HSC Chemistry Question Thread

[angelahchan]

Hey there!

I know this has been answered already, but any ideas on how to solve this? Wouldn't mind getting linked to the place to save someone the time of solving it (again)

c1v1= c2v2
c1= 0.1
v1= 0.09 (converting it into liters)
c2 = 10^-2 = 0.01
0.1 * 0.09 = 0.01 *x
therefore (0.1*0.09)/0.01 = x
x= 0.9
which is 900mls. But this is the final volume including the initial 90 mls.
Therefore 900-90 = 810. Answer is C

[kauac]

Hey there!

I know this has been answered already, but any ideas on how to solve this? Wouldn't mind getting linked to the place to save someone the time of solving it (again)

Saw this and thought I'd give it a try!

I'm assuming from my calculations that C is the correct answer... If not, let me know!

To start off with, if you have a solution with a pH of 1, then changing the pH to 2 will require a 10x decrease in the concentration of hydrogen ions, as the pH scale is a logarithm. As HCl is a strong acid and completely dissociates in water, you can assume that the 0.1 molarity given in the questions is also the hydrogen ion concentration.

Decreasing the hydrogen ion concentration by 10 would give it a value of 0.01 - you can check this using the pH=-log[H+] formula. Hence this is the molarity to aim for in the following calculation:

C1V1            = C2V2
0.1 x 0.09     = 0.01V2
0.009            = 0.01V2
V2                 = 0.009 / 0.01
                     = 0.9 L or 900mL

Using the original concentration and volume, as well as the intended final concentration, 900mL is found to be the final volume of solution needed to obtain a pH of 2. Since the solution is already 90ml, then you can simply subtract 90 from 900 to give you the amount of water need for the dilution, which is:   900 - 90 = 810mL (option C).

Hope this helps!

[winstondarmawan]

Hello!
Can someone please go over the formation of ozone in the troposphere and the associated formula? I'm missing it in my notes..
TIA.

[toasted]

Hello!
Can someone please go over the formation of ozone in the troposphere and the associated formula? I'm missing it in my notes..
TIA.

Formation:
O2 --UVc--> 2O. note O. denotes a free oxygen radical. UVc is 'Catastrophic' and hence its absorption is beneficial
O.+O2---->O3
Degradation:
O3--UVb--> O2 + O. UVb is 'Bad'

Hope that helps!

[bsdfjnlkasn]

Hey there!

I thought the answer was C for this, I can't see how the change in oxidation states is what is given in B..

I knew it was ethanol, but what calculations would we be expected to calculate to find this out?

For pH, I thought it was B? Or is it A because there's been a 100 fold increase in volume as opposed to 10 which is what the 2.4 pH would indicate ?

Also, aren't neutralisation reactions exothermic? So if temperature increases, the reactants will be favoured i.e. less ionisation and so pH increases? (answer is A)

[Natasha.97]

Hey there!
I thought the answer was C for this, I can't see how the change in oxidation states is what is given in B..

Hi!


C is incorrect: A decrease in the oxidation state means that there is a gain in electrons. In this case, Zn gives electrons to Oxygen, so the oxidation state of Zn changes from 0 to +2.

B is correct: The total oxidation number of a compound is 0. In this case, the oxidation state of O₂ is -4 (2 x -2), so to balance it out, the oxidation state of Mn is +4. In the compound Mn₂O₃, the oxidation number of O₂ is -6 (3x-2) so the oxidation state of Mn is 6÷2, which is +3.

Hope this helps

[MisterNeo]

Hey there!
I thought the answer was C for this, I can't see how the change in oxidation states is what is given in B..

I knew it was ethanol, but what calculations would we be expected to calculate to find this out?

-Find how many moles of every fuel is present in 1000 grams using m/MM.
-Multiply each fuels' moles with their respective heat of combustion values to obtain the amount of energy released from 1kg.
-The smallest value would be the answer.

For pH, I thought it was B? Or is it A because there's been a 100 fold increase in volume as opposed to 10 which is what the 2.4 pH would indicate ?

Also, aren't neutralisation reactions exothermic? So if temperature increases, the reactants will be favoured i.e. less ionisation and so pH increases? (answer is A)

They should be exothermic. For this question, it requires you to interpret the graph to see if you can understand a graph. So, the answer would be A since pH decreases as more H+ are ionised.

[bsdfjnlkasn]

Hey there!

Just wanted to clarify a few things to do with nuclear chem as a part of the production topic. Do we have to know anything for this outcome: "Process information from secondary sources to describe recent discoveries of elements"? I sort of just assumed that they would give us data to interpret or something, has anyone seen any questions that might relate to this outcome where they haven't given you data? Because should I then memorise some examples of recent element discoveries?

Do we have to know the specifics of the different radiation detectors? How many is everyone learning?

What sort of questions can they ask us to answer with regards to interpreting the screen-shotted graph?

Any help would be super appreciated!

[f_tan]

Can anyone help explain what a dot means in an equation?
I came across this equation: CuSO4.5H2O --> CuSO4 + 5H2O for the dehydration of copper sulphate crystals, and i'm not sure what it means.
Thanks!

[MisterNeo]

Can anyone help explain what a dot means in an equation?
I came across this equation: CuSO4.5H2O --> CuSO4 + 5H2O for the dehydration of copper sulphate crystals, and i'm not sure what it means.
Thanks!

The dot shows a number of water molecules attached to a hydrated salt. It’s chemically bonded to the salt but shown in a special way using the dot. Concentrated sulfuric acid will remove those water molecules from the blue hydrated salt to form a white anhydrous salt of copper sulfate.

Hey there!
Just wanted to clarify a few things to do with nuclear chem as a part of the production topic. Do we have to know anything for this outcome: "Process information from secondary sources to describe recent discoveries of elements"? I sort of just assumed that they would give us data to interpret or something, has anyone seen any questions that might relate to this outcome where they haven't given you data? Because should I then memorise some examples of recent element discoveries?
Do we have to know the specifics of the different radiation detectors? How many is everyone learning?
What sort of questions can they ask us to answer with regards to interpreting the screen-shotted graph?
Any help would be super appreciated!

I haven’t seen a question on recent discoveries, but I’m not particularly sure if they will. It’s best to prepare one just in case. All you would need is a brief description of how it is produced using what particles/technology, and half-life/emissions. For radiation detectors, most people are doing Geiger counter or Scintillation counter, but I don’t think they’ll ask for more than one. From the graph, they would expect you to know that too many neutrons causes -beta decay, too little causes +beta decay, and too heavy nucleus causes alpha/gamma. They wouldn’t ask you something very specific to the graph unless it was supplied.

[arunasva]

The dot shows a number of water molecules attached to a hydrated salt. It’s chemically bonded to the salt but shown in a special way using the dot. Concentrated sulfuric acid will remove those water molecules from the blue hydrated salt to form a white anhydrous salt of copper sulfate.I haven’t seen a question on recent discoveries, but I’m not particularly sure if they will. It’s best to prepare one just in case. All you would need is a brief description of how it is produced using what particles/technology, and half-life/emissions. For radiation detectors, most people are doing Geiger counter or Scintillation counter, but I don’t think they’ll ask for more than one. From the graph, they would expect you to know that too many neutrons causes -beta decay, too little causes +beta decay, and too heavy nucleus causes alpha/gamma. They wouldn’t ask you something very specific to the graph unless it was supplied.

well it came up in the cssa trials.

[mary123987]

HEY guys for AAS does anyone knnow wether or not the graph is a curve or line of best fit as i have been told from numerous sources varying things from my understanding for HSC level it is a line of best fit but in reality it is actually a curve of best fit is that true ?

[MisterNeo]

HEY guys for AAS does anyone knnow wether or not the graph is a curve or line of best fit as i have been told from numerous sources varying things from my understanding for HSC level it is a line of best fit but in reality it is actually a curve of best fit is that true ?

The graph should have a straight line (connecting the dots) because the absorbance and concentration are proportionate to each other, and the plotted points should be collinear.

[mary123987]

Thankyou !

[roygbivmagic]

Hi! Please can you explain how to do this question?
'What volume of 0.005 mol L-1 KOH is required to neutralise 15mL of 0.0005 mol L-1 H2SO4?'
The equation is H2SO4 + 2KOH -> K2SO4 + 2H2O so isn't the number of moles of H+ already equal to the number of moles of OH-? In the answers they found the number of moles of H2SO4 and multiplied that by 2 to find number of moles of H+ and found how many mL of KOH gives this many moles. But what I don't understand is that aren't H+ and OH- already in a 1:1 ratio? Why do you have to multiply H+ by 2? Do you just disregard molar ratios for pH calcs?
Hopefully that made sense! Thank you