# UNIT IV PPT #6 - Titration Curves.ppt

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UNIT IV Titration Curves

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESMixing an acid and a base produces a solution which can be acidic, basic, or neutral depending on the relative amounts of reactants. NOTE: In acid base reactions, if one or both of the reactants are “strong” then the reaction will go to completion. Only when both reactants are “weak”, will you get an equilibrium situation. Titrations always require reactions which go to completion (single arrow), so acid/base titrations will always have either a strong acid, a strong base, or both.

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESExample If 3 moles of NaOH are mixed with 1 mole of HCl, what will happen? NaOH + HCl  H2O + NaCl

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESExample 10.00 mL of 0.100M NaOH is mixed with 25.00 mL of 0.100 M HCl. Find the pH of the final (resulting) mixture. Balanced equation: Initial moles of NaOH: Initial moles of HCl:

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESExcess moles: [H3O+] = [HCl] in the final mixture: pH = Note: Moles of acid or base may be determined from solids samples as well using molar mass.

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESExample 40.00 mL of 0.100 M NaOH is mixed with 25.00 mL of 0.100 M HCl. Calculate the pH of the resulting solution.

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESThink of a diprotic acid as releasing 2 protons (H+’s) to the base. (NOTE: even though we learned that diprotic acids like H2SO4, donate only 1 proton completely, that was to WATER, not to a STRONG BASE. A STRONG BASE will take both the protons from H2SO4!) Dissociate bases to find out the number of OH- ions they provide.

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESExample 15.00 mL of 0.100 M H2SO4 is mixed with 12.50 mL of 0.200 M NaOH. Calculate the pH of the resulting solution. Balanced equation: Dissociations:

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FINDING THE PH OF MIXTURES OF ACIDS AND BASESInitial moles of NaOH: Initial moles of HCl: Excess moles: [H3O+]= pH = Hebden Textbook page 143 Questions #58-68

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TITRATION CURVESTitration Curves: a plot of pH as a function of volume of added titrant.

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STRONG ACID – STRONG BASE TITRATION CURVESWe can calculate the pH of the mixture in the beaker throughout the titration. First, we separate the process into 3 stages: INITIAL POINT: Acid before any base is added EQUIVALENCE POINT: Equivalence (Stoichiometric) Point FINAL POINT: Base in excess

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STRONG ACID – STRONG BASE TITRATION CURVES1. INITIAL POINT The beaker contains 25.00 mL of 0.100 M HCl. Calculate the pH.

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STRONG ACID – STRONG BASE TITRATION CURVES2. EQUIVALENCE POINT 0.100 M NaOH is added to 25.00 mL of 0.100 M HCl. Find the volume of base added and calculate the pH.

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STRONG ACID – STRONG BASE TITRATION CURVESTHE SALT FORMED FROM A SA-SB TITRATION IS ALWAYS NEUTRAL. Since there is no SA, no SB, and just H2O and a NEUTRAL salt, the pH of the solution formed will be 7.00. At the Equivalence (Stoichiometric)Point of a SA—SB Titration, the pH is always = 7.00

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STRONG ACID – STRONG BASE TITRATION CURVES3. FINAL POINT 26.00 mL of 0.100 M NaOH is added to 25.00 mL of 0.100 M HCl. Find the pH of the resulting solution.

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STRONG ACID – STRONG BASE TITRATION CURVES

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WEAK ACID – STRONG BASE TITRATION CURVESAgain, this type of titration has the 3 stages: INITIAL POINT: Acid before any base is added EQUIVALENCE POINT: Equivalence (Stoichiometric) Point FINAL POINT: Base in excess

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WEAK ACID – STRONG BASE TITRATION CURVES1. INITIAL POINT Find the pH of 25.00 mL of 0.10 M CH3COOH before any base is added to it.

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WEAK ACID – STRONG BASE TITRATION CURVESWe see that for a WEAK ACID – STRONG BASE titration, the pH before the base is added is higher (ex. 2.87) than it was for a SA-SB titration (where the pH before the base is added is 1.00). For the same concentration, the weaker the acid, the HIGHER the pH will start out!

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WEAK ACID – STRONG BASE TITRATION CURVESA note about the BUFFER REGION: 10.00 mL of 0.100 M NaOH is added to 25.00 mL of 0.10 M CH3COOH.

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WEAK ACID – STRONG BASE TITRATION CURVESWhat we are left with is a mixture of a weak acid (CH3COOH) and the salt of its conjugate base (NaCH3COO). A mixture of a weak acid and a weak base (the salt of its conjugate base) is called a BUFFER SOLUTION.   As we will see later, a buffer solution is a solution which maintains the pH at a fairly constant value. This causes the titration curve to decrease in slope during this stage. The area on the curve is called the “Buffer Region”. (In Chem12, we will not need to be able to calculate the pH in a buffer solution.)

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WEAK ACID – STRONG BASE TITRATION CURVES2. EQUIVALENCE POINT 0.100 M NaOH is added to 25.00 mL of 0.100 M CH3COOH.

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WEAK ACID – STRONG BASE TITRATION CURVESBut, this time we must consider the salt (NaCH3COO) that is produced  because it is NOT neutral! NaOH + CH3COOH  H2O + NaCH3COO This salt that is produced (NaCH3COO) dissociates to form Na+ (spectator) and CH3COO- which undergoes base hydrolysis in water.

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WEAK ACID – STRONG BASE TITRATION CURVESWe can now use the hydrolysis equation and an ICE table to calculate the [OH-] and then pOH and then pH:

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WEAK ACID – STRONG BASE TITRATION CURVES For a WEAK ACID – STRONG BASE Titration, the pH at Equivalence Point is ALWAYS > 7 This is because, when a weak acid reacts with a strong base, you always produce the conjugate base of the weak acid, which is BASIC.

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WEAK ACID – STRONG BASE TITRATION CURVES3. FINAL POINT Looking at the Balanced equation: NaOH + CH3COOH  H2O + NaCH3COO Once NaOH is in excess, you will have some STRONG BASE (NaOH) and some WEAK BASE (CH3COO-) in the resulting mixture. The OH- contributed by the weak base ( CH3COO-)was significant when there was no other base present (EP), but once a strong base (NaOH) is present, the OH- contributed by the weak base is insignificant compared to that produced by the NaOH. So, the titration curve past the EP for a WA/SB Titration is the same as it is for a SA/SB Titration (where NaOH is in excess).

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Last Updated: 8th March 2018