# Thread: HSC 2016 Chemistry Marathon

1. ## Re: HSC Chemistry Marathon 2016

Originally Posted by InteGrand
You appear to be off by a factor of 10 compared to Flop21's reported answer. What method did you use? Did you try the method I posted (I have t actually typed numbers into a calculator but the method should be right)?
Hey can you help me with my question I posted above?

2. ## Re: HSC Chemistry Marathon 2016

Nevermind, I got the answer for such a simple question lol.

3. ## Re: HSC Chemistry Marathon 2016

Originally Posted by InteGrand
You appear to be off by a factor of 10 compared to Flop21's reported answer. What method did you use? Did you try the method I posted (I have t actually typed numbers into a calculator but the method should be right)?
Hey,
Yeah, ur right I read wrong and thought no. of cu atoms was 4.6* 10^24 instead of 10^23. Not sure about ur method tbh, cuz it doesn't make sense to me ahahahahaha
I just did mass cu2s/mm cu2s = moles of Cu/2
= (4.6/6.022) / 2
Mass Cu2S = [(4.6/6.022 )*mm cu2S] / 2

4. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Flop21
61g apparently
btw is this 4 uni chem? (and which one?)

5. ## Re: HSC Chemistry Marathon 2016

The river water downstream from a factory was suspected of having lead ion and barium in contamination.
Analysis of a sample of this water by atomic absorption spectroscopy found its absorbance valuet the main lead wavelength to be 0.078.
The absorbance value of a standard laboratory sample, known to have a concentration of 5.85 ppm of lead was 1.087.

Calculate the concentration of lead in ppm in the river water sample

ALSO when calculating Total Dissolved Solids (TDS), the formula is use is:

( Mass (Beaker + Residue) - Mass (Beaker) ) / ( Mass (Beaker + Water ) - Mass (Beaker) )

what would be the units of the result i get from this calculation?? and how do i convert this number to ppm. Thanks!

6. ## Re: HSC Chemistry Marathon 2016

Absorbance is a linear function of concentration no matter what.

5.85/1.087 = [Pb2+]/0.078

[Pb2+] = 0.41977920883 mol L^-1 but you should round to 2 s. f. according to the question.
________________________

That would give you the mass of the residue. You know that n=m/M and C=n/V (V is the volume of the water you originally had)

7. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Occupied
btw is this 4 uni chem? (and which one?)
I'm doing an intro chem course at uni (chem 1001 at UNSW). So it's mainly HSC level chem, stuff to prepare us for the first year chem courses everyone takes.

8. ## Re: HSC Chemistry Marathon 2016

I understand what the numbers on the left of each element symbol mean, but how do I work out this question when it has the symbols on the right (what are the symbols on the right, e.g. 3- and how do I use that to work out the question?)

9. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Flop21
I understand what the numbers on the left of each element symbol mean, but how do I work out this question when it has the symbols on the right (what are the symbols on the right, e.g. 3- and how do I use that to work out the question?)

The symbols on the right tell us the net charge, as mentioned here: https://en.wikipedia.org/wiki/Ion#Notation.

10. ## Re: HSC Chemistry Marathon 2016

Oh I understand! Because the net charge is negative by 3, meaning it has 3 extra electrons.

11. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Flop21
I understand what the numbers on the left of each element symbol mean, but how do I work out this question when it has the symbols on the right (what are the symbols on the right, e.g. 3- and how do I use that to work out the question?)

Can someone please explain what the net charge has on the nucleus? Meaning like what's the relationship between the neutrons, protons and electrons. Does having a net charge affect the number of protons and neutrons? Kind of confused

12. ## Re: HSC Chemistry Marathon 2016

Originally Posted by RachelGreen
Can someone please explain what the net charge has on the nucleus? Meaning like what's the relationship between the neutrons, protons and electrons. Does having a net charge affect the number of protons and neutrons? Kind of confused
Electrons are negative, protons are positive. So if its net charge is 3-, it's 3 negative, and what is negative? Electrons are negative, so it must have more electrons than protons to give it a negative charge.

13. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Flop21
Electrons are negative, protons are positive. So if its net charge is 3-, it's 3 negative, and what is negative? Electrons are negative, so it must have more electrons than protons to give it a negative charge.
yeh this
basically in a neutral atom the number of ur protons = ur electrons. in a charged electron the number of ur protons =/= electrons. if it's a positive charge, u have more protons cuz protons are positive. if its a neg charge, u have more electrons cuz electrons r negative (they have neg charge).

neutrons just hold the nucleus together -> like if u think about it, like charges repel and opposite charges attract right? there isn't any electrons in the nucleus - nucleus only has protons and neutrons. theoretically protons should repel other protons (cuz like charges repel) but they don't cuz of the neutrons. ur number of neutrons can be found by minusing atomic number from atomic weight.

In hsc chem from memory u don't really deal with neutrons other than isotopes

net charges dont affect the number of ur protons because any element only ever has a set numer of protons. if u change the number of protons, ur making it into another element. e.g a neutral hydrogen has 1 proton and 1 electron. if u add a proton somehow, ur making it into Helium+ (He+) which has 2 protons and 1 electron. Net charges only affect number of electrons -> ur not changing number of neutrons or protons.

14. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Occupied
yeh this
basically in a neutral atom the number of ur protons = ur electrons. in a charged electron the number of ur protons =/= electrons. if it's a positive charge, u have more protons cuz protons are positive. if its a neg charge, u have more electrons cuz electrons r negative (they have neg charge).

neutrons just hold the nucleus together -> like if u think about it, like charges repel and opposite charges attract right? there isn't any electrons in the nucleus - nucleus only has protons and neutrons. theoretically protons should repel other protons (cuz like charges repel) but they don't cuz of the neutrons. ur number of neutrons can be found by minusing atomic number from atomic weight.

In hsc chem from memory u don't really deal with neutrons other than isotopes

net charges dont affect the number of ur protons because any element only ever has a set numer of protons. if u change the number of protons, ur making it into another element. e.g a neutral hydrogen has 1 proton and 1 electron. if u add a proton somehow, ur making it into Helium+ (He+) which has 2 protons and 1 electron. Net charges only affect number of electrons -> ur not changing number of neutrons or protons.
Well I mean, in order to be able to deal with isotopes you need to know what the mass number actually is first (no. of protons + no. of neutrons) so that probably comes first

15. ## Re: HSC Chemistry Marathon 2016

Originally Posted by leehuan
Well I mean, in order to be able to deal with isotopes you need to know what the mass number actually is first (no. of protons + no. of neutrons) so that probably comes first
yh true true

16. ## Re: HSC Chemistry Marathon 2016

With reference to the underlying chemistry and with relevant equations, assess the impacts on society of TWO uses of ethanol. (7 marks)

.

18. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Speed6
With reference to the underlying chemistry and with relevant equations, assess the impacts on society of TWO uses of ethanol. (7 marks)
More goon, okayish fuel.

Is that acceptable?

19. ## Re: HSC Chemistry Marathon 2016

Originally Posted by someth1ng
More goon, okayish fuel.

Is that acceptable?
A better answer would include a judgement for the first point

20. ## Re: HSC Chemistry Marathon 2016

Originally Posted by someth1ng
More goon, okayish fuel.

Is that acceptable?
BOSTES just called me, this is what they said:

Candidates need to improve in these areas:

A satisfactory level in literacy skill is necessary before commencing study in the field of HSC Chemistry.

21. ## Re: HSC Chemistry Marathon 2016

Calculate the number of oxygen atoms present in 0.843 mol of silver sulfate

22. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Flop21
Calculate the number of oxygen atoms present in 0.843 mol of silver sulfate
The formula for silver sulfate is Ag2SO4. Knowing this, this question becomes similar to the previous ones you posted here.

23. ## Re: HSC Chemistry Marathon 2016

Originally Posted by InteGrand
The formula for silver sulfate is Ag2SO4. Knowing this, this question becomes similar to the previous ones you posted here.
Can you help please, instead of 0.843 mol of silver sulfate, let's do 0.697 mol.

So is the calculation...

4(#of oxygen atoms per mole of silver sulfate) *217.33157 (g of silver sulfate we are looking at)
___________
311.81 (molar mass of silver sulfate)

= 2.788

what is this result in? do I use this to * by the mole number?

24. ## Re: HSC Chemistry Marathon 2016

What.

4 moles of oxygen are in 1 mol of silver sulfate
Therefore 4*0.697 = 2.788mol of oxygen is present in 0.697mol of silver sulfate

So moles of oxygen = 2.788mol

Thus the number of atoms in oxygen is just 2.788 * 6.022*10-23 because by definition $n=\frac{N}{N_A}$

Once you have the moles and you want the number of atoms going back to the mass is pointless. Rather, if they gave you the mass then you'd want to use n=m/M to get the value for n

25. ## Re: HSC Chemistry Marathon 2016

Originally Posted by Flop21
Can you help please, instead of 0.843 mol of silver sulfate, let's do 0.697 mol.

So is the calculation...

4(#of oxygen atoms per mole of silver sulfate) *217.33157 (g of silver sulfate we are looking at)
___________
311.81 (molar mass of silver sulfate)

= 2.788

what is this result in? do I use this to * by the mole number?
Had a closer look at the Q. The answer is simply 4*0.697*NA, where NA is Avogadro's number, which is on the HSC Chemistry formula sheet.

This is because in 0.697 mol of silver sulfate, there are 4*0.697 mol of oxygen atoms (from the silver sulfate formula).

Now that we know how many moles of oxygen atoms there are, we just multiply this by Avogadro's number to find the actual number of oxygen atoms.

Edit: Done above

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