# Chapter 19 Self-Ionization of Water and pH. Strong vs. Weak Strong: 100% dissociation of H + or OH – – Strong electrolytes (conduct electricity very well)

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Chapter 19 Self-Ionization of Water and pH

Strong vs. Weak Strong: 100% dissociation of H + or OH – – Strong electrolytes (conduct electricity very well) Weak: Less than 100% dissociation of H + or OH – – Weak electrolytes (conduct electricity somewhat) Strong, weak, or nonelectrolytes (2 min)

Examples 1) Strong acids: H+7A, H 2 SO 4, HClO 4, HNO 3 2) Weak acids: HCN, HC 2 H 3 O 2 3) Strong bases: 1A+OH, Sr(OH) 2, Ba(OH) 2 4) Weak bases: NH 3 (ammonia) = NH 4 OH (ammonium hydroxide), CH 3 NH 2

Self-ionization of Water In neutral water, (1)H 2 O + H 2 O ↔ H 3 O + + OH – (2) [H 3 O + ] = [OH – ] = 1×10 -7 mol/L at 25 ˚C (3) [H 3 O + ] · [OH – ] = (1×10 -7 ) · (1×10 -7 )=1×10 -14 = K w

Acidic or Basic Solutions K w =1×10 -14 always maintained [H 3 O + ][OH - ] pure water [H 3 O + ]=1×10 -7 mol/L[OH - ]= 1×10 -7 mol/L acidic solution [H 3 O + ]>1×10 -7 mol/L[OH - ]< 1×10 -7 mol/L basic solution [H 3 O + ]<1×10 -7 mol/L[OH - ]> 1×10 -7 mol/L

Example The concentration of H 3 O + in blood is 4.0x10 -8 M. Is blood acidic, basic, or neutral? What is the concentration of OH - ions in blood?

pH pH = power of hydrogen ions Acid strength scale Mathematically, pH = ‒ log [H 3 O + ] Lower the pH = stronger acidic solution Higher the pH =stronger basic solution pH acidic solutionpH < 7 pure waterpH = 7 basic solutionpH > 7

Which is stronger acidic solution? 1)pH = 5 vs. pH = 8.2 2)pH = 7 and pH = 10 3)[H 3 O + ] = 10 -4 vs. [H 3 O + ] = 10 -9 4)[OH - ] = 10 -5 vs. [OH - ] = 10 -7

Draw the following

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