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ACID-BASE TITRATIONS Lesson 10

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Acid-Base Titrations PART I: what is a titration? how is it performed? what tools are needed? PART II: Perform Titration calculations

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A titration is a neutralization reaction!

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Neutralization A neutralization reaction occurs when a strong acid reacts with a strong or weak base (or vice versa) 2HCl (aq) + Ba(OH) 2(aq) BaCl 2(aq) + 2H 2 O (l) Double replacement reaction that produces a salt & water.

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Everyday neutralizations The odour of fish and other seafood is caused by basic compounds called amines, we add vinegar (acetic acid) or lemon juice to neutralize the base and produce a more pleasant smell :)

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Titration Titrations are: Acid-base neutralization reactions Goal is to find an unknown concentration Reaction stops at the equivalence point What is the equivalence point? Point in a reaction where Moles of acid (H + ) = moles of base (OH - ) according to mol ratio in equation(stoichiometry)

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Equivalence Point = Stoichiometric Point Point in titration where the ratio of the moles between acid/base exactly equals the ratio of the coefficients of the acid/base in the balanced reaction equation Ratio of COEFFICIENTS = Ratio of MOLES PRESENT @ this point the indicator changes colour

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Titration How do you perform a titration? Using a known concentration and volume of a solution Add it slowly to the solution with the unknown concentration You will stop when the equivalence point is reached How do you know when to stop? The colour will change! ( you use indicators)

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How it looks like

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Criteria for Titrations Only one reaction can occur between the solutes contained in the two solutions. The reaction between those solutes must go rapidly to completion There must be a way of signalling/identifying the equivalence point ( use indicators)

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Keep in mind The equivalence point: Total number of moles of H + from the acid is equal to the total number of moles of OH - from the base Titrations require very precise instruments and measurements. Usually require 3 trials. You average your best trials to get your results you may discard a bad trial

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Standard solutions/Titrants A successful titration requires that the concentration of the standard solution/titrant be very accurately known. There are two kinds: 1. Primary Standards 2. Secondary Standards

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1. Primary Standard A standard solution that can be easily prepared because it is Stable non-deliquescent soluble available in a highly pure form.

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2. Secondary Standard If the solute Is not available in a highly pure form can readily undergo a reaction with H 2 O or CO 2 Then the solution has to be standardized/calibrated to accurately determine its concentration. This is done by titrating the solution against a primary standard!

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A standard solution must have a known molarity A primary standard is made by weighing a pure solid and diluting in a volumetric flask. A secondary standard requires a titration to calibrate its concentration. Suitable for primary standard Ex Reason solid bronsted basesNa 2 CO 3 pure solid solid weak acids H 2 C 2 O 4 pure solid Not suitable for Primary Standard Ex Reason Arrhenius bases NaOH Hygroscopic Strong acids HCl volatile liquids or gases

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Primary StandardSecondary Standard H 2 C 2 O 4(s) HCl (g) C 6 H 5 COOH (s) Na 2 CO 3(s) H 2 SO 4(l) KC 6 H 5 COO (s) NH 3(g) NaOH (s)

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Titration calculations

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Titrations 2HCl (aq) + Ba(OH) 2(aq) BaCl 2(aq) + 2H 2 O (l) M= # moles 1 L Rearrange to get moles #moles = Molarity x Volume #moles = M x L

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Ex #1 : Calculating Concentration A 25.0 mL sample of H 2 SO 4 requires 46.32 mL of a standard 0.203 M NaOH solution to reach the equivalence point. Calculate the original [H 2 SO 4 ]

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Ex# 2 : Calculating Volume What volume of a standard Sr(OH) 2 solution with a pH of 13.5 is needed to neutralize 25.0 mL solution of 0.432 M HCl ?

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TRY: What volume of a standard Ca(OH) 2 solution with a pH of 13.5 is needed to neutralize 15.0 mL solution of 0.4 M HCl ?

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Ex# 3 : Calculating Molar Mass A 0.328 g sample of an unknown monoprotic acid, HA, is diluted in water and titrated with a standardized 0.1261 M NaOH solution. If 28.10 mL of the basic solution is required to reach the equivalence point, calculate the molar mass of the acid.

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Practice Problems Page 158 94, 97, 101, 105* (has a table of 3 trials, do not forget to discard a bad trial first)