HSC Chemistry Question Thread

[bananna]

Hi...

When drawing structural formulae for molecules in exam, do we need to draw individual bonds between atoms in a functional group? E.g. a bond between a oxygen and hydrogen atom in a hydroxyl group. Or can we just draw the groups in condensed form, such as OH, as long as they are in the appropriate order for bonding in the molecule?

my teacher said when drawing the structural formula of citric acid, you must draw out all bonds, including O-H

so yep, I think they are strict with structural formula and drawing out all necessary bonds. When they specifically ask for structural formula, that is.
otherwise, its fine to draw the condensed form

[winstondarmawan]

Hello, I'm struggling with knowing when exactly a line of best fit should be linear or a curve.
For example:
https://scontent-syd2-1.xx.fbcdn.net/v/t34.0-12/23146248_1365590483566471_2141030131_n.jpg?oh=25754963e74fcebb116b007aaf0d0367&oe=59F9EB63

[MisterNeo]

Hi was just wondering for chem monitoring : how many quantitative /qualitative tests do we need to know  practical-wise
is 1 each enough?
so like turbidity for quality and acidity for quantity?
thanks
also what are some other qualitative tests?

For the water quality tests (hardness, turbidity, TDS, etc), one qualitative and quantitative for each of them would be fine. So for turbidity, you can talk about Nephelometric Units as quantitative, and the appearance of water as qualitative. Or for hardness, precipitating calcium and magnesium would be qualitative to prove hardness, whereas soap lather height or gravimetric analysis would show the extent of hardness. There are also the cation/anion tests, where flame and precipitate tests are qualitative, and gravimetric analysis is quantitative. \

Hi...
When drawing structural formulae for molecules in exam, do we need to draw individual bonds between atoms in a functional group? E.g. a bond between a oxygen and hydrogen atom in a hydroxyl group. Or can we just draw the groups in condensed form, such as OH, as long as they are in the appropriate order for bonding in the molecule?

I would draw out all bonds. Especially for ethanol solvent questions, where you would need to show the polar nature of ethanol on the -OH group. The only exception I would make is for drawing benzene rings on styrene and the -OH groups on cellulose/glucose.

[statues]

This is from the 2016 MC Q11

Why is the Iupac name
2-bromo-2-chloro-1,1,1-triflouroethane (D)
preferable over
1-bromo-1-chloro-2,2,2-trifluoroethane (A)

[angelahchan]

This is from the 2016 MC Q11

Why is the Iupac name
2-bromo-2-chloro-2,2,2-triflouroethane (D)
preferable over
1-bromo-1-chloro-2,2,2-trifluoroethane (A)

There's this rule called sum of smallest locants or something. D is 2-bromo-2chloro-1,1,1,trifluoroethane not 2-bromo-2-chloro-2,2,2-triflouroethane (so maybe you read over the wrong answer)
So the sum of the numbers for A is 1+1+2+2+2= 8
and the sum of numbers for D is 2+2+1+1+1 = 7
You have to choose the naming that gives the smallest sum-  (I have no clue why, but that's a rule in the naming  system), which is why the answer is D

[statues]

There's this rule called sum of smallest locants or something. D is 2-bromo-2chloro-1,1,1,trifluoroethane not 2-bromo-2-chloro-2,2,2-triflouroethane (so maybe you read over the wrong answer)
So the sum of the numbers for A is 1+1+2+2+2= 8
and the sum of numbers for D is 2+2+1+1+1 = 7
You have to choose the naming that gives the smallest sum-  (I have no clue why, but that's a rule in the naming  system), which is why the answer is D

ah cool beans thanks:)

[liya1234]

For the bromine water prac equations, is it okay to refer to bromine water as Br2(aq)? Will we lose marks if we don't use BrOH

[MisterNeo]

For the bromine water prac equations, is it okay to refer to bromine water as Br2(aq)? Will we lose marks if we don't use BrOH

I've always used Br_2(aq). In the HSC, they will accept it even though BrOH is the scientifically correct method.

[Baylsskool]

For the dot point, name a synthetic replacment for natural product in industrial chemistry, does guano being replaced by superphosphate as a fertiliser count???

[MisterNeo]

For the dot point, name a synthetic replacment for natural product in industrial chemistry, does guano being replaced by superphosphate as a fertiliser count???

I wouldn't do a fertiliser. I've seen questions in Trial papers (maybe a HSC paper) where they can specify not to use a fertiliser or a fossil fuel (it's to stop people from using Haber process). Most people do Styrene Butadiene, if you need an alternative.

[J.B]

HI,
In Titration Questions I also get confused which is in the Burette, and which is pipetted into the conical flask. So I was wondering what terminology they use to let you know?

Thank you

[Dante1091]

HI,
In Titration Questions I also get confused which is in the Burette, and which is pipetted into the conical flask. So I was wondering what terminology they use to let you know?

Thank you

Remember that the Base in Burette, while the Acid + (indicator) are in the conical flask

Remember B for Burette and Base

[J.B]

Remember that the Base in Burette, while the Acid + (indicator) are in the conical flask

Remember B for Burette and Base

Thank you!
I was just very confused, because we did it both ways at my school.

[beau77bro]

ok for the adding H2SO4 step - what if there is Ca, how do u know if the precipitate means there is Ca alone or Ca and Ba cations?

[mbdtHSC]

ok for the adding H2SO4 step - what if there is Ca, how do u know if the precipitate means there is Ca alone or Ca and Ba cations?

Add NaF to the sample, if it's Ca2+ then CaF2 white ppt forms