Ionic Equilibria, pH and Buffers Flashcards
What is an electrolyte solution?
A solution that has ions present and can conduct electricity
Can you easily measure the equilibrium constant of a strong electrolyte? Why?
No. Ions are almost completely dissociated
What is the calculation for equilibrium constant?
K = [C][D]/[A][B]
If the equilibrium of a solution lies to the RHS, what is the value of K?
> 1
If the equilibrium of a solution lies to the LHS, what is the value of K
< 1
What is the value of Kw?
1 x 10^-14
What type of solution has a [H+] of less than 1 x 10^-7?
Acidic
What type of solution has a [H+] of more than 1 x 10 -7?
Alkaline
Give the equation for pH? (2 possible answers)
pH = -log10[H+]
OR
pH = log10(1/[H+])
T or F? If you have a higher [H+], you have a higher pH
False
What can limit the activity of H+? What is this ability called?
Presence of other ions.
Ionic strength
What is the equation relating to H+ activity?
pH = -log {[H+ x [f]}
How does a pH sensor take a measurement?
Determines the potential difference between measuring electrode and reference electrode
What type of solution will give a positive potential difference
Acidic
What type of solution will give a negative potential difference?
Basic
What two buffers would you use to calibrate a basic solution?
pH 7 and pH 10
What is the relationship between Ka/b and strength of acid/base
The larger the Ka/b , the stronger the acid/base
What is the difference between a strong and weak acid?
A strong acid is fully ionised whereas a weak acid is not
What is the relationship between pH and pKa for an acid?
pH = 0.5 pKa - 0.5 log c
What is the relationship between pH and pKb
pH = 0.5 (pKw + pKa + log c)
Give some characteristics of salts
Ionic H2O soluble High melting point Crytalline solids Can increase the H2O solubility of a drug
What type of salt is produced by a reaction between a strong acid and a strong base?
Neutral salt
Give an example of a strong acid/strong base reaction and explain why the salt is acidic, basic or neutral
HCl + NaOH -> NaCl + H2O
NaCl dissociates into Na+ and Cl-. Neither of these ions react with water so salt is neutral
What type of salt is produced by a strong acid/weak base reaction
Acidic Salt
Give an example of a strong acid/weak base reaction and explain why the salt is acidic, basic or neutral
HCl + NH3 -> NH4+ + Cl-
Cl- doesn’t react with water. NH4+ donates protons to water forming NH3 and H3O+. As H3O+ is a hydrated proton, pH of solution falls (becomes more acidic)
What type of salt is produced in a weak acid/strong base reaction?
Basic salt
Give an example of a weak acid/strong base reaction and explain why the salt is acidic, basic or neutral
CH3COOH + NaOH -> CH3COO-Na+ + H2O
Na+ doesn’t react with water but CH3COO- accepts proton to form CH3COOH + OH-. Increased OH- concentration causes pH to rise
What type of salt is produced in a weak acid/weak base reaction?
Neutral salt
Give an examples of a weak acid/weak base reaction and explain why the salt is acidic, basic or neutral
CH3COOH + NH3 -> NH4+ + CH3COO-
NH4+ and CH3COO- both react with water to form OH- and H+/H3O+ so pH remains constant
What is an amphiprotic salt?
A salt that can act as both an acid and a base (can accept and donate protons)
Give a reaction to calculate pH of a amphiprotic salt
pH = 1/2(pKa1 + pKa2)
What is a buffer?
A solution that resists changes in pH when small amounts of acid or base are added
What two combinations can make up a buffer
A weak acid and it’s salt (conjugate base) or a weak base and it’s salt (conjugate acid)
How does a buffer stabilise pH
It acts as a proton store or sink.
Give an equation that predicts the behavior of a buffer
pH = pKa + log ([SALT]/[ACID])
At 1/2 end point of a titration of weak acid with a strong base what is pH equal to?
pKa
When is buffer capacity at its highest?
When pH = pKa
What is the buffer capacity?
The number of moles of strong acid/base required to produce an increase of 1 on the pH scale
Give an explanation of a biological buffer
At neutral pH, amino acids exist as zwitterions. In an acidic environment, COO- accepts proton. In a basic environment, NH3+ donates proton
What is the calculation for the percentage of an acidic drug that is ionised in the body?
%ionised = 100/1+antilog(pKa-pH)
What is the calculation for the percentage of a basic drug that is ionised in the body
%ionised = 100/1+antilog(pH-pKa)
Give examples of functional groups that indicate a drug may be acidic
Carboxylic acids, phenol groups, cyclic amides, sulfonamide
Give examples of functional groups that indicate a drug may be basic
Amine
How can the pH of the end point of a titration be estimated for an acidic indicatior
pKa + 3
How can the pH of the end point of a titration be estimated for a basic indicator
pKa - 3
