5.1.11: Calculating pH of strong and weak acids

Question 1

How do you calculate pH from H+ concentration?

Answer 1

pH= -log[H+(aq)]

Question 2

How do you calculate H+ concentration from pH?

Answer 2

[H+(aq)] = 10^-pH

Question 3

A low pH value means a ___[H+(aq)].

A high pH value means a ___[H+(aq)].

Answer 3

Large

Small

Question 4

A pH change of 1 changes [H+(aq)] by ___ times.

Answer 4

10

Question 5

An acid with a pH of 2 contains ____ times the [H+(aq)] of an acid with a pH of 5.

Answer 5

1000 (10^3)

Question 6

In a strong monobasic acid, how do you calculate

pH.

Answer 6

-[H+(aq)] = [HA(aq)]
A strong monobasic acid has virtually complete dissociation in water so [H+(aq)] is approximately equal to [HA(aq)].
-pH can them be calculated using pH= -log[H+(aq)]

Question 7

How do you work out the pH of a weak acid?

Answer 7

• [H+(aq)] = (square root) Ka x [HA(aq)]

- pH can them be calculated using pH= -log[H+(aq)]

Question 8

What approximation is made when calculating the pH of a weak acid?

Answer 8

-It is assumed that so little of the original has dissociated that the concentration of the acid at eqm. is effectively the same as the concentration of the original amount of acid.

Question 9

Why is the approximation that
[HA(aq)]eqm. = [HA(aq)] undissociated
not exactly true and when can this approximation be used.

Answer 9

• There will be some acid that does dissociate.
• However, if this amount is less than 5% of the total for [HA] undissociated, it is deemed safe to use the approximation.
• If more than 5% dissociates, the approximation cannot be used.

Question 10

How do you calculate Ka of a weak acid and what information do you need?

Answer 10

• pH (can be found using a pH metre)
• Concentration of the weak acid
• Use Ka=[H+(aq)]^2 / {HCOOH(aq)}