Lecture 15

Question 1

When is the ratio to observe colour change 1/10 and when is it 10/1

Answer 1

Acid to basic (low pH to high pH) –> [In-]/[HIn] = 1/10
Basic to acid (high to low) –> ratio = 10/1
–> flips

Question 2

Which formula is used to express when the colour change will be observed when TITRATING AN ACIDIC SOLN containing an indicator using a BASE

Answer 2

pH = pKa - 1 (soln shows colour change at this pH)

pH = pKa + log (1/10)

Question 3

Which formula is used to express when the colour change will be observed when TITRATING A BASIC SOLN containing an indicator using an ACID

Answer 3

pH = pKa + 1 (soln shows colour change at this pH)

pH = pKa + log(10/1)

Question 4

Why is it important to choose the right indicator for strong acid-base titrations

Answer 4

For different indicators, the volumes will be VERY similar
–> pH shoots up but volume barely changes

Question 5

Why is it important to choose the right indicator for weak acid or base titrations

Answer 5

For different indicators, there will be VERY different volumes
–> may be a colour change before the actual eq pt

Question 6

What is the solubility product eq

Answer 6

Ksp = [P]^n

Question 7

What does the solubility represent in the ICE table

Answer 7

The change (x)

Question 8

What is solubility

Answer 8

An equilibrium position
–> will change if common ion is present

Question 9

What is the solubility product

Answer 9

constant at any one temp

Question 10

When can you compare the Ksp of salts

Answer 10

When they produce the same number of ions
–> highest Ksp = most soluble
EX:
AgI
CuI
CaSO4
1:1:1

Question 11

What do you do when you can’t compare Ksp

Answer 11

Must calculate “x” for each (to find solubility)

Question 12

What would happen to the solubility if there were no common ion

Answer 12

Solubility would be higher (would go completely to the right)

BUT common ion effect decreases solubility
–> doesn’t go as far to the right

Question 13

How do you know if solubility increases or decreases in acidic soln

Answer 13

1. Must contain the conjugate of weak acid for possibility to recombine, thus lowering it (soln must then go to the right) sooo solubility increases if it must be more dissolved

TRAP: if contains a conjugate of strong acid, WONT INCREASE OR DECREASE

Question 14

What is the ion product (and eq)

Answer 14

Q (uses initial concentration)

Q = [P]^n (initial)

Question 15

How are Q and Ksp related and how do you know if a precipitate forms

Answer 15

Q > Ksp –> precipitate forms
(not enough ions to form precipitate, stay in soln)
Q < Ksp –> NO precipitate forms
(too many ions, saturated)

Question 16

How to do precipitation problems

Answer 16

IMPORTANT DIFFERENCE: MUST ADD VOLUMES OF BOTH IN BF WHEN CONVERTING TO MOLES AND BACK TO [ ]!!

1. Find Q and compare to Ksp to see if precipitate forms
2. FIGHT TO THE DEATH (between 2 atoms that must [ ] must be found)
   –> in mol!!
   –> BR
   AR
   Final [ ]
   –> LR goes to 0 and excess remains (wins fight)
3. ICE table (with winner of fight starting at final [ ] found and loser starting at 0)
   –> find x (solubility)

Question 17

What is selective precipitation (for questions asking what precipitates first)

Answer 17

When something precipitates first (lower [ ])
- Find [ ] of ion added when mixed with both ions in solution
–> The one with the lower concentration of ion added will precipitate first cuz less is require for a solid to form

Question 18

At which position on graph is there the best buffer

Answer 18

When pH = pKa (1/2 eq pt)