Buffers, Titrations, Solubility

Question 1

common ion effect

Answer 1

the shift in the position of an equilibrium upon addition of a substance that provides an ion already involved in that equilibrium

Question 2

buffers

Answer 2

solutions that resist changes in pH when limited amounts of acid or base are added

Question 3

where does the buffer’s resistance to change in pH arise from?

Answer 3

the presence of appreciable concentrations of weak acids and its conjugate (weak) base

Question 4

addition of OH- to a buffer

Answer 4

the acid in the buffer will neutralise added strong base

Question 5

addition of H3O+ to a buffer

Answer 5

the base in the buffer will neutralise added strong acid

Question 6

a good buffer contains

Answer 6

the conjugate acid and base in similar amounts
- source of protons: eg HA to neutralise incoming bases
- sink of protons : eg A- to neutralise incoming acids

Question 7

Henderson-Hasselbach equation

Answer 7

the equation highlights that the pH of a buffer solution has a value close to the pKa of the weak acid

Question 8

how to make a buffer

Answer 8

1. select a weak acid with a pJa similar to desired pH
2. A) mix equal amounts of acid and its conjugate base or B) start with the weak acid and neutralise half of it with a strong base
3. adjust to desired pH by adding small amounts of strong acid/base

Question 9

buffer capacity

Answer 9

the molar amount of acid or base which the buffer can handle without significant changes in pH

Question 10

why can’t buyers tolerate the addition of infinite amounts of strong acid or base?

Answer 10

after enough external acid or base has been added to deplete the base or acid in the system, the buffer is destroyed

Question 11

how would you approach quantitative problems of neutralisation and buffers?

Answer 11

1. figure out which major species remain after any strong acids/bases act
   - assume these reactions go to completion
   - it’s often convenient to work in amount (moles)
2. determine the concentrations of any minor species that the major species generate via equilibrium reactions
   - work in concentrations (volume is constant)

how?
have both an acid and its conjugate base -> buffer -> HH
have only a weak acid/base -> acid/base dissociation -> ICE
have either H3O+ or OH- directly remaining from step 1 -> done

Question 12

titrant

Answer 12

known concentration

Question 13

analyte

Answer 13

unknown concentration solution

Question 14

equivalence point

Answer 14

point at which stoichiometrically equivalent quantities of acid and base have been mixed together

Question 15

strong acid/strong base titration

Question 16

weak acid/strong base

Question 17

acid-base indicator

Answer 17

a substance that changes colour in a specific pH range. indicators exhibit pH-dependent colour changes because they are weak acids and have different colours in their acid (Hln) and conjugate base (In-) forms

HIn + H2O <–> H3O+ + In-

Question 18

strong base/strong acid

Question 19

weak base/strong acid

Question 20

solubility

Answer 20

amount of solute that dissolves in a given amount of solvent (mol/L)

Question 21

formula for ionic compounds dissociating into ions in solution as they dissolve

Question 22

saturated solution

Answer 22

we’ve added enough solid so that some remains at the equilibrium, where ions are (re)crystallising and dissolving at the same rates.

Question 23

equilibrium constant (ion-product) expression

Answer 23

solubility product, Ksp

Question 24

Ksp

Answer 24

a measure of how much of an ionic compound has dissolved at equilibrium
= [M^n+]^m[X^y-]^x

Question 25

Precipitation of ionic compounds

Answer 25

AB (s) <–> A+ (aq) + B-(aq)
-> dissolution
<- precipitation

Ksp = [A+]eq[B-]eq
Qsp = [A+]t[B-]t

Question 26

compare Qsp to Ksp

Answer 26

Qsp>Ksp: solution is supersaturated and precipitation will occur
Qsp=Ksp: solution is saturated and equilibrium exists already
Qsp<Ksp: solution is unsaturated and dissolution will occur

Question 27

how does pH affect solubility

Answer 27

if the compound contains the conjugate anion of a weak acid, the addition of a strong acid will increase solubility