The acid dissociation constant, Ka

Question 1

How do u write Ka constants?

Answer 1

• KA = products / reactants

Question 2

What are the units of Ka?

Answer 2

moldm-3

Question 3

What do Ka values tell us about equillibrium?

Answer 3

• the larger the ka value is, the further equillibrium is to the right

Question 4

What do Ka values tell us about the strength of the acid?

Answer 4

• The greater the ka value, the greater the dissociation of the acid and therefore the greater strength of the acid

Question 5

What do Pka values tell us about the strength of an acid?

Answer 5

• The smaller the Pka value, the larger the Ka and the stronger the acid
• The larger the Pka value, the smaller the Ka value and the weaker the acid

Question 6

How can we find Pka?

Answer 6

pka = -logka

Question 7

How do we find Ka from Pka values?

Answer 7

Ka = 10-Pka

Question 8

What are the two approximations that can be made to simplify ka values of weak acids?

Answer 8

Approximation 1
- from the ka equation, HA dissociates to produce equal values of H+ and A-, therefore we can assume that H+ at equillibrium is equal to A- at equilibrium
Approximation 2
- from the ka equation, we assume that HAs value is smaller at equilibrium compared to when it is undissociated. However, as the dissociation of HA is small as it is a weak acid we can assume that the concentration of HA at the start is greater than H+

Question 9

What can the ka value of weak acids be simplified to?

Answer 9

[H+]2/ [HA]

Question 10

How do we calculate PH from Ka?

Answer 10

• write the ka expression
• simplify it to make hydrogen the subject
• H+ = sqaure root of ka x HA
• then use PH = -log[H+]

Question 11

What are ways we can determine Ka?

Answer 11

• by preparing a standard solution of a weak acid of a known conc
• measuring the PH of the standard solution using a PH meter

Question 12

What are two situations when the approximations break down?

Answer 12

1. The first approximation assumes that the dissociation of water is negligible. At 25 degrees, the concentration of hydrogen ions is 10-7. Therefore, if ph is greater than 6, the dissociation of water is significant and affects the concentration of hydrogen ions. So this approximation breaks down for very weak acids or very dilute solutions
2. The second approximation assumes that as it is a weak acid, the concentration at the start of HA is greater than the concentration at equillibrium. [HA(aq)]start- [H+(aq)]eqmThis breaks down when there is a strong acid and there is a real difference between HA and H+ ions.

Question 13

When is Ka fixed?

Answer 13

At 298K