[20.3] the acid dissociation constant Ka

Question 1

describe how to form the equation fo the acid dissociaton constant, Ka

Answer 1

• dissociation of any weak acid, HA, is HA ⇌ H+ (aq) + A- (aq)
• Ka = [products] / [reactants]
• units = mol dm⁻³

Question 2

equation for Ka

Answer 2

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

Question 3

how is Ka standardised?

Answer 3

Ka values are usually recorded at 25°C

Question 4

why does Ka need to be standardised?

Answer 4

it changes with temperature

Question 5

the larger the numerical value of Ka the…

Answer 5

• further along the equilibrium is to the right
• greater the strength of the acid
• greater the dissociation

Question 6

why is pKa used instead of Ka?

Answer 6

• difficult to compare numbers with negative indices
• easier form of seeing how weak an acid is

Question 7

equation for pKa using Ka

Answer 7

pKa = -log Ka

Question 8

equation for Ka using pKa

Answer 8

Ka = 10⁻ᵖᴷᵃ

Question 9

describe the values for Ka and pKa for a strong acid

Answer 9

the stronger the acid:

• the bigger the Ka
• the smaller the pKa