Determination of Amount of Acid by Back Titration. Experiment 4. Experiment 4

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Cameron Dwain Miles
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1 Determiatio of Amout of Acid by Back Titratio Experimet 4 Goal: Experimet 4 Determie amout of acid that ca be eutralized by a commercial atacid Method: React atacid tablet with excess stomach acid (HCl) Perform a back titratio usig stadardized NaOH solutio 1

2 Big Picture 1) Determie amout of acid (HCl) i flask HCl, iitial = [HCl] V HCl used 2) Neutralize some HClwith atacid tablet (CaCO 3,Mg(OH) 2 ) eutralize d, tablet =? 3) Titrate remaiig acid with base (NaOH) HCl titrated = M NaOH V NaOH 4) Fid amout of acid eutralized by tablet eutralize d, tablet HCl, iitial = HCl, titrated Acid-Base Defiitios Acids geerate H + i water H + doors excess H + Bases geerate OH - i water H + acceptors Excess OH - 2

3 Equilibrium i Water K = K w = [H 3O ][OH ] = [H ][OH = 1 10 o eq ] 25 Small K equilibrium favors reactats K w Review: H + ad OH = [H ][OH ] + Kw [H ] = - [OH ] [OH - ] = K [H w + ] = [H + ] to i water [OH - ] to i water 3

4 Review: As [H + ] rises, [OH - ] falls OH - OH - OH - H + H + H Acidic Neutral Basic ph + = -log [H ] 0 to 14 i water 4

5 Relatioships H + H OH - 2 O [H + ] >[OH - ] [H + ] =[OH - ] [H + ] <[OH - ] Acidic solutio Neutral solutio Basic solutio Strog Acids (exp. 4) 100% dissociatio / good H + door equilibrium lies far to right HCl H + + Cl - Before dissociatio After dissociatio Relative # moles HCl H + Cl - HCl H + Cl - 5

6 Weak Acids (exp. 5) <100% dissociatio / ot-as-good H + door equilibrium lies far to left HCO 3 - H + + CO 3 2- Before dissociatio After dissociatio Relative # moles HCO 3 - H + CO 3 2- HCO 3 - H + CO 3 2- Acid Dissociatio Costat Relative stregth of acid HA A - + H + K a = + [H ][A [HA] - ] Strog acids: Weak acids: K a is huge K a is small 6

7 Idicators are weak acids HI H + + I - 7

8 Overview HCl (aq) stomach 0) Stadardize NaOH with KHP 1) Fid moles of acid, H+ HCl (aq) H + (aq) + Cl - (aq) (H + is H 3 O + ) Overview Atacid Mg(OH) 2 + CaCO 3 2) Neutralize some with atacid Mg(OH) HCl Mg Cl - + 2H 2 O HCl (aq) CaCO HCl Ca Cl - + H 2 O + CO 2 8

9 Overview NaOH titrat 3) Back-titrate leftover HCl with NaOH HCl + NaOH Na + + Cl - + H 2 O Leftover HCl (aq) Experimetal Stages HCl eutralized by tablet = HCl iitial HCl titrated = [HCl] iitial V HCl used iitial HCl cosumed by atacid fial fial = 0 HCl cosumed by titratio 9

10 Part 1 Stadardizatio of NaOH Fid molarity of NaOH g KHP (calculated i prelab) m KHP i ~50 ml H 2 O + drops BTB Titrate with NaOH V NaOH Give: ~0.5 M eq. pt.: NaOH = KHP MM KHP =204.23g/mol Calculate: NaOH NaOH 3 trials Calculate average M NaOH M NaOH Part 2 Stadardizatio of HCl Fid HCl molarity Exactly 10mL HCl + drops BTB V HCl Titrate with NaOH V NaOH Give: [NaOH] M eq. pt.: HCl = NaOH Fid: NaOH used = [NaOH]. V NaOH Calculate: HCl titrated i 10 ml M HCl Repeat Calculate average M HCl M NaOH 10

11 Example data Stadardizatio Part 1 NaOH Trial g KHP ml NaOH M NaOH averages Use M NaOH i parts 2 (HCl stadardizatio) ad 3 (titratio) Stadardizatio Part 2 HCl Trial ml HCl ml NaOH M HCl averages Use M HCl i part 3 (titratio) Part 3 Atacid tablet reactio Rise all glassware Record mass of 4 atacid tablets Label ml Erlemeyer flasks Add exactly 25 ml HCl ad 1 tablet to each Getly boil 5 miutes Cool ad add 2 drops BTB idicator First trial = guide for 3 more Titrate with NaOH to equivalece poit Save as stadard color Repeat more 3 times 11

12 Why is heatig ecessary? Heat removes CO 2 made i HCl/atacid reactio CaCO HCl CaCl 2(aq) + H 2 O + CO 2 CO 2 + H 2 O H 2 CO 3(aq) HCO 3- + H + Removig CO 2 allows maximum amout of acid to be eutralized Part 3 Acid Neutralized by Tablet NaOH + HCl remaiig NaCl (aq) + H 2 O Give: [NaOH], [HCl] M NaOH, M HCl Fid: NaOH used V eq. pt.: NaOH = HCl Calculate: HCl remaiig HCl HCl titrated = NaOH used = [NaOH]. V NaOH 12

13 Example data Titratio Part 3 HCl Trial g tablet ml HCl ml NaOH mol HCl titrated averages avg mol eutralized/g tablet Theoretical amout eutralized by tablet g base MM mol ratio mol HCl eutralized Mg(OH) CaCO sum %differece 1.58 Part 3 HCl eutralized by tablet HCl eutralized by tablet = HCl iitial HCl titrated = [HCl] iitial V HCl used iitial HCl cosumed by atacid HCl titrated = M NaOH V NaOH HCl cosumed by titratio fial 13

14 Part 3 Reportig Results Acid eutralized by tablet 110 mg Mg(OH) HCl MgCl 2(aq) + H 2 O + CO mg CaCO HCl CaCl 2(aq) + H 2 O + CO 2 1.) Per mass HCl experimetal g tablet 3 good trials average, σ 2.) % acid eutralized relative to predicted average % of moles HCl experimet al 100% calculated actually eutralized HCl theoretic al vs. predicted stoichimetry Abstract Results Report Sample calculatios icludig Part 1: idividual [NaOH] + average & σ Part 2: idividual [HCl] + average & σ Part 3: moles HCl iitially moles HCl titrated with NaOH moles HCl eutralized by tablet moles HCl per mass tablet % predicted Discussio/review questios 14

15 Strog acid-strog base titratios 13.0 At equivalece poit: ph 7.0 Equivalece poit mol H + = mol OH - H+ preset = OH- added Vol NaOH added (ml) 15

Steward Fall 08. Moles of atoms/ions in a substance. Number of atoms/ions in a substance. MgCl 2(aq) + 2 AgNO 3(aq) 2 AgCl (s) + Mg(NO 3 ) 2(aq)

Steward Fall 08. Moles of atoms/ions in a substance. Number of atoms/ions in a substance. MgCl 2(aq) + 2 AgNO 3(aq) 2 AgCl (s) + Mg(NO 3 ) 2(aq) Dealing with chemical stoichiometry Steward Fall 08 of Not including volumetric stoichiometry of Chapter 6.0x10 A 6.0x10 Mol/mol ratio from balanced equation B 6.0x10 6.0x10 s, Equations, and Moles: II