HSC Chemistry Question Thread

[kiwiberry]

Qu: is a solution made using equal volumes of sulfuric acid and sodium sulfate a buffer solution and why?

Also, I'm not sure if there is such a thing as a brief rundown/explanation on titrations, but if there is, would anyone please mind helping me out? I'm pretty confused by the whole deal! Thanks

Hey, sorry for the wait!!
A solution made with sulfuric acid and sodium sulfate would not be a buffer solution because 1) a buffer must be made using a weak acid and 2), sulfate is not the conjugate base of sulfuric acid.

This is a brief outline of the steps in titration, does this help a bit?

Maybe I'll try with a different question
I'm really stuck with this and if someone could possibly help me that would be awesome!!

The sulfate concentration in a sample of water can be determined gravimetrically by:
- first adding acid to the sample to react with carbonate ions
- then adding excess barium ions
- filtering off, drying and weighing the BaSO4 precipitate formed.

Barium carbonate is insoluble. Adding acid before adding barium ions prevents BaCO3 precipitating with the BaSO4. Write a balanced equation for the reaction in which H+ ions destroy a carbonate ion releasing a gas.

Thanks so much!!

The equation will be
The gas released will be carbon dioxide

[kiwiberry]

What is a valid equation to explain addition polymerisation?

I've tried to use Ethene(s) monomers --> Polyethylene through

n(C₂H₄) --> (C₂H₄)_n but I'm not sure if it's quite correct.

Can someone please provide a valid equation?

Much appreciated!!!! 

That equation is perfectly fine to use to describe the overall process!! If you want, you can also use these equations for initiation and propagation

[kiwiberry]

Hi, just a quick question on graphing.

If the question does not specify, do we draw a line of best fit or join the dots? I've gotten mixed messages across the years so I'm not entirely sure.
And also same with curves of best fit.

Thanks!

I would say draw a line/curve of best fit if they don't specify

[LaraC]

Awesome - thanks kiwiberry!!

[mxrylyn]

Hey!

Does Polystyrene polymerisation need a free radical/ initiator?

[kiwiberry]

Hey!

Does Polystyrene polymerisation need a free radical/ initiator?

I believe it does - but you don’t need to know about this

[kiwiberry]

Hey!

Does Polystyrene polymerisation need a free radical/ initiator?

I believe it does - but you don’t need to know about this

[arii]

I'm not sure about part (ii) of both. I'm pretty sure I did the identification right in (i).

[Calley123]

Hey,
Can someone please explain Q28 c) of 2008 to me??
- why is the titre ( 0.02345L ) used to calculate the moles of Na2CO3 ( i think this is standard solution- thus, the known conc.)  rather than 0.025 L ?
Im confused about entire question since I always thought that the unknown conc. of solution  was the titrant and known conc.. the standard solution???


http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2008exams/pdf_doc/chemistry-notes-08.pdf

Thanks

[Mate2425]

Hi could someone please tell me the cathode (reduction) and anode (oxidation) half equations for the  Dry Cell and the Fuel Cell.
Many thanks 

[RuiAce]

Hi could someone please tell me the cathode (reduction) and anode (oxidation) half equations for the  Dry Cell and the Fuel Cell.
Many thanks 

I found the dry cell one with a Google search. But I have no clue with the fuel cell because I swear there's more than one of them.

Hey,
Can someone please explain Q28 c) of 2008 to me??
- why is the titre ( 0.02345L ) used to calculate the moles of Na2CO3 ( i think this is standard solution- thus, the known conc.)  rather than 0.025 L ?
Im confused about entire question since I always thought that the unknown conc. of solution  was the titrant and known conc.. the standard solution???


http://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2008exams/pdf_doc/chemistry-notes-08.pdf

Thanks

0.025L was the volume of the acid, not the base.

In this experiment, presumably 25 mL of acid was in the conical flask, and it was the Na₂CO₃ base that was in the burette. (Else, the words "average titration volume" would be bs.) So whereas you use 0.025L to calculate the volume of HCl, you'd need 0.02345L for the volume of the Na₂CO₃

[beeangkah]

Hi could someone explain how to get the answer for this?

Q:
A student tests some solid metal oxides. All 4 oxides react with sulphuric acid. Only one of the oxides reacts with sodium hydroxide. This oxide could be:
a) Al₂O₃
b) MgO
c) Fe₂O₃
d) BaO

Thanks!

[kiwiberry]

Hi could someone explain how to get the answer for this?

Q:
A student tests some solid metal oxides. All 4 oxides react with sulphuric acid. Only one of the oxides reacts with sodium hydroxide. This oxide could be:
a) Al₂O₃
b) MgO
c) Fe₂O₃
d) BaO

Thanks!

Hey!!
For the oxide to react with NaOH, it has to be either an acidic oxide (reacts with bases) or amphoteric oxide (reacts with both acids and bases). MgO, Fe2O3 and BaO are all basic oxides (left side of the periodic table), so won't react with NaOH. Al2O3, however, is an amphoteric oxide, meaning it will react with NaOH, so the answer is A

[RuiAce]

I'm not sure about part (ii) of both. I'm pretty sure I did the identification right in (i).

Taking a stab at this, for part a) you could use an acetic acid/acetate ion buffer, whilst for b) you could use a hydrogen carbonate ion/carbonate ion buffer. Because I reckon the former would be acidic by nature whilst the latter is basic by nature, yet you're still putting in an acid with its conjugate base.

(The idea is that for B, both \(HCO_3^-\) and \(CO_3^{2-} \) are basic, so they'd show some resistance over the basic range. But you'd still need an acid-base conjugate pair. Whereas for A, presumably \(CH_3COOH\) is more acidic than \(CH_3COO^-\) is basic, so it'd probably be more stable on the acidic side of things. Although tbh, I asked the chemistry team and we really thought that this wouldn't be necessary for the HSC.)

[Calley123]

I found the dry cell one with a Google search. But I have no clue with the fuel cell because I swear there's more than one of them.
0.025L was the volume of the acid, not the base.

In this experiment, presumably 25 mL of acid was in the conical flask, and it was the Na₂CO₃ base that was in the burette. (Else, the words "average titration volume" would be bs.) So whereas you use 0.025L to calculate the volume of HCl, you'd need 0.02345L for the volume of the Na₂CO₃

Thanks !