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# titration curve watch

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1. can anyone tell how to find the range of ph values of a mixture when its acting like a buffer from the titration curve?

or how to kind Ka using the curve? plzzz
2. (Original post by kriztinae)
can anyone tell how to find the range of ph values of a mixture when its acting like a buffer from the titration curve?

or how to kind Ka using the curve? plzzz
the horizontal region

because when alkali is added, the pH value is almost constant
3. (Original post by keisiuho)
the horizontal region

because when alkali is added, the pH value is almost constant
thats what i thought! but on the mark scheme its wrong! argg
but i believe you coz it makes more sence
how abt ka?
4. how do u calculate the pH of the buffer?
5. (Original post by tammypotato)
how do u calculate the pH of the buffer?
no hehe
dont worry just go study!
6. (Original post by kriztinae)
no hehe
dont worry just go study!
lol no im asking how do u calcul8 the ph of a buffer coz im stuck.
7. (Original post by kriztinae)
thats what i thought! but on the mark scheme its wrong! argg
but i believe you coz it makes more sence
how abt ka?
it must be a titration between a weak acid and a strong base.
From the titration curve, find the pH when half of the volume of the base at the end point is added.
at that point, half of the acid is neutralised.
so [HA] = [A-]
Ka = [H+]
pKa = pH at that point
8. (Original post by tammypotato)
lol no im asking how do u calcul8 the ph of a buffer coz im stuck.
oh lol im good at this!
well
ka = [H+][A-]/ [HA]

you will be given Ka, maybe the conns of A- and HA if not you will be given enough information to calculate them!
then you find [H+] using that equation
then use ph = -long [H+]
got it?
9. (Original post by kriztinae)
oh lol im good at this!
well
ka = [H+][A-]/ [HA]

you will be given Ka, maybe the conns of A- and HA if not you will be given enough information to calculate them!
then you find [H+] using that equation
then use ph = -long [H+]
got it?
there is also an easier way of learning this: u could just use this equation:
ph= pka- log(acid/salt)...
10. (Original post by kriztinae)
oh lol im good at this!
well
ka = [H+][A-]/ [HA]

you will be given Ka, maybe the conns of A- and HA if not you will be given enough information to calculate them!
then you find [H+] using that equation
then use ph = -long [H+]
got it?
yeah i thought it would b more dificult as its buffers lol
11. (Original post by Tanaz)
there is also an easier way of learning this: u could just use this equation:
ph= pka- log(acid/salt)...
cool
12. (Original post by tammypotato)
lol no im asking how do u calcul8 the ph of a buffer coz im stuck.
ok , basically without the graph the hydrogen concentration is equal to ka multiplied by the concentration of the acid/concentration of the salt
PH=-log concentration of H+ ions
13. (Original post by kriztinae)
oh lol im good at this!
well
ka = [H+][A-]/ [HA]

you will be given Ka, maybe the conns of A- and HA if not you will be given enough information to calculate them!
then you find [H+] using that equation
then use ph = -long [H+]
got it?
1) moles acid
2) moles alkali
3) moles acid left = moles acid - moles alkali
4) concentration acid left

Then substitute the values into the Ka=[H+][A-]/[HA] and rearrange for [H+]

With A- being the concentration of alkali added and HA being the concentraion of acid left.

Then work out the pH through logging.

If the alkali is found to be in excess just work out the conctration of the excess and from that work out pOH, with pH = 14 - pOH.
14. (Original post by zazy)
ok , basically without the graph the hydrogen concentration is equal to ka multiplied by the concentration of the acid/concentration of the salt
PH=-log concentration of H+ ions
If you don't know that now you're screwed!
15. hey everyone, I have to go now
good luck!
16. (Original post by kingpong)

If the alkali is found to be in excess just work out the conctration of the excess and from that work out pOH, with pH = 14 - pOH.
Arrrrgh, are they gonna give questions where the damn thing's in excess? *sob*
17. (Original post by keisiuho)
hey everyone, I have to go now
good luck!
arg noo ucant go! ur one of the smart people who can help me!!
ok well good luck!! you be fine!!
Arrrrgh, are they gonna give questions where the damn thing's in excess? *sob*
Alkali in excess is eaaaaaaasy, just work out the conc of the alkali in excess, pOH, then pH, it's plain sailing.

Remember we can only get questions on calculating the pH of acid buffers, not alkaline buffers.

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