Chapter 16 - Acids and Bases

Question 1

When solving questions about acid base equilibria of polyprotic acids, what is one thing to absolutely remember?

Answer 1

The final [H30+] should be the addition of the first dissociation AND the section dissociation. Please don’t forget this!

Question 1

A buffer is prepared by adding 150 mL of 1.0 mol L-1 NaOH to 250 mL of 1.0 mol
L-1 NaH2PO4. What amount (mol) of HCl must be added to this buffer solution to change the pH by 0.18
units? If necessary, assume the total volume remains unchanged at 400 mL

Answer 1

1. Calculate concentrations of OH- and H2PO4- after dilution but before neutralization. Using C2 = C1V1/V2
2. Calculate concentrations after neutralization

H2PO4 = 0.625 - 0.375 = 0.25 (all of OH reacts because strong base)
acid is reduced by moles of base
HPO42- = 0.375 (increases by the amount of strong base)
this equals moles of strong base

Setup HH equation 1 and HH equation 2 (for when the pH is changed)

Since you’re adding acid the pH will decrease by 0.18 so -0.18

So we setup the two equatiosn as pH2 - pH1, trying to solve for the ratio of [A-]/[HA]

You get a ratio of 1/1 which means the value are equal. Total concentration of both components (conjugate acid and base) is always equal to the inital concentration before neutralization, remains constant. Since they are equal, just divide this total concentration by 2.

Find inital and final moles of HA or A- and calcualte teh difference to find the moles needed

Question 2

Would the % error increase or decrease if the temperature was increased to 𝟑𝟎𝟎 °𝑪

Answer 2

Real pressure increases faster than the deviation from ideal behaviour (higher value). So % error decreases with increasing temperature as the system behaves more closely to ideal conditions

Question 3

Acid-Base Definitions

Arrenenius
Bronsted-Lowry
Lewis

Answer 3

Arrenenius
- Acid (produces H)
- Base (produces OH)
Bronsted-Lowry
- Acid (proton donor)
- Base (proton acceptor)
Lewis
- Acid (electron pair acceptor)
- Base (electron pair donor)

Question 4

Describe the relationship between [H+] and [HA]0 for strong and weak acids

Answer 4

• For strong acids, they are about equal because dissociates completely
• For weak acids, concentration of acid initally is significantly higher than proton concentration

Question 5

Define buffer region

Answer 5

half equilalence points, concentration of acid equals that of conjugate base

pH = pka

Question 6

Outline two ways to make a buffer

Answer 6

1. Equal moles of acid and conjugate base
2. Partial neutrlaization: weak acid + strong base to generate conjugate base

Question 7

When can you neglect the x value to simplify quadratic?

x + 0.05

Answer 7

For the dissociation of a weak acid, where the change in concentration is not significant

So long as the x/10^-7 > 100

You would neglect the 0.05 for very strong acids or bases where the inital concentration is so small to the change