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Chem_215_Lecture_11_Color

Chem_215_Lecture_11_Color - 8.2 Buffer Solutions Buffers...

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1 8.2: Buffer Solutions Buffers resist large pH changes upon addition of an acid or base. Contains: – Weak acid + conjugate base or – Weak base + conjugate acid One component neutralizes acids; the other neutralizes bases. Unbuffered: 1.00 L of Pure Water: Add 10.0 ml of 2.0M HCl Buffer: 1.00 L of 1.50M Acetic Acid/ 1.20M Sodium Acetate: Add 10.0 ml of 2.0M HCl pH = 1.69 pH = 4.65 pH = 7.00 pH = 4.63
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2 Buffer solutions consume added acid or base
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Henderson-Hasselbalch Equation For a buffer solution containing a weak acid and its conjugate base: Notes: Equation was derived by assuming M - x M. [conjugage base]/[acid] should lie between 0.10 and 10.0 Molarity of each component should exceed K a by factor of 100. 1.50M Acetic Acid / 1.20 M Sodium Acetate: K a = 1.8 x 10 -5 ; pK a = 4.74 [ ] [ ] 65 . 4 09 . 0 74 . 4 5 . 1 1.2 log 4.74 pH 10 = = + = [ ] [ ] HA A log pK pH - 10 a + =
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4 Preparing Buffers [ ] [ ] HA A log pK pH - 10 a + =
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5 8.3 Exact Treatment of Buffer Solutions When the pH of a buffer is near 7.0, it may be necessary to consider the autoionization of water.
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