• Best of luck to the class of 2025 for their HSC exams. You got this!
    Let us know your thoughts on the HSC exams here

chemists please help... how did i just find this out? (3 Viewers)

C2H6O

Well-Known Member
Joined
Oct 18, 2024
Messages
1,190
Gender
Male
HSC
2025
ok so apparently in a titration you cant make a standard using naoh crystals cause they absorb water... so the weighed crystals would not be 100% naoh.
now what if... youre trying to titrate acetic acid? you have to use a strong base... NaOH and KOH are out of the picture, theyre hygroscopic... Ca(OH)2 is insoluble... tf do you do then? im pretty sure you cant use a secondary to titrate, but if you could... how would you titrate the NaOH? using HCl? that doesnt come in solid form. HNO3? that comes in a hydrated form... so wtf do you do then? every titration ive ever done is a lie
if they ask for the method for titrating some vinegar imma crash out
 

99.95dreams

Well-Known Member
Joined
Jan 11, 2025
Messages
310
Gender
Female
HSC
2025
i dont think naoh needs to be pure. if it is acting as the titrant, it just has to be of known molarity (done through earlier titration with the actual primary standard). so you are right in saying that naoh cannot be a primary standard because it is hygroscopic, but that is why we titrate it first with something like khp. and once it is standardised, then it can be used in a titration with acetic acid - not because it's pure, but because we know the concentration accurately, which is the basis of any titration
 

C2H6O

Well-Known Member
Joined
Oct 18, 2024
Messages
1,190
Gender
Male
HSC
2025
i dont think naoh needs to be pure. if it is acting as the titrant, it just has to be of known molarity (done through earlier titration with the actual primary standard). so you are right in saying that naoh cannot be a primary standard because it is hygroscopic, but that is why we titrate it first with something like khp. and once it is standardised, then it can be used in a titration with acetic acid - not because it's pure, but because we know the concentration accurately, which is the basis of any titration
can i ask if you were taught this?
 

Trial&Error

Well-Known Member
Joined
Nov 18, 2024
Messages
216
Gender
Female
HSC
2025
you use some primary standard to make a secondary standard. So you'll make NaOH secondary standard meaning you make it "fresh" and of an exact known value by titrating it with a non-hygroscopic, non-volatile, highly pure, perfectly known mass primary standard.

for example use KHP which is a common primary standard and titrate it with NaOH to find its exact concentration. And then use it straight away in your titration with the strong acid.
 

C2H6O

Well-Known Member
Joined
Oct 18, 2024
Messages
1,190
Gender
Male
HSC
2025
Guys this is the first time ive heard of KHP did yall google this or did i not get taught 😭
 

Trial&Error

Well-Known Member
Joined
Nov 18, 2024
Messages
216
Gender
Female
HSC
2025
Guys this is the first time ive heard of KPH did yall google this or did i not get taught 😭
You can use any primary standard but KHP is a common one.
I was taught that we normally use secondary standards because they're cheaper and easier to work with for certain chemicals. So you only need a bit of a primary standard like KHP and you can make a lot of a secondary standard.

But secondary standards cannot be stored for long periods of time and need to be made right before the titration. But in school labs we just use ready made chemicals whcih havent been standardised and assume. but in hsc you would write that. edit: I meant "wouldn't"

So you would mention that making the secondary standard using a primary standard will increase validity and stuff as well.

There's also some past hsc questions which standardise the titrant first and you have to include it in calculations. So your basically doing 2 titrations.
 

99.95dreams

Well-Known Member
Joined
Jan 11, 2025
Messages
310
Gender
Female
HSC
2025
that is why many hsc problems have 2 titrations. the first one is to standardise the naoh, and the second one is to use that same naoh as a titrant.
 

wizzkids

Well-Known Member
Joined
Jul 13, 2016
Messages
432
Gender
Undisclosed
HSC
1998
what if you're trying to titrate acetic acid? you have to use a strong base
NaOH is notoriously bad as a primary standard.
As you say in its solid form it is extremely hygroscopic, so you can't accurately measure its mass.
As soon as a solution is prepared it starts to react with atmospheric CO2 so it is slowly being neutralised by carbonic acid.
You don't have to use a strong base to titrate acetic acid.
Anhydrous sodium carbonate NaCO3 is the preferred base for preparing a primary standard before titrating acetic acid.
 

Users Who Are Viewing This Thread (Users: 2, Guests: 1)

Top