HSC Chemistry Question Thread

[MisterNeo]

Can someone pls explain why for q 12 2015 hsc the answer is a and not d?

Hey
The top number represents the mass number, which is the total protons plus neutrons. The bottom number represents atomic number, which is the total protons.
Since the plutonium-239 gains X to become plutonium-241, we know that the protons haven't changed because it's still plutonium. So it must have gained neutrons to increase its mass number by 2.
Now plutonium-241 is unstable (too many neutrons), and decays into americium-241 by changing a neutron to a proton and an electron.
Think of a neutron as a proton combined with an electron (they cancel each other). To become a proton, it kicks out the electron to become positive. _{(This isn't scientifically correct but it's how I remember it)}
Hence, the answer is A because X is a neutron and Y is an electron.
Hope this helps

[Mathew587]

Oh right I see. It pretty much undergoes neutron bombardment. Thanks

[justwannawish]

Hi!

Not too sure about the first question, sorry!

- Separating a variety of mixtures
- Intermolecular(dispersion/dipole-dipole/hydrogen) and Intramolecular(ionic/covalent/metallic) forces
- Development of the Periodic Table
- SOLUBILITY OF SALTS
- Diamond/Graphite comparison

Hope this helps!

Thank you for your help!

Is there a generally standard way to set out moles questions? With or without limiting reagents

Mod edit: Formatting

[Natasha.97]

Thank you for your help!

Is there a generally standard way to set out moles questions? With or without limiting reagents

Mod edit: Formatting

Hi!
Usually, the question gives values that allow the moles to be determined for one particular species. I would set it out with the general formula (m/M), then the molar ratio between the known substance and the unknown. Refer to the questions below:

If 15.65g of Calcium Carbonate was completely decomposed by heat, what volume of carbon dioxide gas would be produced if measured at SLC?

Spoiler

-Equation: CaCO_3(s) → CO_2(g) + CaO_(s)
- Mass (g) = 15.65g
- Molar Mass of CaCO₃ = 40.08 + 12.01+ 48 = 100.09
- Moles (n) = m/M = 15.65/100.09 = 0.1564 mol
- n(CaCO₃) : n(CO₂) = 1 : 1, so n(CO₂) = 0.1564 mol
∴ Volume of CO₂ = 0.1564 × 24.8 = 3.88L at SLC

What volume of hydrogen gas at SLC would be produced if 10.00g of lithium metal was reacted with sulphuric acid?

Spoiler

Equation: 2Li_(s) + H₂SO_4(s) →H_2(g) + Li₂SO_4(aq)
- Mass (g) = 10.00g
- Molar Mass of Li = 6.941g/mol
- Moles (n) = m/M = 10.00/6.941 = 1.441 mol
- n(Li) : n(H₂) = 2 : 1, so n(H2) =  1.441/2 = 0.7204
∴ Volume of H₂ = 0.1564 × 24.8 = 3.88L at SLC

Hydrated sodium carbonate is represented by the formula Na₂CO₃ ∙ xH₂O, where x is an integer. When 100.00 g of this compound was heated strongly to drive off the water, the anhydrous compound left behind had a mass of 37.04g. Calculate the number of water molecules, x, in one mole of hydrated sodium carbonate.

Spoiler

                          Na₂CO₃ ∙ xH₂O → Na₂CO_3(s) + xH₂O_(g)
Water
- Mass (g) = 100.00 – 37.04 = 62.96g
- M (H₂O) = (2 × 1.01) + 16.00 = 18.02g/mol
- Moles (n) = m/M = 62.96/18.02 = 3.495 mol

Sodium Carbonate
- Mass (g) = 37.04g
- Molar Mass (M) = (2 × 22.99) + 12.01 + (3 × 16.00) = 105.99g/mol
- Moles (n) = m/M =  37.04/105.99 = 0.349 mol

Ratio
n(H₂O) : n(Na₂CO₃) = 3.495 : 0.349 = 10 : 1
x = 10
∴ 10 moles of water in one mole of hydrated sodium carbonate

Hope this helps

[caprese_]

Hi,
answer is iii and iv. Why are there no hydrogen  bonds between the molecules?
Ty v much

[MisterNeo]

Hi,
answer is iii and iv. Why are there no hydrogen  bonds between the molecules?
Ty v much

Hey

Hydrogen bonding only occurs if two molecules have a hydrogen atom bonded to a highly electronegative atom (N, O, halogens).
Ethanol has a hydrogen bonded to an oxygen, which is partially positive because the oxygen is more electronegative and will pull the electrons closer to itself.
CHF₃ has its hydrogen bonded to a carbon, which has almost the same electronegativity as hydrogen. This doesn't allow hydrogen bonding since the hydrogen isn't positively charged and will not attract to ethanol's oxygen.

Basically, you look at what the hydrogen is bonded with.

[winstondarmawan]

Hello!
For the production of LDPE, I have come across some sources which say it is created with the absence of a catalyst, and other sources which say it is created with an organic catalyst.
Which is more correct and should be used in the HSC?
TIA.

[Natasha.97]

Hello!
For the production of LDPE, I have come across some sources which say it is created with the absence of a catalyst, and other sources which say it is created with an organic catalyst.
Which is more correct and should be used in the HSC?
TIA.

Hi!

I've asked this exact question to my teacher, and they said that there is no catalyst in LDPE production. A peroxide initiator e.g. benzoyl peroxide is used, but it is not a catalyst as it becomes part of the polymer. In the production of HDPE, a Ziegler-Natta catalyst is used e.g. Titanium Chloride.

Hope this helps

[anotherworld2b]

I am not sure how to do this question. Can I have help please?

[VanillaRice]

I am not sure how to do this question. Can I have help please?

n(NaOH) = cV = 0.0126 x 0.390 = 0.004914 mol
n(sulfuric acid) = 1/2 x n(NaOH) = 0.002457 mol
Dilution factor is 1000/25 = 40
n(sulfuric acid)_intial = 0.024578 x 40 = 0.09828 mol
m(sulfuric acid) = 98g/mol x 0.09828 mol = 9.63144g
So, there was 9.63144g of actual sulfuric acid

Total mass = 19.6mL x 1.67g/mL = 32.732g

So, % mass of sulfuric acid = 9.63144/32.732 x 100 = 29.4%

Hope this helps

[winstondarmawan]

Hello!
Can someone go through the thought processes for answering a question like Q1.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/21291632_1316215258503994_317323894_n.jpg?oh=658bc156d33a2b45eb8b183a5415ef5c&oe=59AE7C18
TIA.

[MisterNeo]

Hello!
Can someone go through the thought processes for answering a question like Q1.
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/21291632_1316215258503994_317323894_n.jpg?oh=658bc156d33a2b45eb8b183a5415ef5c&oe=59AE7C18
TIA.

Hey
For questions like these, there isn't really a specific approach but you would need to know that:
-Addition monomers are usually unsaturated (double/triple C-C bonds).
-Condensation monomers are usually saturated (single C-C bonds)
So the answer can't be C.

After that, my main approach is to do trial-and-error to see what works, like try doing a quick sketch or do it visually. Also note that the monomers alternate each time. It just takes practice. 

[winstondarmawan]

Are we required to know the structural two-step reaction which shows how sulfuric acid acts as a catalyst?
(For hydration of ethene and dehydration of ethanol.)
Edit: Why do we need a water reaction medium for fermentation?
TIA.

[MisterNeo]

Are we required to know the structural two-step reaction which shows how sulfuric acid acts as a catalyst?
(For hydration of ethene and dehydration of ethanol.)
Edit: Why do we need a water reaction medium for fermentation?
TIA.

In the HSC course, we don't need to know how it reacts with the catalyst. All you need to know is what the catalyst is.
Water is used because:
-Moist conditions
-It dissolves glucose because yeast is a powder form and can't do anything to another solid.
-Water dilutes the ethanol produced because yeast dies in high conc. of ethanol.

[Aussie1Italia2]

Hi,

Can you please explain to me what validity is again? I'm trying to answer a past HSC paper question and its asking me to discuss the validity of a student's investigation.

Thank you!!!