1. 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

2. 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

3. Buffers

4. 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

5. 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.

8. 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

9. 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)

10. 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)

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

12. 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.

14. 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

15. Buffer Calculations
Q1 What is the pH of 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

16. 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

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