Chemistry Examination Discussion

[Stick]

Is it technically wrong though? :S

[lzxnl]

If you multiply all three Ka's together, you get some absurdly small number.
From memory, it was like 10^-3, 10^-8, 10^-13

And I think we can see that the acidity constant of phosphoric acid is unlikely to be on the order of 10^-24

[Stick]

Grr. These errors will be the end of me. XD

[DefyingGravity]

Couldn't you define Ka as the percentage of products?
So you get the conjugate base of the acid (H3O+), then it goes through another reaction, get more H3O+, then get more? It may be small, but that is definitely not equal to Ka1... I don't know...

[thushan]

Does anyone have a copy of the exam? Thushy wants to get his hands on it

[achre]

Grr. These errors will be the end of me. XD

If it makes you feel any better, I guessed 'A' for almost every MC question on electrochem.

[beastly101]

How did you find the exam? I feel it was pretty easy in comparison, but the increased abundance of worded questions may have increased the potential of chemistry assessors to be pedantic XD I suspect the A+ cutoff will be around ~105/120. What do you guys think?

I think the cutoff will be a little higher, the exam wasn't that difficult in comparison to the others, but who knows how the rest of the state went!! The multiple choice were a little tricky, not too bad though!

[Mafioso]

Why would choosing only k3 be wrong?

[achre]

Why would choosing only k3 be wrong?

Because k3 was tiny, from memory. k1 was the largest value, hence it was the value most composed of reactants (H3PO4) and the best approximation of whatever was being asked.
guys we need this exam, hasn't some other site posted it yet?

[DefyingGravity]

If it makes you feel any better, I guessed 'A' for almost every MC question on electrochem.

What? Did you run out of time?

[achre]

What? Did you run out of time?

I was very, very underprepared.
Sorry for scaling the whole school down.

[lzxnl]

Choosing k3 only takes into account the dissociation of the weakest acid (not including water; HPO4- is almost as weak as water) and does not take into account the dissociation of the strongest acid, H3PO4.

Let's do this quantatively.
I don't have the actual Ka's, so I'm going to go off 10^-3, 10^-8 and 10^-13 as I remember those as the correct order of magnitudes.
Let's assume we have a 1.0 M solution of H3PO4.

Then, by our weak acid assumption, Ka=10^-3 = [H+]^2/1
[H+]=10^-3 =>[H+]=roughly 0.032

Technically this means our assumption is getting close to dodgy, but let's disregard that.
Then, let's do the same assumption for the second dissociation. We have 0.032 M H2PO4-
So, using Ka=10^-8 this time, Ka=10^-8 = (0.032+[HPO4-])[HPO4-]/0.032 M
Note that we can't assume [H+]=[HPO4-] because we already start off with a lot of H+ in comparison to how much HPO4- we'll get. Just look at the Ka of 10^-8
But, we can assume that 0.032 is much greater than [HPO4-], so we have 10^-8=0.032*[HPO4-]/0.032
I think we can see that the concentration of HPO4- is going to be around 10^-8, and thus [H+]=0.032+[HPO4-]=still around 0.032

You can probably see, by now, that the second dissociation isn't going to count much. Similarly, the third dissociation will be even more negiglible.

Generally, for polyprotic weak acids, only the first dissociation really impacts the pH.

[DefyingGravity]

I was very, very underprepared.
Sorry for scaling the whole school down.

Hahahaha. Don't worry about it! I'm pretty sure we have a really strong cohort so as long as your sacs average above 75% I'd say, A+

[Planck's constant]

That was a ridiculously easy exam...

I am not sure you are allowed to say this around here

[Edward21]

If A+ for last year's Unit 3, that felt of similar difficulty to this year's, had 90% A+ then about 108+/120 could be the cutoff?