Determination of Amount of Acid…by Back Titration Experiment 4

Goal: Determine amount of acid that can be neutralized by a commercial antacid Method: React antacid tablet with excess stomach acid (HCl) Perform a back titration using standardized NaOH solution

Big Picture 1) Determine amount of acid (HCl) in flask 2) Neutralize some HClwith antacid tablet (CaCO 3,Mg(OH) 2 ) 3) Titrate remaining acid with base (NaOH) 4) Find amount of acid neutralized by tablet

Acid-Base Definitions Acids generate H + in water H + donors excess H + Bases generate OH - in water H + acceptors Excess OH -

Equilibrium in Water Small K  equilibrium favors reactants

Review: H + and OH - [H + ] 1  10 0 to 1  in water [OH - ]1  to 1  10 0 in water

Review: As [H + ] rises, [OH - ] falls H+H+ H+H+ H+H+ OH Acidic Neutral Basic 0 to 14 in water

Relationships H+H+ OH - [H + ] >[OH - ] Acidic solution Basic solution [H + ] =[OH - ] [H + ] <[OH - ] Neutral solution H2OH2O

Strong Acids (exp. 4) 100% dissociation / good H + donor equilibrium lies far to right HCl  H + + Cl - HCl H + Cl - Relative # moles Before dissociation After dissociation

Weak Acids (exp. 5) <100% dissociation / not-as-good H + donor equilibrium lies far to left HCO 3 - H + + CO 3 2- HCO 3 - H + CO 3 2- Relative # moles Before dissociation After dissociation HCO 3 - H + CO 3 2-

Acid Dissociation Constant Relative strength of acid Strong acids:K a is huge Weak acids:K a is small

Indicators are weak acids HIn H + + In -

Overview 0) Standardize NaOH with KHP 1) Find moles of acid, n H+ HCl (aq)  H + (aq) + Cl - (aq) (H + is H 3 O + ) HCl (aq) “stomach”

Overview 2)Neutralize some with antacid Mg(OH) HCl  Mg Cl - + 2H 2 O CaCO HCl  Ca Cl - + H 2 O + CO 2 Antacid Mg(OH) 2 + CaCO 3 HCl (aq)

Overview 3) Back-titrate leftover HCl with NaOH HCl + NaOH  Na + + Cl - + H 2 O NaOH titrant Leftover HCl (aq)

Experimental Stages n final n initial HCl consumed by antacid HCl consumed by titration n HCl neutralized by tablet = n HCl initial ─ n HCl titrated

Part 1 Standardization of NaOH Find molarity of NaOH g KHP (calculated in prelab) m KHP in ~50 mL H 2 O + drops BTB Titrate with NaOHV NaOH Given:~0.5 M eq. pt.:n NaOH = n KHP MM KHP =204.23g/mol Calculate:n NaOH n NaOH 3 trials Calculate average M NaOH  M  NaOH

Part 2 Standardization of HCl Find HCl molarity Exactly 10mL HCl + drops BTB V HCl Titrate with NaOHV NaOH Given:[NaOH] M eq. pt.:n HCl = n NaOH Find:n NaOH used = [NaOH]. V NaOH Calculate:n HCl titrated in 10 mLM HCl Repeat Calculate average M HCl  M  NaOH

Example data Use M HCl in part 3 (titration) Use M NaOH in parts 2 (HCl standardization) and 3 (titration)

Part 3 Antacid tablet reaction Rinse all glassware Record mass of 4 antacid tablets Label mL Erlenmeyer flasks Add exactly 25 mL HCl and 1 tablet to each Gently boil 5 minutes Cool and add 2 drops BTB indicator First trial = guide for 3 more Titrate with NaOH to equivalence point Save as standard color Repeat more 3 times

Why is heating necessary? Heat removes CO 2 made in HCl/antacid reaction CaCO HCl CaCl 2(aq) + H 2 O + CO 2 CO 2 + H 2 O H 2 CO 3(aq) HCO H + Removing CO 2 allows maximum amount of acid to be neutralized

Part 3 Acid Neutralized by Tablet  NaOH + HCl remaining  NaCl (aq) + H 2 O Given:[NaOH], [HCl]M NaOH, M HCl Find:n NaOH used V eq. pt.:n NaOH = n HCl Calculate:n HCl remainingn HCl  n HCl titrated = n NaOH used = [NaOH]. V NaOH

Example data

Part 3 n HCl neutralized by tablet n final n initial HCl consumed by antacid HCl consumed by titration n HCl neutralized by tablet = n HCl initial ─ n HCl titrated

Part 3 Reporting Results Acid neutralized 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 average,  2.)% acid neutralized relative to predicted % of moles actually neutralized vs. predicted stoichimetry 3 good trials

Report Abstract Results Sample calculations including Part 1:individual [NaOH] + average &  Part 2:individual [HCl] + average &  Part 3:moles HCl initially moles HCl titrated with NaOH moles HCl neutralized by tablet moles HCl per mass tablet % predicted Discussion/review questions

Strong acid-strong base titrations At equivalence point: mol H + = mol OH - n H+ present = n OH- added