The Common Ion & Buffering. Common Ion Effect the ionization of an acid (or base) is limited by the presence of a significant amount of its conjugate.

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Presentation transcript:

The Common Ion & Buffering

Common Ion Effect the ionization of an acid (or base) is limited by the presence of a significant amount of its conjugate base (or acid) HA + H 2 O A - + H 3 O + Why?

Common Ion Problem 1.What is the pH of 0.10 M solution of HF? (K a = 7.2 x10 -4 ) 2.What is the pH of a solution that is 0.10 M HF and also contains 0.15 M F - ?

BUFFERS maintain pH! A buffer is a solution with a __________ pH, adding acid or base to a buffer doesn’t significantly affect the pH Requirements: 1) Need an acid to absorb added _____ and a base to absorb added ____ 2) The acid and base ________________ with each other. (so conjugate pairs are used)

Suitable Buffers An ideal buffer is prepared from ______ amounts of a weak acid and its conjugate _______ The pH of a buffer ______ depends on the K a and can be raised or lowered by altering the ratio of acid/base present The action of a buffer is special case of the common ion effect, and allows for a shortcut!!!!!

Equilibrium Expression Weak Acid: K a = Solve the expression for [H 3 O + ]

Henderson-Hasselbalch Equation Weak acid and conj. base

Common Ion Problem Again Use the H-H Equation to solve this problem again. 1.What is the pH of a solution that is 0.10 M HF and also contains 0.15 M F - ? (K a = 7.2 x10 -4 )

Henderson-Hasselbalch Equation Recall that _______ buffers have equal amounts of acid and conj. base. If the K a of acetic acid is 1.8E-5, what is the pH if a buffer is prepared having equal concentrations of acetic acid and acetate?

Preparing Buffer Solution The desired pH must be close to the pKa of the acid Increase the pH by adding more _______ and decrease the pH by adding more _______

H-H Equation Problem 1 Which acid/conjugate base pair would be most suitable to buffer a pH of 7.5? Weak AcidK a Acetic acid 1.8 X Dihydrogen phosphate 6.2 x10 -8 Hydrogen phosphate 3.6 x10 -13

H-H Equation Problem 1 cont. What is the ratio of conjugate base to weak acid needed to have a pH of 7.5?

H-H Equation Problem 2 A buffer solution contains 0.20 M acetic acid and 0.50 M acetate. What is the pH?

Use the H-H Equation pH of Buffers pH of solutions with Common Ions (saves you the ICE table) Acid/Base Titrations

Practice Problem Problem 2 continued. Suppose you have 500 mL of the acetic acid/acetate buffer system. What is the pH after you add 25.0 mL of 1.0 M HCl to the buffer?