Ionic Equilibria

Ionic Equilibria

Acid-Base- Acid/ Base/ Salt/ Buffer- Titration Curve/ Indicator

Solubility - Dissolving - Precipitation

Acid/ Base Equilibria

p = -log10

At 25°C:

pH + pOH = pKw = 14

pKa + pKb = 14

[H+][OH–] = 10-14

Ka Kb = Kw = 10-14

- only valid at 25⁰CIonic Equilibria

1. Determine what is in solution:

- Acid/ Base/ Salt/ Buffer

2. Apply the corresponding eqns

ApproachProblem Solving

Acid/ Base

strong acid weak acid

strong base weak base

[H+] = [HA]

pH = -log [HA]

[OH–] = [B]

pOH = -log [B]

[H+] = Ka [HA]

pH = -log [H+]

[OH–] = Kb [B]

pOH = -log [OH–]

How to determine if salt is acidic/ basic/ neutral?

1. Split the salt into its ions.2. Determine the parent acid/ base of the ions:

Salt

Acid Base SaltStrong Strong NeutralStrong Weak Acidic

conj acid of weak base hydrolyses H2O to form H+

Weak Strong Basicconj base of weak acid hydrolyses H2O to form OH–

Weak Weak Depends

SaltNature of salt Formed from Finding pH

Neutral strong acid + strong base

= 7

Acidic strong acid + weak base

treat as weak acid;conj acid of weak base hydrolyses H2O to form H+

use weak acid formulaBasic weak acid +

strong basetreat as weak base;conj base of weak acid hydrolyses H2O to form OH–

use weak base formula

How to calculate pH of a salt solution:

1. Find Ka/ Kb of the ion from Kb/ Ka of the parent acid/ base use Kw

2. Find [salt] =

[H3O+] = Ka × [salt] [OH–] = Kb × [salt]

n (limiting reagent)

Vtotal

Vtotal = Vacid + Vbase

Salt

pH = pKa + log [salt][acid]

pOH = pKb + log [salt][base]

Buffer

Type Formed from pHAcidic weak acid +

conjugate base< 7

Basic weak base + conjugate acid

> 7

1. Determine which species is reacted and which is formed.2. Calculate new amounts (in moles)3. Substitute into buffer equation.

How to calculate pH of a buffer on adding small amounts of H+ and OH–:

Buffer

1. When using buffer equation, just substitute no. of moles of salt and acid/base since total volume is the same and cancels out.

2. Look out for [salt] = [acid] which simply means pH = pKa (same

for basic buffer)3. When unsure if buffer is acidic or basic, look at pKa

e.g. HCO3–/CO3

2– pKa = 10since pKa is large, buffer is more basic than acidic use basic buffer formula

pH = pKa + log nsalt/Vnacid/V

Buffer

There are three important points in a titration curve.e.g. titration of weak acid against strong base

Titration Curve

Titration Curve

Titration curve What to observe What can be found

1 Initial pt - pH [H+] due to dissociation of acid

2 Equivalence pt - pH

- volume of base

[H+] due to hydrolysis of salt;Given [base], [acid] can be found and vice versa

3 Half equivalence pt

- pH pKa of acid

Calculate pH at various points in titration curve:1. At the beginning2. In the middle (sometimes at half eq. pt)3. At equivalence pt4. Beyond eq. pt.

Strategy1. Determine what is in solution

- Acid/ Base/ Salt/ Buffer2. Use the relevant eqns

Titration Curve

Titration Curve

Titration curve What is in solution Equations

Initial pt weak acid

Between initial pt and equivalence pt

acidic buffer

At equivalence pt

acidic salt

Beyond equivalence pt

strong base

pH = pKa + log [salt][acid]

pH = -log Ka × [salt]

pH = -log Ka × [acid]

pOH = -log [B]

e.g. titration of weak acid against strong base