HSC Chemistry Question Thread

[noonedoesnt]

why is it that the strength of an acid/base does not affect the volume required for titration (rather, the concentration of the acids affects the volumes)? E.g. if both acid/base solutions are 0.1M it does not matter whether one or the other is weak or strong - the same volumes for neutralisation will be required.

This makes sense calculation wise for tritrations to me (n=cv), but, I thought that for neutralisation, it's based on H⁺ + OH^- --> H₂O, and the concentration of H+ in weak acids is less than the concentration of the acid, wouldn't this affect the volume neeeded for neutralisation?

but it doesn't. ._.

I believe in a strong acid + strong base the equal moles of H+ and OH- neutralise, and the reaction goes to completion.

However in a weak acid + strong base, the equation is an equilibrium equation and the strong base forces the creation of more H+ as it reacts with OH-, thus the same volumes are required.

That's what I think anyways, Rui will be able to elaborate a bit more.

[RuiAce]

why is it that the strength of an acid/base does not affect the volume required for titration (rather, the concentration of the acids affects the volumes)? E.g. if both acid/base solutions are 0.1M it does not matter whether one or the other is weak or strong - the same volumes for neutralisation will be required.

This makes sense calculation wise for tritrations to me (n=cv), but, I thought that for neutralisation, it's based on H⁺ + OH^- --> H₂O, and the concentration of H+ in weak acids is less than the concentration of the acid, wouldn't this affect the volume neeeded for neutralisation?

but it doesn't. ._.

This is the exact same reaction that occurs in all titrations. Which is why we never use a weak base and weak acid because that will cause concentrations to matter too much.

At the instant one of the species is strong, things happen. Note that the weak one, say, weak acid, does not fully ionise by itself.

But what happens when you add a strong base? The OH-'s there are fully ionised. Clearly the OH- will react with the H+.

Thus the weak acid lost H+. Hence by Le chatelier's principle, the system will want to replace the most H+, thus more of it gets ionised.

But this H+ also gets used by the OH-. Hence the weak acid is ultimately forced to keep ionising until all the OH- is gone. So ultimately the H+ gets forced into ionising completely because the base is strong and is, in a way, "hydroxide abundant"

[wesadora]

ohhhhhhhh! So they strong reagent forces the other to ionise more than it normally would, so a complete neutralisation can occur? And the only that changes is the equivalence point (e.g. weak acid strong base ph at equiv. > 7).
legends

[kimmie]

Hey for these types of Qs, how do you know that 0.16 turns to -0.16?
I uploaded what I did but the answer is 1.20 V

[wesadora]

Hey for these types of Qs, how do you know that 0.16 turns to -0.16?
I uploaded what I did but the answer is 1.20 V

(Image removed from quote.)



(Image removed from quote.)

remember, the table is of standard reduction potentials. Since SO2 is being oxidised, its -0.16V (reduction potential of SO4^2- is 0.16V, as you read off the table, but in this case the reaction is SO2 being oxidised (backwards reaction to what's shown on your reduction table- so SO₂ + 2H₂O --> SO₄^2- + 4H⁺ + 2e^-...and so you flip the sign and it's -0.16V). 1.36V + (-0.16V) = 1.2

[JemexR]

Just a quick converting question

How do you convert mg/L -> mL?

It is the 2009 Multiple choice Q 15

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf

[ProfLayton2000]

Is it true that nuclear reactors are used to make radioisotopes with atomic number <= 95 and accelerators (cyclotrons and linear accelerators) make radioisotopes with atomic number > 95?

[wesadora]

Just a quick converting question

How do you convert mg/L -> mL?

It is the 2009 Multiple choice Q 15

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf

for that question, you gotta convert mg/L to mg/10L (as per the Q) and so times mg by 10. Convert mg to g (times by 0.001). use n=m/M to find number of moles (M = molar mass of oxygen, m = mass of oxygen in grams, which you worked out before).
Then, use V = n x 24.79, to find the volume in Litres. Convert Literes to mL. It's tricky, and not a straight conversion - but that's the process and it just requires you to know how your equations all link together REAAAALLY well.

[wesadora]

Is it true that nuclear reactors are used to make radioisotopes with atomic number <= 95 and accelerators (cyclotrons and linear accelerators) make radioisotopes with atomic number > 95?

Nuclear reactors are used for neutron bombardment, which can be used for elements up to atomic number 98 (californium). Accelerators can be used for ANYTHING - you can make commercial radioisotopes less than atomic number 95 in linear accelerators, e.g. Nitrogen-14 + Helium-4 --> Fluorine-18. But, you can also use them to collide larger nuclei to make super large nucleuses (and pretty much all transuranic elements) beyond atomic number 98 that can't be produced in nuclear reactors (by neutron bombardment).

[RuiAce]

Is it true that nuclear reactors are used to make radioisotopes with atomic number <= 95 and accelerators (cyclotrons and linear accelerators) make radioisotopes with atomic number > 95?

You can use whatever you want to make <=95 but for >95 you're bombarding too many protons and you're kinda forced to use accelerators. Because how are you going to get some element with atomic number 113 to stay there without decaying whilst you bombard with neutrons.

Though nuclear reactors would be more common than accelerators for <=95 I suppose

[JemexR]

For Q 18 of the 2010 paper, why is the answer C?

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2010exams/pdf_doc/2010-hsc-exam-chemistry.pdf

For D : When the Ammonium and Chloride dissociate, wouldn't the positively charged ammonium ion ionise with the hydroxide in the water, creating water and ammonia?

And for HSC 2006 Q8, given that they have the same concentration, wouldn't that mean that they are equally dilute/concentrated?

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2006exams/pdf_doc/chemistry_06.pdf

[proficles]

Hi, this 2005 question has the answer as B, which i agree with. However i don't understand why option D is wrong, do electrons not flow through the voltmeter? Thanks in advance

[kimmie]

Hey for this, I understand it's an exothermic reaction so the answer can be B or D only but how is the answer D? Do we look at the moles or something?

[lha]

Just wondering, what raw mark do you have to get for it to scale to 90?

[RuiAce]

Just wondering, what raw mark do you have to get for it to scale to 90?

Use the raw marks database to see historic data.