Calculations pH1819 web

[Pages:16]03.10.2018

pH calculations

MUDr. Jan Pl?ten?k, PhD

Br?nsted-Lowry concept of acids and bases

? Acid is a proton donor ? Base is a proton acceptor

HCl(aq) + H2O(l)

Acid

Base

H3O+(aq) + Cl-(aq)

Conjugate acid

Conjugate base

H2O(l) + NH3(aq)

Acid

Base

NH4+(aq) + OH-(aq)

Conjugate acid

Conjugate base

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Which of the following are conjugate acid-base pairs?

A) HCl, NaOH B) H2O, OHC) H2SO4, SO42D) H2SO3, HSO3E) HClO4, ClO3F) H3C-NH2, H3C-NH3+

Autoionization of water

Water is amphoteric as it can behave both as acid and base

2 H2O(l)

H3O+(aq) + OH-(aq)

Ion-product constant for water: Kw = [H3O+][OH-] = [H+][OH-]

In pure water at 25 ?C: [H+] = [OH-] = 1.0 ? 10-7 mol/L

Kw = (1.0 ? 10-7 mol/L)?( 1.0 ? 10-7 mol/L) = 1.0 ? 10-14 mol2/L2 Constant!

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pH

pH= -log10(activity of H+) pOH= -log10(activity of OH-) Ion-product of water (constant!):

pH + pOH = 14

Activity = f . c f is activity coefficient, f ................
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