MM). Can use it to verify MM obtained from titration. If MM s from regular titration and titration differ by % or more, you have either made a weighing error, or, may not have plotted vs the Delivered Volume.

6 // The pk a values in this table should be viewed as approximate. Measured values may differ from the tabulated values by. units or more. Note on Unknown Acids We will NOT assign as unknowns polyprotic acids with two pk a s What can cause Errors in this Exercise?. SAMPLE WEIGHT (~ mg) Weigh BY DIFFERENCE!!!!!!!!!!! Suppose you use a watch glass to weigh and lose mg of sample in transfer - / = -% ERROR that 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 the manual in accordance with the above! X. TITRATION (~ ml) Miss End Point by. ml ( drops). / = % ERROR IN MM. /. = % ERROR IN HALF TITRATION UME & % ERROR IN pk a 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 FROM CONTAINER From the graph, we have determined: Molar Mass =. pk a =. COMPOUND FORMULA MOLAR MASS FROM GRAPH Acetic acid Propanoic acid HC H O HC H O.... Molar. Mass X. of Acid =.. mg/mmol Crotonic acid HC H O.. Vol Half Equiv. ml dl-lactic acid HC H O... /. =. Chloroacetic acid Half Equiv... pk a of AcidAssuming it is monoprotic. pk a AN EXAMPLE

7 // You weigh. mg of your unknown for the phenolphthalein titration. It requires. ml of. M NaOH to reach the pink end point. To use ml of the same NaOH, you will need: ( ) *. /. = mg You weigh. mg of your unknown for the titration. Your expected end point is at:. mg /. mg/ml =. ml of.m NaOH [ You begin using drop increments of NaOH around. ml of NET ADDED NaOH (not buret reading) ]. Sig Figs are determined by the precision with which Titration curve -. you mg read of Weak the Acid graph with. M NaOH You read the volume axis at least to the nearest. ml Half Eq Pt. ml Volume of. M NaOH added (ml) End Pt. ml. mg of your unknown requires. ml of. M NaOH to reach the end point:. ml X. mmol/ml =. mmol of NaOH were used, and MM =. mg /. mmol = mg/mmol How does that compare with the phenolphthalein titration?. mg/(. ml x. mmol/ml) = mg/mmol A percent deviation of X / = % pk a : from (at half equiv pt) pk a =. The tiration curve has produced: MOLAR MASS = g/mol ±? & pk a =. ±? To what acid do these data correspond? What are the expected errors in these two numbers? In the mass TABLE range I: WEAK -, ACIDS Table I shows: ~. Oxalic acid dihydrate..,. Potassium hydrogen oxalate.. Glutaric acid..,. dl Malic Acid..,. Potassium dihydrogen phosphate..,. Potassium bisulfate.. Sodium bisulfate hydrate.. Sodium dihydrogen phosphate..,. Adipic acid..,. ~.. Our numbers are ±?. ±? The precision of the Molar Mass determination is certainly not sufficient to distinguish the two most likely alternatives. What should you do? Report: My unknown is either: Potassium Bisulfate or Sodium Bisulfate Hydrate As long as your unknown is one of the alternatives you name, your answer will be viewed as correct. (However, you still may be penalized for the precision in your determination e.g. how close were the Molar Masses from the preliminary and titrations? )