Acid-Base Titrations

Titrating a Strong Acid with a Strong Base

Textbook Reference pp. 595 - 599

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Titration is a common laboratory technique used to determine the concentration of a solution

Titrant is the solution in the buret

Standard Solution a solution of KNOWN concentration, usually in the buret

Sample is the solution being analyzed in a titration (the “unknown” concentration)

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Endpoint is the point at which the indicator changes colour

Before endpoint At endpoint After endpoint (faint pink) (overshot!)

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Equivalence Point the measured quantity of titrant recorded at the point at which chemically equivalent amounts have reacted (a.k.a. the stoichiometric point)

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Titration Technique Using a Buret VIDEO#1

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• Titration Technique Using a Buret VIDEO#2

For most neutralization reactions, there is no visible signs that a reaction is occurring

An acid-base indicator is a substance that changes colour in acidic and basic solutions

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Acid-Base Indicators

Majority of acid-base indicators are weak, monoprotic acids, the undissociated weak acid is one colour, and its conjugate base is a different colour

H (indicator) (aq) H+ + (indicator)- (aq)

Colour 1 Colour 2

For example, phenolphthalein is an indicator often used for reactions between strong acids and bases. It is colourless between pH 0-8 and turns pink between pH 8-10

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Endpoint is the point at which the indicator changes colour

Before endpoint At endpoint After endpoint (faint pink) (overshot!)

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Standard Solution is one of the reactants, you must know the precise and accurate concentration of the standard solution.

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Titration ExampleAn acid rain sample containing sulfurous acid

was analyzed in a laboratory using a titration with a standard solution of sodium hydroxide. Use the following evidence given in Table 1 to determine the concentration of the sulfurous acid.

Trial 1 2 3

Final buret reading (mL) 15.3 30.5 45.5

Initial buret reading (mL) 0.2 15.3 30.5

Volume of NaOH(aq) added 15.1 15.2 15.0

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Table 1: Titration of 20.0 mL of H2SO3(aq) with 0.150 mol/L NaOH(aq)

Balanced Chemical EquationH2SO3(aq) + 2NaOH(aq) Na2SO3(aq) + 2H2O(l)

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SOLUTION

What was the average NaOH(aq) used to titrate 20.0 mL sulfurous acid?

vNaOH = 15.1 + 15.2 + 15.0

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= 15.1 mL

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Where are you starting? Where are you going?H2SO3(aq) + 2NaOH(aq) Na2SO3(aq) + 2H2O(l)

Stoichiometry Time!!!

Answer: 0.0566 mol/L H2SO3

Titration Practice (ws)