[xKurisuh]

Hi, analytical chemistry pleb here.

We've learnt the derivations for pH and pOH. I'm specifically referring to the ones for weak acids and weak bases
But how do you know which one to use in the context of an exercise?

It was never really explained in my textbook how to know which one to derive and then use.
Would anyone be able to explain and give me easy examples on how to know whether to use the formula for a weak acid or a weak base? Do you look at what you start with or do you look at what you have to get, or does it have anything to do with the concentration of your compounds or the pKa / pKb of them?


I hope I was able to get my question / problem across since English isn't my native language.
I apologise for any poor wordings.
Thank you in advance!

[Borek]

Not sure what you mean. Can you give an example?

[xKurisuh]

Not sure what you mean. Can you give an example?

E.g HCl + NaOH -> NaCl + H2O

How am I supposed to know what pH formula to use? Do I calcute the pOH here or the pH

[Borek]

This is not a solution, this is a reaction equation. Reactions don't have pH, solutions do. First, find out what was the reaction result and what the solution contains, then we can talk about how to calculate pH.

[xKurisuh]

Oh ok hi again, I have found two exercises I'm confused about.

1. 10 ml H3PO4 (74 m/m%) + 11,6971g NaOH (87 m/m%) in 990 ml H2O
I'm assuming for this one you use the pH formula for an ampholyte bc you get: H3PO4  +  NaOH  →  NaH2PO4   +  H2O. Altho you do start with an acid (H3PO4) or does that not have anything to do with it?                 

2. 5g Na2CO3 + 250ml H2O
I'm assuming here you use the pOH formula of a weak base to get the pH

[Borek]

1. 10 ml H3PO4 (74 m/m%) + 11,6971g NaOH (87 m/m%) in 990 ml H2O
I'm assuming for this one you use the pH formula for an ampholyte bc you get: H3PO4  +  NaOH  →  NaH2PO4   +  H2O. Altho you do start with an acid (H3PO4) or does that not have anything to do with it?

Start with the stoichiometry, once you know what is present in the solution we will think what to do next.

Not sure what "11,6971g NaOH (87 m/m%)" means, this is rather ambiguous - either 11.6971 g of NaOH (then 87% info needs to be ignored), or 11.6971 g of NaOH solution (then 87% is an important part of the data).

5g Na2CO3 + 250ml H2O
I'm assuming here you use the pOH formula of a weak base to get the pH

Yes, this is a base solution, so pOH is the way to go.

[xKurisuh]

m/m% is the amount of grams in 100 g. It's not used quite often over here either, not sure why we use it. Think it's just to confuse the students.

https://imgur.com/gallery/apipORC?fbclid=IwAR0pTJ9BlGT7fDKQsEZIl7rRU1FSvjS-MJA-ryMGoT7kgILIXhcvU-rvotA

This is what I've done so far and here is where I get stuck, do I add more NaOH to NaH2PO4 now or?

Thanks already for the help

[AWK]

10 ml 74 % H₃PO₄ contains about 0.12 to 0.13 mol H₃PO₄.

[xKurisuh]

So what I calculated for H3PO4 is incorrect? I'm not sure how to do it otherwise since that's the only data we got and are allowed to use

[AWK]

This is a solution. You should know its density (~1.6 to 1.7 g/cm³)

[xKurisuh]

Oh ok I see, weird how that wasn't given, I'll remake it. Thank you!

[xKurisuh]

This is my second attempt, hopefully, it's right, wouldn't know how to do it otherwise

https://imgur.com/gallery/710yxdv?fbclid=IwAR2_xxcLmzVrLuVXSkb8fTEgXqxPzx_lM5XIVu5aooIWSt3rStoM6RKj4eA

[AWK]

You took density for pure solid acid instead of 74 % solution.

[xKurisuh]

You took density for pure solid acid instead of 74 % solution.

I'm not quite sure what you mean I'm afraid, with the density known, this is about the only way I know how to get the answer. How would I take 74% of the solution instead?

[AWK]

It is impossible to measure 10 ml of solid phosphoric acid containing impurities. It is very viscous. Commerciał solutions of this acid contain about max. 80 % of acid.