HSC Chemistry Question Thread

[angelahchan]

Hi,
I was told that  50000 is considered 5 significant figures in chemistry. Is this true?

Yes. A 0 becomes included as a significant figure once any number besides 0 is in front of it.
e.g. 0.0327 to 2 s.f. = 0.033
but 0.3027 to 2 s.f. = 0.30. The 3 = the first significant figure, and the 0 behind it is the second.

[MisterNeo]

Also can someone explain to me avogrado's number and how we use it?
Thanks

In chemistry, we usually work with moles. Avogadro's Number is the amount of particles in one mole, 6.022 x 10²³.
-One mole of carbon has 6.022 x 10²³ atoms of carbon, which weigh 12.01 grams because that's one mole of its 12.01g/mol molar mass.
-One mole of fluorine has 6.022 x 10²³ atoms of fluorine, which weighs 19 grams. However, there is fluorine GAS, which is 6.022 x 10²³ of F₂ molecules, which weigh 38 grams because there are twice as many fluorine atoms.
-One mole of ethanol has 6.022 x 10²³ molecules of ethanol, which weigh 46.068 grams because it's made of 2 x 6.022 x 10²³ atoms of carbon, 6 x 6.022 x 10²³ atoms of hydrogen, and 6.022 x 10²³ atoms of oxygen.
Avogadro's Number is most prevalent in Prelims. In HSC, it's all done in moles (1mol, 2mol, etc) and they may occasionally ask "How many particles are there in..." for multiple choice.

[J.B]

In chemistry, we usually work with moles. Avogadro's Number is the amount of particles in one mole, 6.022 x 10²³.
-One mole of carbon has 6.022 x 10²³ atoms of carbon, which weigh 12.01 grams because that's one mole of its 12.01g/mol molar mass.
-One mole of fluorine has 6.022 x 10²³ atoms of fluorine, which weighs 19 grams. However, there is fluorine GAS, which is 6.022 x 10²³ of F₂ molecules, which weigh 38 grams because there are twice as many fluorine atoms.
-One mole of ethanol has 6.022 x 10²³ molecules of ethanol, which weigh 46.068 grams because it's made of 2 x 6.022 x 10²³ atoms of carbon, 6 x 6.022 x 10²³ atoms of hydrogen, and 6.022 x 10²³ atoms of oxygen.
Avogadro's Number is most prevalent in Prelims. In HSC, it's all done in moles (1mol, 2mol, etc) and they may occasionally ask "How many particles are there in..." for multiple choice.

Thank you

Do you know a year that they asked this?

[Fahim486]

Hey so I'm having trouble understanding this question so could someone pls explain it. Thanks!

[angelahchan]

Hi, could someone  please help me  with question 19 of the 2015 paper? thanks!
https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-chemistry.pdf

[toasted]

Hi, could someone  please help me  with question 19 of the 2015 paper? thanks!
https://www.boardofstudies.nsw.edu.au/hsc_exams/2015/exams/2015-hsc-chemistry.pdf

Hey!
This question is tricky in the sense that they give us more information than required to solve the question.

Pb+2NaCl ---> PbCl2 + 2Na
through mole ratios n(Pb)=n(PbCl2)=n/2(Nacl) however Pb can be the limiting agent and this means that n(pb) does NOT necessarily equal n/2(NaCl)
n(Pb)= n(PbCl2)
=0.13/(207.2+35.45x2)
=0.000467
n(Pb)= 0.000467
conc(Pb)~0.000467/0.05
~ 9.3x10^-3
=C

Hope that helped!

Mod Edit: Removed link.

[angelahchan]

Hey!
This question is tricky in the sense that they give us more information than required to solve the question.

Pb+2NaCl ---> PbCl2 + 2Na
through mole ratios n(Pb)=n(PbCl2)=n/2(Nacl) however Pb can be the limiting agent and this means that n(pb) does NOT necessarily equal n/2(NaCl)
n(Pb)= n(PbCl2)
=0.13/(207.2+35.45x2)
=0.000467
n(Pb)= 0.000467
conc(Pb)~0.000467/0.05
~ 9.3x10^-3
=C

Hope that helped!

thanks so much! the thing I'm confused about is the moles of lead ions being the same as moles of PbCl2, is this the case for all molecules?

[toasted]

thanks so much! the thing I'm confused about is the moles of lead ions being the same as moles of PbCl2, is this the case for all molecules?

Yes it would be true for all molecules, due to mole ratios. However, you have to be incredibly careful of limiting reagents for the reactants. 

Hey so I'm having trouble understanding this question so could someone pls explain it. Thanks!

Hey similar to the question above, this is one where nesa is simply trying to confuse you with useless information.
Essentially, what it's testing is whether you know that the wavelength of light released from the lamp matches the wavelength of the light at the detector. This is necessary because AAS essentially measures the absorption, and if the wavelengths were not the same, it would be unreliable. Hence the answer is A.

Mod Edit: Merged posts.

[winstondarmawan]

Hello!
When asked to draw glucose, is it okay to use the condensed structural formula?
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/23113018_1364885916970261_476464333_o.jpg?oh=f3edc646ed78d2cdbaab29b8e7893f92&oe=59F8A60B
Also, how recent does the recently discovered elements have to be?
TIA.

[bridie_2345]

Hey there do we need to know the complete structural formula for a soap, eg. sodium tristearate or just the condensed structural formula?
thanks

[angelahchan]

Hey there do we need to know the complete structural formula for a soap, eg. sodium tristearate or just the condensed structural formula?
thanks

I think it's better if you memorise the full structural formula just in case- i did a past question today on saponification which asked for including an equation, and the sample answer included the complete structural formula

[bsdfjnlkasn]

n(Pb)= 0.000467
conc(Pb)~0.000467/0.05
~ 9.3x10^-3
=C

Hope that helped!

Hey! how did you go between these steps?

[toasted]

Hey! how did you go between these steps?

Hey!
Im assuming its from the moles to the concentration? So we're given in the question that the sample collected from the pond was 50ml. We know that C=n/V where V is in litres so we get C=0.000467/0.05.

Hope that makes sense now

[_denise]

hi,
I've been having a lot of trouble with calculation questions, mainly because I'm not sure which formulae to use and which steps I'm supposed to take to get to the answer. Just wondering if anyone has any tips on how to approach them?
Thanks!

[Mymy409]

How do you do questions 14 and 15 from 2009 HSC? TIA!

https://www.boardofstudies.nsw.edu.au/hsc_exams/hsc2009exams/pdf_doc/2009-hsc-exam-chemistry.pdf