HSC Chemistry Question Thread

[RuiAce]

so any inert metal can have any type of gas but it depends on what the electrolyte is? eg. Pt metal reducing Hydrogen ions to hydrogen gas

Yeah that's right. It depends on what the electrolyte is.

[amandali]

im still not 100% sure in terms of rounding


so for b)  is it necessary to write the actual number of moles (line 2) or do i just need to write number of moles rounded to 3 sig fig only?

i was taught to use variable like x or y to show examiner that im using the actual numbers not the rounded one. So for part c)  will it be wrong if i write in the rounded number (but using the actual number if im not using variables)?
And should the value for n(HCl) initial and n(HCl remaining) (not the answer but part of the working) be in 3 sig fig?

[RuiAce]

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im still not 100% sure in terms of rounding


so for b)  is it necessary to write the actual number of moles (line 2) or do i just need to write number of moles rounded to 3 sig fig only?

i was taught to use variable like x or y to show examiner that im using the actual numbers not the rounded one. So for part c)  will it be wrong if i write in the rounded number (but using the actual number if im not using variables)?
And should the value for n(HCl) initial and n(HCl remaining) (not the answer but part of the working) be in 3 sig fig?

In your working out, you are meant to use the maximum amount of sig figs possible.

When it comes to answer, you look at the data you used in that question or in advance to arrive at your answer. The lowest number is your s.f. value.

For b), you need to write both. This is what you do.
1. Punch the numbers into the calculator
2. Write down your unrounded answer (don't forget units - I know you didn't here)
3. KEEP that unrounded answer in your calculator if you need it for later
4. Write down the rounded answer (3 s.f.) remembering the units again
5. For the next part, use your unrounded answer.

That let = x thing is up to you. Writing an unrounded answer can just take a lot of time, which is the benefit of introducing an 'x'. I never did though.

[amandali]

ques: explain nature of X^(-2) ion using the graph     
i dont understand why the answer says it s a moderately weak conjugate base




shouldnt this be A since  bromo needs to be mentioned before chloro  (alphabetical order) or is it by arragrment from carbon 1

[jakesilove]

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ques: explain nature of X^(-2) ion using the graph     
i dont understand why the answer says it s a moderately weak conjugate base


(Image removed from quote.)

shouldnt this be A since  bromo needs to be mentioned before chloro  (alphabetical order) or is it by arragrment from carbon 1

For your first question, I wouldn't have said it was a conjugate base either; but we can tell that it's a weak base. If we titrate a strong acid with a strong base, we expect the equivalence point to sit at about a pH of 7. Strong acid + weak base = pH of 4-5. Strong base + weak acid = pH of 8-10. Therefore, based on the graph (and since NaOH is a strong base), H2X is going to be a weak acid!'


As for your second question, in my mind anyway, it should definitely be A based on HSC rules. I think the answer may be wrong!

[RuiAce]

Yeah that's A. Something's up about that one.

[conic curve]

A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula for this compound?

[RuiAce]

A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula for this compound?

Compare the percentage composition to the molar masses of the relevant substances for the sake of this question. The comparison should be done so using division, because that's the only way we can get something useful.

Note that molar masses tell us which atoms have more mass on a broader scale.

Mg: 23.3%/24.31 = 0.958% = approx 1%
S: 30.7%/32.07 = 0.957% = approx 1%
O: 46%/16.00 = 2.875% = approx 3%

Comparing ratios - Ratio of Mg to S to O is
Mg:O:S
1:1:3 (approximately)

Therefore i predict it to be MgSO₃.

A bit poorly worded. Feel free to point out confusion.

[jakesilove]

A compound is found to contain 23.3% magnesium, 30.7% sulfur and 46.0% oxygen. What is the empirical formula for this compound?

Hey! So, essentially we just need to compare their masses and find some values that make approximate sense. Let's say that the formula is



We know that mm(Mg)=24.3g, mm(S)=32.1g, mm(O)=16g. So, what can we tell from all this?





This is just the maths that pops out of the info we know. Whilst is COULD be helpful, there's an easier way. Notice that there is twice as much Oxygen as there is Magnesium (approximately). How can we get a ratio of Mg:O that is 1:2 by mass? Well, if we take three atoms of Oxygen, that will weight 48g. Half of 48g is 24g, which is basically the 24.3g we need! So, the empirical formula LOOKS LIKE it will take three moles of Oxygen for one more of Magnesium.

Now, to work in the sulfur. The mass needs to be between the 24.3g of Magnesium, and the 48g of Oxygen. Well, looks like it already is! If we double the moles of Sulfur, the mass will be GREATER than that of Oxygen, which we don't want. Okay, so let's check out what we've discovered and see if it makes sense.



The total mass of the above is 104.4g. For Mg, 24.3/104.4=0.23, exactly as much as desired. For Sulfur, 32.1/104.4=0.307, exactly as expected. Finally, for Oxygen, 3*16/104.4=0.46, exactly as expected.

You'll note that there was a level of intuition in the above answer. I remember there being a move definitive way; let me think for a bit, and I'll try remember it!

Jake

[jakesilove]

Compare the percentage composition to the molar masses of the relevant substances for the sake of this question. The comparison should be done so using division, because that's the only way we can get something useful.

Note that molar masses tell us which atoms have more mass on a broader scale.

Mg: 23.3%/24.31 = 0.958% = approx 1%
S: 30.7%/32.07 = 0.957% = approx 1%
O: 46%/16.00 = 2.875% = approx 3%

Comparing ratios - Ratio of Mg to S to O is
Mg:O:S
1:1:3 (approximately)

Therefore i predict it to be MgSO₃.

A bit poorly worded. Feel free to point out confusion.

And that would be the more definitive way

[conic curve]

Sorry to get a bit off track but how do you study for sciences like Phys and chem?

I know english is all about writing practice essays, memorising quotes, practicing adapting, reediting your essay/creative etc. I know maths is all about practice and learning from your mistakes through past papers but what about Chemistry and Physics?

[RuiAce]

Sorry to get a bit off track but how do you study for sciences like Phys and chem?

I know english is all about writing practice essays, memorising quotes, practicing adapting, reediting your essay/creative etc. I know maths is all about practice and learning from your mistakes through past papers but what about Chemistry and Physics?

Same as maths.

Just that you're practicing how to write 6 mark responses and mark maximisation instead of a bunch of equations

[conic curve]

Same as maths.

Just that you're practicing how to write 6 mark responses and mark maximisation instead of a bunch of equations

Oh...what about looking through notes?

[RuiAce]

Oh...what about looking through notes?

Oh right. Notes actually do contribute in some way or another in physics/chemistry.

Whereas maths you just need to jump into past papers, once you get into HSC you'll find you may spend some time revising notes and gradually shift into past papers.

[conic curve]

Oh right. Notes actually do contribute in some way or another in physics/chemistry.

Whereas maths you just need to jump into past papers, once you get into HSC you'll find you may spend some time revising notes and gradually shift into past papers.

I thought for maths, notes are helpful at first when you're getting introduced to new abstract ideas and then over time you don't need it as much

Notes, I thought were only good for basic understanding for physics and chemistry and over time, it wouldn't be needed as much