chem mon manage woes (1 Viewer)

[paper cup]

For the lawn fertiliser prac: I don't get it, why do you need to add HCl to the fertiliser when really all you need is to add the barium to precipitate the sulfate out...why does it need to be acidified?
Salt pHs:
Why are acidic cations the way they are - there is no explanation in the text. And why are there no basic cations?

 

[Dreamerish*~]

For the lawn fertiliser prac: I don't get it, why do you need to add HCl to the fertiliser when really all you need is to add the barium to precipitate the sulfate out...why does it need to be acidified?
Salt pHs:
Why are acidic cations the way they are - there is no explanation in the text. And why are there no basic cations?

I don't think it matters. In Chemistry in Context, the question asked on sulfates in fertiliser said it was dissolved in water.

Anyway, I'm sure we don't have to know why we use HCl.

Acidic cations? What do you mean?

 

[wanton-wonton]

Oh, apparently there are two methods of doing this prac. (I'm not sure.)

By precipitation and by titration. Which one is .... 'better'/easier to learn?

 

[Dreamerish*~]

Oh, apparently there are two methods of doing this prac. (I'm not sure.)

By precipitation and by titration. Which one is .... 'better'/easier to learn?

I would go with precipitation.

I hate titration to bits.

 

[richz]

u use hcl becuz it helps it dissolve the fertiliser. U also use that acid instead of any other one becuz u add BaCl, and since that already has cl it doesnt mix chemicals together

 

[paper cup]

there is a dot point on the pH of salt solutions, and a table in the chem contexts book on acidic/neutral/basic ions - this is, as someone mentioned, a hard dot pt - and the neutral ions come from strong acids and bases, the basic anions come from weak acids...I think, and the acidic anions come from polyprotic acids (why?!) and the acidici cations from weak bases...I hate this dot pt, I am so screwed does anyone know anything about salts containing hydrogen and how they work? And yeah, there are no basic cations, why is this?

 

[Captain Gh3y]

It appears to be because of what they're the conjugate base or acid of. So you can just use the rules,
The conjugate base of a strong acid is an extremely weak base,
And the conugate base of a weak acid is a weak base,
and same way with acid/base switched.

The dotpoint doesn't mean we have to memorise which anions/cations are acidic/basic. (...does it?)

"acidic anions come from polyprotic acids (why?!) "
Well, polyprotic is > 1 H+, so say... Sulfuric acid (most obvious example, but still)
H2SO4 --> H⁺ + HSO4^- so there's an anion that's still acidic, so when an acid gives up a single H+ and it's still got some left, it's an anion that's acidic.

Neutral ions come from strong acid and bases because they're things like... I dunno, Cl^- is the conjugate base of HCl, and applying the rule "the conjugate base of a strong acid is an extremely weak base", so Cl- will be an exteremely weak base, ie. effectively neutral.

Next part... Basic anions coming from weak acids, again we use the rule "the conjugate base of a weak acid is a weak base", like, maybe Acetic acid, it gives the acetate ion, which is a weak base that's basic enough to make a solution... basic. Yeah (it's 2am.)

"acidici cations from weak bases" That'd be from a reaction between a strong acid and a weak base, giving an extremely weak conjugate base and a weak conjugate acid, so the salt has pH < 7...

 

[Libbster]

you add HCl to make sure u precipate out sulfate, not phosphates that are present. if we remember that in dilute acid solutions, barium ions produce a precipitate with sulfate but not phosphate. This is because HPO4 2- (sorry can't get subscript/superscript ) is a much weaker acid than HSO4 -. if we think in terms of equilibrium, the hydrogen phosphate ion is so weak that equilibrium lies far to the left, thus producing insufficient phosphate ions to precipate with barium. thus we ensure accuracy of the experiment, otherwise the result could have been too high!