Acid-Base and pH II Flashcards by Casi Jones

What does pKa show?

How strong an acid is

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-log10 [ ]

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[ ]

concentration of an acid

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is completely dissociated / ionised in water

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How to calculate concentration of an acid (strong acid)?

Ka = [A-] [H30+] / [HA]

base is the same as the salt and acid

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RECAP Kw

[10^-7]^2

= 10^-14

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Calculate concentration of strong base use Kw

pH = -log10 [H3O+]

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Acid molarity

m/Mr

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Calculate moles:

m/Mr
unit: g/mol or M

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Conc [] = n/v

c=n/v
dm3 for volume

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Weak acid and weak base:

equilibrium / not completely dissosiated

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Weak ACID always has…

Conjugated BASE

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Weak BASE always has…

Conjugated ACID

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How to calculate pH or weak acids

pH 1/2 pKa - 1/2 log [HA]

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Calculate the pH of a 0.010 M solution of aspirin at 25C
Ka of aspirin is 3.2 x 10^-4 at this temp

pKa = -log10 Ka

(Ka of weak acid)

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pH of weak acid

1/2 pKa - 1/2log [HA]

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In a weak acid, not completely dissociated in water [HA] can be considered the same as…

original weak acid

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Define equilibrium:

A state in which opposing forces or influences are balanced.

“the task is the maintenance of social equilibrium”

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Calculate weak base: NH3

pH = pKw -1/2 pKb = 1/2 log [B]
pKb -log10 Kb =
pH

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Henderson-Hasselbalch equation:

e.g acid conc of 2 = acid has been transformed to one of conc of base
acid = 1
conj base = 1
pH = pKa

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pKa

the Ph at which it is exactly half dissociated

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If we increase the pH (more basic)?

acid becomes more ionised pH > pKa dissociated

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If we lower pH (more acidic)?

acid becomes less ionised pH< pKa UNdissociated

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if pH is SMALLER than pKa

The acid will be in an undissociated form

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If pH is 4

4 = 5 + log

Ionised form = conjugated base

When talking about the WEAK acid

When in ionised form use which equation?

H-H equation

pH = pKa + [A-] / [HA] at pH 4

log [A-] / [HA] = pH - pKa =

Original acid

1 - conjugated base [HA] = 1 - X

Acid-base reactions:

strong acid + strong base

Neutral base

strong acid + weak base

Acidic salt

Weak acid + strong base

basic salt

Weak acid + weak base

depends which one is stronger

NaCl + H2O >>> Na+ + Cl-

Neutral solution - cannot react further with water

Cl- + H2O <> HCl = HO-

Cl- = very weak conjugate base of HCl Reaction with water can be neglected

HCl (strong acid) = NH3 (weak base <> NH4Cl {acid salt}

No H2O = weak bases tend not to have a hydroxide

Weak acid + weak base = depends on which one is stronger

Ka cation > Kb anion = solution acidic

kb anion > ka cation =

solution basic

Ka & Kb similar =

solution is close to neutral (pH approx. 7)

Acid-base titration (neutralisation)

quantitive analysis of the concentration of an unknown acid or base solution

Strong base added to weak acid =

solution containing the weak acid and its conjugate base until the initial weak acid is completely neutralised by the base

H-H equation

pH = pKa = log [base] / [acid] This is the conjugate base [A-]

equivalence point:

acid-base (added >>> equal stoichiometric amounts

When is reaction completed??

all acid has been converted to its conjugated base

Half equivalence point: half of the total amount of base (added) needed to neutralise the acid has been added

pH = pKa of weak acid

[H3O+] =

(initial amount moles of acid) = (amount of moles of base added) / (volume of acid) + (volume of base added)

After neutralisation:

[OH-] >

Acid-base graph:

1) pH low at start 2) rises sharply 3) rises very slowly

Equivalence point

the point on the graph where the curve is steep (straight up)

What would be equivalence point in string acid or base?

pH = 7

what is the equivalence point with weak acid or base?

Ranges

Indicator

The best one is in the range of the equivalence point

Buffer

A solution usually containing an acid and a base, or a salt, that tends to maintain a constant hydrogen ion concentration

Buffer is made of

HIGH concentration of weak acid CH3CO2H <> CH3CO2- + H+

And a completely ionised strong base

CH3Co2-Na+ > CH3CO2- + Na+

The H+ in the buffer will react with...

the Na+ from the strong acid

Preparing a buffer:

Which pKa is the most relevant for biological systems and why? To which dissociation does it correspond?

pKa = 7.21 / pKa = 2.21 X pKa = 12.67 X

Which relative concentrations do we need to have the maximum buffering power??

Function as a buffer and control the pH to 7.0

IN CLASS TEST: What is the pH of a buffer solution containing 0.042M NaH2PO4 and 0.058M Na2HPO4?

Use H-H equation

How much does pH change if we add 1mL of 10M NaOh to 1L of this buffer solution?

M = mol / vol, mol = M x V 0.01 mol NaOH titrate (neutralise) 0.01 mol NaH2PO4 and form 0.01 mol Na2HPO4 >>> new conc: 0.032 M NaH2PO4, 0.068M Na2HPO4