CALCULATING THE pH OF ACID/ALKALI MIXTURES 1 Calculate initial moles of H+ and OH¯ 2 Calculate which one is in excess 3 Calculate its concentration in the combined solution 4 Convert concentration to pH 1 Calculate initial moles of acid and alkali 2 Calculate the excess moles of OH¯ 3 Calculate the OH¯ concentration in the combined solution 2 Calculate the excess moles of acid 3 Calculate the moles of anion formed (same as the alkali used up) 4 Use the value of Ka for the weak acid to calculate the value of [H+] 5 Convert concentration to pH

Q1 Calculate the pH after 20cm3 of 0. 1M HCl is added to 25cm3 of 0 Q1 Calculate the pH after 20cm3 of 0.1M HCl is added to 25cm3 of 0.1M NaOH Q2 Calculate the pH after 22cm3 of 0.1M CH3COOH is added to 25cm3 of 0.1M NaOH Q3 Calculate the pH after 20cm3 of 0.1M KOH is added to 25cm3 of 0.1M CH3COOH

Buffers

Learning outcomes You should be able to: understand the action of acidic and basic buffer solutions qualitatively in terms of equilibria be able to calculate the pH of a buffer solution know some uses of buffer solutions

Buffer resist changes in pH A buffer is a solution that resists changes in pH -when small amounts of acid or base added / -when it’s diluted.

Acidic Buffer CH3COONa(aq) Na+(aq) + CH3COO-(aq) CH3COOH (aq) H+ (aq) + CH3COO-(aq) CH3COONa(aq) Na+(aq) + CH3COO-(aq) Weak Acid – Partially dissociates to: Salt of weak Acid –Fully dissociates to: CH3COOH (aq) H+ (aq) + CH3COO-(aq) Large quantities of the weak acid and conjugate base are present. The amount of salt remaining is negligible The metal ion is not part of the buffering system and so is not shown here but is still present in the solution

Buffer, shift, equilibrium, restore, conjugate, acid, base, salt. YOUR TASK Suggest how a buffer solution is made and how it works using the key terms: Buffer, shift, equilibrium, restore, conjugate, acid, base, salt. CH3COOH (aq) H+ (aq) + CH3COO-(aq)

Lots of undissociated weak acid Lots of CH3COO- from dissociated salt Addition of H+ (acid) CH3COOH (aq) H+ (aq) + CH3COO-(aq) Lots of undissociated weak acid Addition of OH- (base)

CH3COOH (aq) H+ (aq) + CH3COO-(aq)

Basic/Alkaline Buffer NH3(aq) + H2O(l) NH4+ (aq) + OH-(aq) YOUR TASK Explain how a buffer solution is made and how it works using the key terms: Buffer, shift, equilibrium, restore, conjugate, acid, base, salt.

How to calculate the pH of buffer solution? A buffer solution contains 0,40 moldm-3 methanoic acid, HCOOH and 0,60 moldm-3 sodium methanoate,HCOONa. For methanoic acid, Ka =1,6ˣ10-4 moldm-3. What is the pH of this buffer? STEPS: -write the expression for Ka of the weak acid; - rearrange the expression and stick in the data to calculate [H+]; - convert [H+] to pH Few assumptions: [HCOONa] = [HCOO-] [HCOOH]equib = [HCOOH]inital

Buffer Calculations Q1 What is the pH of 100.0 mL buffer solution which is 0.100M in HC2H3O2 and 0.100M in NaC2H3O2 ? Ka =1.74 ˣ 10-5 mol.dm-3 pH buffer = 4.76 Q2 What is the new pH after 2.00 mL of 0.100M NaOH is added to this buffer? STEPS Equation Initial moles of acid and conjugate base in buffer is _________ Initial moles of OH- added is _______________ Final moles of acid and base / ICF table New volume of solution New concentrations of [OH-] and [CH3 COO-] and [CH3 COOH] New pH, click for answer ANSWER : 4.78 Q3: What is the new pH after 2.00 mL of 0.100M HCl is added to the same buffer in Q1? ANSWER : 4.74

Buffer Calculations OH- + HC2H3O2 → C2H3O2- + H2O What is the new pH after 2.00 mL of 6.00M NaOH is added to the original buffer? OH- + HC2H3O2 → C2H3O2- + H2O

What is necessary to work on? 1 What has been learned? 2 What remained unclear? 3 What is necessary to work on?