Ionic Equilibria Guide to Calculating pH and Solubility

Ionic Equilibria (I) Acids and bases

Ionic Equilibria (I) 1. How do we identify acids and bases?

Ionic Equilibria (I) ,[object Object],[object Object],[object Object],[object Object],1. How do we identify acids and bases? Tip : It may be helpful for beginners to memorise some common acids and bases.

Ionic Equilibria (I) 2. How do we identify conjugate acids and bases?

Ionic Equilibria (I) ,[object Object],[object Object],[object Object],2. How do we identify conjugate acids and bases? Acid Conjugate Base – H + + H + Base Conjugate Acid + H + – H +

Ionic Equilibria (I) 3. How do we know whether an acid/ base is strong or weak?

Ionic Equilibria (I) 3. How do we know whether an acid/ base is strong or weak? Strong acids/ bases ionise completely in aqueous solution. Weak acids/ bases ionise partially in aqueous solution. ,[object Object],[object Object],[object Object],[object Object],[object Object]

Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base?

Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (I) Learn the important terms/ relations ,[object Object],[object Object],[object Object],[object Object],pH + pOH = 14 pK a + pK b = 14 [H + ][OH – ] = 10 -14 K a . K b = K w = 10 -14

Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (II) Determine strong or weak Strong acids ionise completely in aqueous solution. [H 3 O + ] = [HA]  pH = - log [HA] Strong bases ionise completely in aqueous solution. [OH - ] = [B]  pOH = - log [B] Strong Acid Strong Base

Ionic Equilibria (I) 4. How do we calculate pH of an acid/ base? (II) Determine strong or weak Weak bases ionise partially in aqueous solution. [OH - ] < [B]  pOH = - log [OH – ] Weak Acid Weak Base Weak acids ionise partially in aqueous solution. [H 3 O + ] < [HA]  pH = - log [H 3 O + ] [H 3 O + ] = K a × [HA] [OH – ] = K b × [B]

Ionic Equilibria (II) 5. How do we know if a salt is neutral, acidic or basic?

Ionic Equilibria (II) ,[object Object],[object Object],[object Object],[object Object],5. How do we know if a salt is neutral, acidic or basic?

Ionic Equilibria (II) 6. How do we calculate the pH of a salt solution?

Ionic Equilibria (II) ,[object Object],[object Object],[object Object],[object Object],6. How do we calculate the pH of a salt solution? [H 3 O + ] = K a × [HA] [OH – ] = K b × [B]

Ionic Equilibria (II) ,[object Object],[object Object],6. How do we calculate the pH of a salt solution? [H 3 O + ] = K a × [HA] [OH – ] = K b × [B] n (limiting reagent) V total V total = V acid + V base

Ionic Equilibria (III) Solubility Equilibria - sparingly soluble salts

Ionic Equilibria (III) 7. How do know if a salt is sparingly soluble?

Ionic Equilibria (III) ,[object Object],[object Object],[object Object],7. How do know if a salt is sparingly soluble?

Ionic Equilibria (III) ,[object Object],[object Object],[object Object],[object Object],[object Object],General Tips

Ionic Equilibria (III) Type 1 Questions K sp solubility conc. of ions Given the value of one of the above (e.g. K sp ), find the other 2 values (solubility and conc. of ions)

Ionic Equilibria (III) ,[object Object],[object Object],[object Object],[object Object],Type 1 Questions (Strategy) K sp solubility conc. of ions Expressing each term in x can help in the interconversion. solubility = x K sp = 4x 3 [Ca 2+ ] = x [F – ] = 2x

Ionic Equilibria (III) ,[object Object],Type 2 Questions Given the conc. of Ca 2+ and F – and K sp , predict whether ppt is formed or Given K sp and conc. of one ion, predict the min. conc of the other ion that will cause precipitation.

Ionic Equilibria (III) ,[object Object],[object Object],[object Object],[object Object],[object Object],[object Object],Type 2 Questions (Strategy)

Ionic Equilibria (I) Tutorial Q1cii) A weak acid dissociates partially producing less moles of H + ions than moles of acid. e.g. 10 moles of a weak acid HA may produce only 0.1 mole of H + But when a weak acid reacts with another reagent (e.g. metal or alkali), the weak acid will react completely. e.g. All 10 moles of HA will be reacted when added to excess NaOH

Ionic Equilibria Guide to Calculating pH and Solubility

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Ionic Equilibria Guide to Calculating pH and Solubility