(L7) Module 1 L7 - Buffers

Question 1

What is a buffer solution?

Answer 1

weak base/conj acid or
weak acid/conj base, at a reasonable concentration …
which will maintain a reasonably constant pH on addition of significant
amounts of H3O+ or OH- ions. (1/2 required statements)

Question 2

How is pH of buffer solution affected by dilution?

Answer 2

pH of buffer solution is unaffected by strongacid/base dilution of the solution.
(2/2 required statements)

Question 3

State the 2 sentences required for full buffer definition.

Answer 3

1.wb/cja or wa/cjb, both at reasonable concentration.. which will maintain a reasonably constant pH on addition of significant
amounts of H3O+ or OH- ions.

2. pH of buffer solution is unaffected by reasonable dilution of the solution.

Question 4

What is equation like for a
Weak acids + Strong base
or a
Strong acid + weak base

Answer 4

They go to completion

CH3COO-(aq) + H3O+(aq) → CH3COOH(aq) + H2O(l)

CH3COOH(aq) + OH-(aq) → CH3COO-(aq) + H2O(l)

Question 5

What is the Henerson-Hasselbach equation

Answer 5

pH = pKa +log [A-]/[HA]
or
pH = pKa + log [base]/[acid]

Question 6

What is henderson-hasselbach for?

Answer 6

finding the pH of any buffer solution.

Question 7

What is the ratio for pH of buffer solutions?

Answer 7

[A-] / [HA]

Question 8

What does it mean when when the ratio [A-] / [HA] = 1 in the buffer solution?

Answer 8

pH = pKa. (because log(1) = 0)

At this pH value, the concentrations of A- and HA in solution are equal, and
the buffer will be equally effective towards the addition of either acid or base.

Question 9

how can henderson-hasselbach equation be expressed in terms of mols?

Answer 9

Question 10

Solubility of Na?

Answer 10

All na compounds?

Question 11

What happens if strong acid/base is added to buffer? What assumptions must be made.

(remember 2. assumption of buffer is that pH will stay relatively same when strong acid/base added)

Answer 11

Before addition of the HCl, the ratio [A-] / [HA] = 1.
The added strong acid will react
stoichiometrically with CH3COO- ion in solution to form CH3COOH

This will then change the [A-
] / [HA] ratio. Need to calculate the individual
concentrations of A- and HA

Question 12

How to calc how much pH will change when a strong acid/base is added to buffer solution?

Answer 12

1. Use concentration table to find concentration.
2. turn into mols.
3. use mols in henderson hasselbach equation

Question 13

How to calc change in pH when 0.100 L of 0.100 mol L–1 HCl is added to 1.00 L of pure water.

Answer 13

1. mols of HCL
2. final [H3O] = mols of HCl/ total volume (water+HCl added)
3. final pH = -log(H3O)
4. inital ph is Kw=[H3O]=1x10^-7
   pH = -log[H3O] = 7
5. change in pH = pHfinal-pHinitial

Question 14

How to make a buffer?

Answer 14

1. Add weak acid with salt of conj base.
2. Strong base + weak acid
   (NaOH + CH3COOH)
   OR
   Strong Acid + Weak base
   (HCl +NH3)

Question 15

Does weak base + strong acid go to completion?
Does weak acid + strong base go to completion?

Answer 15

yesH

Question 16

How to find pH of created buffer system?

Answer 16

1. find mols of each before they were mixed
   2.use concentration table
2. chuck em into henderson hasselbach

Question 17

What is buffer capacity?

Answer 17

When enough OH- or H3O is added, the weak acid/weak base will be used up and no more conj’s base/acids can be produced.
This means the amount of H3O and OH- starts increasing.
pH changes significantly.
We say that buffer capacity has been reached.