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Acid-Base

Titrations

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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

• Endpoint is the point at which the indicator changes colour

• Titration Technique Using a Buret VIDEO#1

3• Titration Technique Using a Buret VIDEO

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Acid-Base Indicators

• 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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• 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

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.

Titration Example

An 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.

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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

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

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SOLUTION

Balanced Chemical Equation

H2SO3(aq) + 2NaOH(aq) Na2SO3(aq) + 2H2O(l)

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

Textbook Questions: p. 400 # 11 and 12

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