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
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
Equilibrium i Water K + - + - = K w = [H 3O ][OH ] = [H ][OH = 1 10 o eq ] 14 @ 25 Small K equilibrium favors reactats K w Review: H + ad OH - + - = [H ][OH ] + Kw [H ] = - [OH ] [OH - ] = K [H w + ] = 1 10 14 [H + ] 1 10 0 to 1 10-14 i water [OH - ] 1 10-14 to 1 10 0 i water 3
Review: As [H + ] rises, [OH - ] falls OH - OH - OH - H + H + H + 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 Acidic Neutral Basic ph + = -log [H ] 0 to 14 i water 4
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
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
Idicators are weak acids HI H + + I - 7
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) 2 + 2 HCl Mg 2+ + 2Cl - + 2H 2 O HCl (aq) CaCO 3 + 2 HCl Ca 2+ + 2Cl - + H 2 O + CO 2 8
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
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 [NaOH] @ 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 NaOH @ 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
Example data Stadardizatio Part 1 NaOH Trial g KHP ml NaOH M NaOH 1 3.0000 29.4 0.4996 2 3.0000 29.1 0.5048 3 3.0000 30.9 0.4754 averages 3.0000 29.8 0.4929 Use M NaOH i parts 2 (HCl stadardizatio) ad 3 (titratio) Stadardizatio Part 2 HCl Trial ml HCl ml NaOH M HCl 1 10.00 20.5 1.0105 2 10.00 20.5 1.0105 averages 10.00 20.5 1.0105 Use M HCl i part 3 (titratio) Part 3 Atacid tablet reactio Rise all glassware Record mass of 4 atacid tablets Label 4 125 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
Why is heatig ecessary? Heat removes CO 2 made i HCl/atacid reactio CaCO 3 + 2 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 NaOH @ eq. pt.: NaOH = HCl Calculate: HCl remaiig HCl HCl titrated = NaOH used = [NaOH]. V NaOH 12
Example data Titratio Part 3 HCl Trial g tablet ml HCl ml NaOH mol HCl titrated 1 1.50 25.00 22.2 0.0143 2 1.51 25.00 21.4 0.0147 3 1.47 25.00 22.0 0.0144 4 1.50 25.00 21.5 0.0147 averages 1.50 25.00 21.8 0.0145 avg mol eutralized/g tablet 0.00972 Theoretical amout eutralized by tablet g base MM mol ratio mol HCl eutralized Mg(OH) 2 0.11 58.33 2 0.0038 CaCO 3 0.55 100.08 2 0.0110 sum 0.0148 %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
Part 3 Reportig Results Acid eutralized by tablet 110 mg Mg(OH) 2 + 2 HCl MgCl 2(aq) + H 2 O + CO 2 550 mg CaCO 3 + 2 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
Strog acid-strog base titratios 13.0 At equivalece poit: ph 7.0 Equivalece poit mol H + = mol OH - H+ preset = OH- added 1.0 0 50.0 100.0 Vol NaOH added (ml) 15