module 5 - the pH of weak acids

Question 1

when HA molecules dissociate, which ions are formed more?

Answer 1

they’re formed in equal quantities

Question 2

how do you work out Ka?

Answer 2

Ka = [h+]^2 / [HA] start

Question 3

how can the ka for a weak acid be determined experimentally?

Answer 3

• prepare a standard solution of the weak acid of known concentration
• measure pH of standard solution using pH meter

Question 4

how do you find ka , if you have acid conc. and ph?

Answer 4

1) find h+ using ph

2) use ka formula to find ka using that h+ conc.

Question 5

what are the 2 approximations in calculations involving weak acids?

Answer 5

• 1st assumes that the dissociation of water is negligible

- 2nd assumes that the conc, of acid is much greater than the H+ conc at equilibrium

Question 6

when does the first approximation about water dissociation break down?? why?

Answer 6

• it breaks down for very weak acids or very dilute solutions
• if pH is more than 6 then h+ ions from water dissociation is significant compared with weak acid dissociation

Question 7

when does the 2nd approximation about water dissociation hold and break?? why?

Answer 7

• holds for weak acids with small Ka values
• breaks when [h+] becomes significant
• not justified for stronger weak acids with a Ka greater than 10^-1 conc and for very dilute solutions