11. Calculating PH of weak and strong acids

Question 1

PH=

Answer 1

-log[H+(aq)]

Question 2

[H+(aq)]=

Answer 2

10 to the power of -PH

Question 3

what does PH value mean

Answer 3

see-saw relationship - one up the other is down
a low PH means large [H+(aq)]
a high PH means a small [H+(aq)]
a PH change of 1 changes [H+(aq)] by 10 times
an acid with PH 2 contains 1000 time the [H+(aq)] of an acid with PH 5

Question 4

calculating PH of strong acid

see paper fc for e.g.

Answer 4

strong monobasic acid- complete dissociation in water

the [H+(aq)] of a strong acid is equal to concentration of the acid

Question 5

calculating the PH of weak acids ***

see paper flash card for e.g.

Answer 5

only partially dissociates
need to work out Ka ([products]/[reactants]
[H+(aq)]= square root of Ka x [HA(aq)]

Question 6

to determine Ka for a weak acid we need

Answer 6

measure PH using PH metre

need concentration of weak acid

Question 7

limitations of scientific approximations***

why is it okay to make approximations i.e. using original concentration of acid when calculating PH of weak acid

Answer 7

despite equilibrium concentration not being used the amount that dissociates from the acid in equilibrium is tiny so doesnt affect results much
however if more than 5% of HA dissociates approximation cant be used