HSC Chemistry Question Thread

[J.B]

Thanks MisterNeo,

I was also wondering, if you receive a question like this
"A student added 1.71g of barium hydroxide to 50.0mL of distilled water. After stirring to dissolve all the solid, the student then added more distilled water to make the final volume 100.0mL of solution. Calculate the pH of the final solution"

And my answer is 13.3 pH. I am a bit confused with how many significant figures to answer to this, as is the 100.0 mL to 1s.f? Meaning that my answer should also be to 1s.f as this is the lowest s.f in the answer?

[RuiAce]

Hey! When writing a molecular formula, the general formula for alkanes is C(n)H(n+2) and for alkenes C(n)H(2n), isn't it? When something is cyclo, does it change it to alkanes: C(n)H(2n) and alkenes: C(n)H(2n-2)? Very confused  thank you!!

Yeah. The reason why it goes down is (informally) due to the fact that you don't have a straight line molecule, but rather a looped one where the ends are joined.

Thanks MisterNeo,

I was also wondering, if you receive a question like this
"A student added 1.71g of barium hydroxide to 50.0mL of distilled water. After stirring to dissolve all the solid, the student then added more distilled water to make the final volume 100.0mL of solution. Calculate the pH of the final solution"

And my answer is 13.3 pH. I am a bit confused with how many significant figures to answer to this, as is the 100.0 mL to 1s.f? Meaning that my answer should also be to 1s.f as this is the lowest s.f in the answer?

Assuming that your calculations are right, it would be to 3 s.f.

100.0mL is correct to 4 s.f.
This is because at the moment you see a 0 after the decimal point, you know it is significant. There's only ambiguity if all the 0's are before the decimal point.

[smshs2017]

Hello, i was past papers and i realised i need help with naming compounds with their proper IUPAC/ systematic name. i have attached an example of a quetsion. if someone could please explain to me how this works, it would be greatly appreciated!!
Thank you.

[MisterNeo]

Hello, i was past papers and i realised i need help with naming compounds with their proper IUPAC/ systematic name. i have attached an example of a quetsion. if someone could please explain to me how this works, it would be greatly appreciated!!
Thank you.

Hey! This area of chem is always very confusing, so glad you asked!
Numbering priority goes to the most electronegative group
So fluorine is most electronegative, so you start counting from the left. (2)
Naming priority is alphabetical, ignoring prefixes like meth-, di-, etc.
So chloro comes before fluoro in the dictionary, thus chloro comes first.
The final answer would be 3-chloro-2-fluorobutane.

[phoebegresham]

Hello!! I was just wondering if I could please get some help with this question! Thanks

'A manufacturer claims his vinegar is 98% pure acetic acid with no additives. 25 mL of this vinegar was diluted to make 250 mL solution. 25.00mL aliquots were titres against a standardised 0.1086 M sodium hydroxide solution. The following tires were obtained during the investigation:
1. V(NaOH) (mL) = 26.25
2. V(NaOH) (mL) = 27.25
3. V(NaOH) (mL) = 27.30
4. V(NaOH) (mL) = 27.20
Calculate the concentration of the original acetic acid and evaluate the manufacturer's claims.'

Thank you!

[J.B]

I was just wondering in the question
 
"Compare TWO technologies used to detect and/or measure nuclear radiation."

Apart from describing the way the two technologies work, would the similarities and differences between say Geiger Counter and Scintillation counter just be:

Similarities: Both can measure the amounts of ionising radiation such as alpha and beta particles, and gamma radiation.
Differences: Their methods, and sensitivity are different.

And is there any way to expand on the above details?

Also I was wondering if someone could tell me there answer for this question:

"5.0g of anhydrous calcium carbonate was reacted with 150mL 0.1 mol.L -1 nitric acid. If the reaction was carried out at 25 o C and 100kPa, calculate the volume of gas that would be produced. And then calculate the pH of the final solution, once the reaction is complete."

As I don't have the answers for this question, but my answer was 0.19L and pH1.

Thank you.

[J.B]

Also, just another question,
For a polymer question when drawing polyvinylchloride and polystyrene, is it best to alternate the sides the chlorine and benzene are on due to their electronegativity forcing them to repel each other. Or just draw them on the same side (i.e bottom or top) to show that the monomer is a repeating unit?

Also can addition polymerisation be between different monomers like condensation polymerisation?
Thanks.

[MisterNeo]

I was just wondering in the question
 

Spoiler

"Compare TWO technologies used to detect and/or measure nuclear radiation."

Apart from describing the way the two technologies work, would the similarities and differences between say Geiger Counter and Scintillation counter just be:

Similarities: Both can measure the amounts of ionising radiation such as alpha and beta particles, and gamma radiation.
Differences: Their methods, and sensitivity are different.

And is there any way to expand on the above details?

Also I was wondering if someone could tell me there answer for this question:

Spoiler

"5.0g of anhydrous calcium carbonate was reacted with 150mL 0.1 mol.L -1 nitric acid. If the reaction was carried out at 25 o C and 100kPa, calculate the volume of gas that would be produced. And then calculate the pH of the final solution, once the reaction is complete."

As I don't have the answers for this question, but my answer was 0.19L and pH1.

Thank you.

For the compare question, maybe (depending on the lines given) talk about how it is more sensitive or how it measures ionised particles in terms of the chemistry (argon becomes charged in Geiger counter and completes circuit).
For the calcium question, I got 0.19L of gas too, but I'm not too sure on the pH since all the acid is used up.
I'm guessing it might be pH 7?

Also, just another question,
For a polymer question when drawing polyvinylchloride and polystyrene, is it best to alternate the sides the chlorine and benzene are on due to their electronegativity forcing them to repel each other. Or just draw them on the same side (i.e bottom or top) to show that the monomer is a repeating unit?

Also can addition polymerisation be between different monomers like condensation polymerisation?
Thanks.

In our school, you will lose marks for not alternating the functional groups in polymers.
Addition polymers can be more than one monomer type, such as SBR (styrene butadiene) where styrene addition polymerises with butadiene monomers.
Hope this helps

Hello!! I was just wondering if I could please get some help with this question! Thanks

'A manufacturer claims his vinegar is 98% pure acetic acid with no additives. 25 mL of this vinegar was diluted to make 250 mL solution. 25.00mL aliquots were titres against a standardised 0.1086 M sodium hydroxide solution. The following tires were obtained during the investigation:
1. V(NaOH) (mL) = 26.25
2. V(NaOH) (mL) = 27.25
3. V(NaOH) (mL) = 27.30
4. V(NaOH) (mL) = 27.20
Calculate the concentration of the original acetic acid and evaluate the manufacturer's claims.'

Thank you!

For this question, you would find the average titre volumes by first eliminating any outliers and averaging the consistent results.
So the average titre is 27.25mL.
-Write out the chemical equation.

-Find the moles of NaOH by multiplying conc. by volume.
-Use stoichiometry to find moles of acetic acid.
-Multiply the moles in 25mL of vinegar by the molar mass of acetic acid.
-Does it equal 98% pure acid? 98% pure acid is 24.5 grams of acetic acid for every 25mL of water (water is 1g/mL).

[winstondarmawan]

Hello! Can someone please clarify buffer solutions for me, like how would you prepare one in a laboratory situation. I know the theory behind it.
TIA

[Mounica]

Hey, Guys
can someone pls tell me, how i would answer this question:
Polyvinyl alcohol can be reacted with acetic acid to form polyvinyl acetate.
(i) describe the reaction conditions you would use to produce polyvinyl acetate in this reaction

[J.B]

Thanks MisterNeo,

I was also just wondering in the alkaline dry cell that I studied the Cathode reaction is:
2MnO2 (s) + H2O (l) + 2e− →Mn2O3 (s) + 2OH− (aq)

But I'm just a bit confused as if the MnO2 is being reduced by gaining 2 electrons, shouldn't its oxidation state go from 4+ to 2+ rather than 4+ to 3+ as seen in this reaction?

Thanks

[Shadowxo]

Thanks MisterNeo,

I was also just wondering in the alkaline dry cell that I studied the Cathode reaction is:
2MnO2 (s) + H2O (l) + 2e− →Mn2O3 (s) + 2OH− (aq)

But I'm just a bit confused as if the MnO2 is being reduced by gaining 2 electrons, shouldn't its oxidation state go from 4+ to 2+ rather than 4+ to 3+ as seen in this reaction?

Thanks

*apologies if I use incorrect terms or don't explain it well/right
If you're just trying to find the oxidation state, you know Mn₂O₃ is overall 0, and each of the O atoms is -2, therefore the oxidation state of the Mn is +3.
Understanding the logic of it: All the atoms are rearranged and molecules are created which have different overall charges.
An H+ atom splits from the H₂O atom leaving it as an OH-, and an O^2- atom splits from one of the MnO₂ atoms to join it, creating another OH- atom. The Mn₂O₃ is left with an overall charge of +2, and the electrons join to make it +0 overall. Each of these reduces each Mn to change it's oxidation state from +4 to +3 as there's one electron for each.
Maybe you also didn't realise there are 2 MnO₂ molecules?
Hope this helps

[itssona]

Heey can anyone explain why ionic substances are soluble in water?

And why covalent network and covalent molecular are insoluble in water?
Thankyou

[MisterNeo]

Heey can anyone explain why ionic substances are soluble in water?

And why covalent network and covalent molecular are insoluble in water?
Thankyou

Ionic substances dissolve in water by dissociating into ions. Ionic molecules always have a positive ion and a negative ion. The hydrogens in water form dipoles with the anion, and the oxygen forms dipoles with the cation.
They essentially rip the ionic molecule apart.
However, you may realise that some ionic substances don't disassociate in water (insoluble precipitates) such as lead chloride, so be careful when explaining how ionic things dissolve.

Covalent molecules aren't all insoluble. They are only insoluble because they are either non-polar or polar.
Sugar is covalent, yet it dissolves in water. This is because sugar (glucose) has those -OH hydroxyl functional groups in its ring-like structure. Those are polar and is able to form hydrogen bonds with water because it is also polar.
Something like...methane...is a covalent molecule and is insoluble in water. This is because it has an even charge distribution and is non-polar, so it does not attract to water in any way but actually repels it.
Basically, it's polar/non-polar of covalent molecules that determine its solubility.
Covalent networks are insoluble because they are chemically bonded to each atom and require a shit-ton of energy to break them (intramolecular). So they remain solid.
Something like ionic lattices are bonded intermolecularly, and can easily be broken up by dipoles, unless an insoluble salt.
Hope this helps

[winstondarmawan]

Hello! Can someone please clarify buffer solutions for me, like how would you prepare one in a laboratory situation. I know the theory behind it.
TIA

Just bumping my post