chemists please help... how did i just find this out? (1 Viewer)

[C2H6O]

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]

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]

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]

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]

Guys this is the first time ive heard of KHP did yall google this or did i not get taught

 

[Trial&Error]

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]

Guys this is the first time ive heard of KHP did yall google this or did i not get taught

i used khp in my depth study for mod 6. i essentially did a whole prac on this back in term 2 lol

 

[99.95dreams]

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.

 

[C2H6O]

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.

oh that makes a lot more sense now thanks!

 

[wizzkids]

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.