HSC Chemistry Question Thread

[RuiAce]

But even if it 'drives it to completion' wouldn't you still only need less OH- to completely neutralise the weak acid?

No. Because since it is driven to completion, the weak acid will "ultimately have the same amount of H⁺ to offer", requiring the same volume.

Note that as the NaOH reacts with the weak acid, H⁺ is taken out of the system. But the weak acid still has its own degree of ionisation, which forms a relationship between the concentration of the acid, and the concentration of H⁺.

With H⁺ gone, this degree of ionisation is disturbed. More H will ionise into H⁺ allowing more of it to react with NaOH.

This process repeats itself and ultimately the same amount of H⁺ is needed to be neturalised with the OH^-

[RuiAce]

Can anyone explain why water samples taken from a fast-running stream and from a swap differ in oxygen concentrations? thanks ))

A swamp is full of useless stuff. Oxygen isn't going to dissolve into that water as easily as a stream which just casually flows. We need common sense here.

[RuiAce]

hey! can i please have some help with these questions :-)

Q15 discussed recently in post #966

(Image removed from quote.)

Also im not sure how to do this q, sry about asking so many q haha... thanks

I only have ideas on how to do this one. It's clearly stretching the boundaries of the HSC course but if you could provide the answer then I can explain it. Cause I cannot guarantee an answer yet

[RuiAce]

hey! can i please have some help with these questions :-)

Observe that in Q18, the molar mass is what we would use at the start.
But instead it is what we're trying to find. Hence, we must work backwards.

NaX + HCl -> NaCl + ?

We don't care about the products. We only care about the reactants.
Note that the 1:1 mole ratio between the reactants is correct. This is because whilst it's obvious that HCl only has one proton, the Na⁺ ion has a +1 charge, hinting that the X ion has a -1 charge to balance it out. So if the mole ratio was not 1:1 there would be an imbalance.

n_HCl = 0.1 * (24.4*10^-3) = 0.00244 mol
Therefore n_{NaX in reaction} = 0.00244 mol
i.e. [NaX] = 0.00244/(20*10^-3) = 0.122 mol L^-1

Note that the concentration of the base must be what's fixed. The same base that was prepared was used in the titration, so there's no dilution factor here.

So since the volume at the start was 100 mL
n_{NaX prepared} = 0.122 * 0.1 = 0.0122 mol

Finally, use n = m/M, where m = 1

[RuiAce]

hey! can i please have some help with these questions :-)

The actual calculations for Q20 are straightforward so I'll briefly explain the weird part.

The equal amounts are consumed in each reaction means that 200g went into equation 1 (to produce CO), whereas the other 200g went into equation 2 (to produce CO₂)

So break down the formula, and figure out what moles of what you need.

The last step is just to add your final masses of CO and CO₂

[wesadora]

Two questions:
1. How can you show NaOH acts as a base by reacting it with water, because it just dissociates into Na+ and OH-... but like, you can react HCl with water to show it's an acid (by donating hydrogen): HCl + H20 --> H3O+ + Cl-....
but NaOH is just NaOH --> Na+ + OH- ?

2. See attached. For monitoring ozone concentrations: how is UV 'backscattered' (i.e. reflected off ozone to be detected by the TOMS). I understand how it detects the differences between UV-B (absorbed by ozone) and UV-A/UV-C (passes through ozone, not absorbed & harmless) but...backscattering? o.O

thx

[RuiAce]

Two questions:
1. How can you show NaOH acts as a base by reacting it with water, because it just dissociates into Na+ and OH-... but like, you can react HCl with water to show it's an acid (by donating hydrogen): HCl + H20 --> H3O+ + Cl-....
but NaOH is just NaOH --> Na+ + OH- ?

2. See attached. For monitoring ozone concentrations: how is UV 'backscattered' (i.e. reflected off ozone to be detected by the TOMS). I understand how it detects the differences between UV-B (absorbed by ozone) and UV-A/UV-C (passes through ozone, not absorbed & harmless) but...backscattering? o.O

thx

1. Technically, if you react something with a base it also suffices to be an acid. As if you react with an acid it suffices to be a base. Which is usually preferred: e.g. NaOH is a base as you can react it with an acid like HCl.

But the reaction with water method is also acceptable and you would show that solid NaOH dissociates into its ions. i.e. You must include the state change from (s) to (aq)


And there is no attachment.

[massive]

for the cyclohexane/cyclohexene practical, are we meant to use liquid bromine (Br₂) or bromine water (BrOH). Also is liquid bromine aq?

[RuiAce]

for the cyclohexane/cyclohexene practical, are we meant to use liquid bromine (Br₂) or bromine water (BrOH). Also is liquid bromine aq?

Liquid bromine is liquid. That's why it's called liquid bromne.

Technically you're using bromine water, which is prepared by reacting, guess what, bromine and water.

But when I asked my chemistry teacher, for the chemical equations you're allowed to write it with either bromine OR bromine water.

[wesadora]

1. Technically, if you react something with a base it also suffices to be an acid. As if you react with an acid it suffices to be a base. Which is usually preferred: e.g. NaOH is a base as you can react it with an acid like HCl.

But the reaction with water method is also acceptable and you would show that solid NaOH dissociates into its ions. i.e. You must include the state change from (s) to (aq)


And there is no attachment.

I see. Also, edited the post now - should be attached

[bethjomay]

For the batteries dot point, do we need to know the anode and cathode as well as the overall?

[RuiAce]

For the batteries dot point, do we need to know the anode and cathode as well as the overall?

Well if you know the half equations for the anode and cathode you can just derive the overall equation on the spot.

But you definitely need to know the half equations at the least, because the dot point mentions the chemistry of the cell.

[jakesilove]

Two questions:
1. How can you show NaOH acts as a base by reacting it with water, because it just dissociates into Na+ and OH-... but like, you can react HCl with water to show it's an acid (by donating hydrogen): HCl + H20 --> H3O+ + Cl-....
but NaOH is just NaOH --> Na+ + OH- ?

2. See attached. For monitoring ozone concentrations: how is UV 'backscattered' (i.e. reflected off ozone to be detected by the TOMS). I understand how it detects the differences between UV-B (absorbed by ozone) and UV-A/UV-C (passes through ozone, not absorbed & harmless) but...backscattering? o.O

thx

Hey! I'm not going to give you a proper answer, because at this stage you don't need one. It isn't important, at all, to understand how TOMS works. You just need to cite off some fancy words that sound right (exactly as you've shown us in the attachment), and that will get you full marks 100% of the time. It's always an 'outline' question, not 'explain', as you're not supposed to understand how the mapping works. So, learn what you've shown us, and move on!

[captainclank]

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)

[jakesilove]

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)

You're absolutely right, and you'll never be given an HSC question where you need to burn Lead! Trial papers are never as good/accurate as HSCs, so I would be focusing on HSC papers from now on