HSC Chemistry Question Thread

[RuiAce]

Wouldn't it be inappropriate to do a flame test when there is a possibility of toxic chemicals (lead) in the solution?

8. When dilute sulfuric acid was added to an unknown solution, L, a white precipitate was formed.
 The possible ions in the unknown are: sodium, calcium, barium and lead.
 What is an additional test that can be done to a fresh sample to determine the exact identity of the
 ion present?

(A) add silver nitrate
(B) do the flame test
(C) add sodium carbonate
(D) test the pH

*Correct answer was B* (James Ruse 2014 Trial)

Obviously sodium is wrong as otherwise there wouldn't be a precipitate.

Leaving us to distinguish between calcium, barium and lead.

Because we're trying to find a cation, adding nitrate does nothing and so A is wrong.
Adding carbonate will cause all three of them to precipitate, so C is wrong
pH is clearly a dud option; what does that do.

B is the correct answer. Note that firstly, lead is something you're testing for, thus if it were a toxic chemical it would be in such high concentrations to the point the blue/white flame is just obvious.
Note that each of the three remaining metals all have distinct flame test colours.

But at the same time, this is a James Ruse paper and the questions are just weird. Factoring in technicalities, flame tests should be a last resort.

[wesadora]

I see. Hold on...so do sacrificial anodes have to be in physical contact? (e.g. magnesium wrapped around an iron nail)
Or say, could a magnesium and an iron strip be next to each other (on opposite sides) of a petri dish with like...an NaCl electrolyte - and the magnesium would still rust preferentially instead of the iron (forcing iron to be a cathode), or would both just do their own things? O.O

hey - sorry jake i think you missed this question On page 64.

[jakesilove]

hey - sorry jake i think you missed this question On page 64.

Sorry! I definitely missed that No, they don't need to be in physical contact, there just needs to be some sort of electrolyte between them. Physical contact will work, but any sort of method of transfer of ions is enough. So, sacrificial anodes to protect piping underground can use the damp soil as an electrolyte? Does that make sense?

[wesadora]

Sorry! I definitely missed that No, they don't need to be in physical contact, there just needs to be some sort of electrolyte between them. Physical contact will work, but any sort of method of transfer of ions is enough. So, sacrificial anodes to protect piping underground can use the damp soil as an electrolyte? Does that make sense?

got it.

[massive]

whats the difference between hydroxy group and hydroxyl group??

[RuiAce]

whats the difference between hydroxy group and hydroxyl group??

Different names for the same thing.

[massive]

Guys you know how ethanol has a non-polar part and a polar end, that's the same for all alcohols right? And is that also the reason why alcohols have a higher melting and boiling point than their corresponding alkanes/alkenes because of the hydrogen bonding that they inherently contain??

[jakesilove]

Guys you know how ethanol has a non-polar part and a polar end, that's the same for all alcohols right? And is that also the reason why alcohols have a higher melting and boiling point than their corresponding alkanes/alkenes because of the hydrogen bonding that they inherently contain??

Well, Methanol probably doesn't quite have a polar/non-polar end.

Plus, I think the important part of Ethanol is that it is EQUALLY polar and non-polar, due to its size. I would hazard a guess that larger molecules, assuming 1 -OH group, go back to being essentially non-polar.

[sharpeye]

Hey there,
Just to clarify about the batteries part of POM - you only need to know EITHER dry cell/lead-acid cell and one of the other batteries listed, right? ie dry cell + button cell? Could they potentially ask for more than that? Thanks!

[RuiAce]

Hey there,
Just to clarify about the batteries part of POM - you only need to know EITHER dry cell/lead-acid cell and one of the other batteries listed, right? ie dry cell + button cell? Could they potentially ask for more than that? Thanks!

Yep that's correct. One of the two, and one of the list.

[angiezhang9]

Yeah not really, they're pretty much produced the same way. They're just two dot points because you need to be prepared to give examples for each.

Thanks RuiAce

[MysteryMarker]

Hey guys, this is just more of a confirmation for one of the conditions implemented in esterification.

For this reaction, I remember reading that the acid and alkanol are used in a 1:3 ratio even if the mol ratio is 1:1. (e.g. ethanoic acid and ethanol) So is this excess used so that as per LCP the equilibrium will shift to the products side as well as to make the acid a limiting reagent?
Also, does it matter whether the acid/alkanol is used in excess, like is one more expensive than the other?

Cheers.

[RuiAce]

Hey guys, this is just more of a confirmation for one of the conditions implemented in esterification.

For this reaction, I remember reading that the acid and alkanol are used in a 1:3 ratio even if the mol ratio is 1:1. (e.g. ethanoic acid and ethanol) So is this excess used so that as per LCP the equilibrium will shift to the products side as well as to make the acid a limiting reagent?
Also, does it matter whether the acid/alkanol is used in excess, like is one more expensive than the other?

Cheers.

I believe that's right; might wanna get Jake to double check. I do recall a mole ratio favouring the alkanoic acid for the sake of shifting the equilibrium.

I think the acid is chosen because it can be neutralised. The alkanol is harder to eliminate in the final mixture when it's used in excess. So we want the alkanoic acid to be in excess and the alkanol to be limiting.

[nimasha.w]

hi! i don't really understand this question

[RuiAce]

hi! i don't really understand this question

Recall that the relationship between absorbance and concentration is linear. When you plot the absorbance on the y-axis and the concentration on the x-axis, you basically get the line y=mx. (In y=mx+b, b=0 because it's obvious that at 0 concentration there is no absorbance.)

The standard solution has concentration 10ppm and absorbance 400. So if you have a substance with absorbance 500 (=5/4 * 400), then your concentration (of copper ions) will be 12.5ppm.

Then we must identify the reaction that takes place
Cu²⁺ + Na₂CO_3(aq) -> CuCO_3(s) + 2 Na⁺

Now, recall that 1 ppm is really just a fancy way of saying 1mg/L
So here, our concentration of Cu is 12.5mg L^-1
i.e. 0.0125 g L^-1

Since our volume was 100mL = 0.1L, we have our mass of Cu²⁺ being 0.00125g

You should be able to take over here. The rest of the calculations are just typical standard stuff: convert to moles and etc.