HSC Chemistry Question Thread

[J.B]

Hi,
I really need to practice back titration calculations.
I was wondering if anyone had a bunch of questions they could send me?

In industrial chemistry:
I was wondering for the electrolysis of NaCl school practical, if using a concentrated solution, would 200mL of 6.0mol/L be suitable for a Hoffman's Voltameter on 6 volts?

And in the modelling of the solvay process practical, my class modelled CaCO3 -> Co2 + CaO, and CaO + H2O -> Ca(OH)2
I have one limitation being we can't perform it at the 900 degrees, but I'm struggling to come up with another one? Any Suggestions?

Also, in soap is the anionic head considered to be a hydrogen bond with water molecules?

Thanks.

[bridie_2345]

Hey there i was wondering if anyone could help me address this dot point in the acidic environment?

perform a first-hand investigation to determine the concentration of a domestic acidic substance using computer-based technologies


thanks!

[angelahchan]

Hey there i was wondering if anyone could help me address this dot point in the acidic environment?

perform a first-hand investigation to determine the concentration of a domestic acidic substance using computer-based technologies


thanks!

Honestly, I  forgot about this one so I did a quick summary- it's just a titration but with a pH probe and data logger, there's sometimes questions on interpreting titration graphs based on this. This is a similar graph to what the prac should've looked like:   I don't think my chem class actually did it with a data logger but I guess we pretended to?

[Nick Seb]

I never really understood how it worked, but how do carbon isotopes in trees and calcium carbonate in coral suggest an acidic oxide increase over the past two centuries [taken from the first edition of the ATARNotes HSC Chemistry complete course notes]?

~Nick

[winstondarmawan]

Hey,
In regards to significant figures, I have been taught it is best to round off the the lowest significant figure that is in the question. However, in some molar calculations involving volume, would values such as 25 degrees have any bearing on the value of SF that we should be rounding off to?

[roygbivmagic]

Hi, how do you solve this question?
"A student set up the following galvanic cell as shown [zinc anode, copper cathode, ZnSO4 and CuSO4 electrolyte solutions, KCl salt bridge.] Both electrodes were weighed before beginning the experiment and each beaker contained 200mL of a suitable electrolyte at 0.100molL-1 concentration. After a short period of time, the student reweighed the copper electrode and found that it had increased in mass by 0.435g. What was the final concentration of the zinc electrolyte?"
(Answer=0.134molL-1)

[liya1234]

this is probably a kinda stupid question but i've somehow forgotten when units i should be using when putting values into formulas (n=m/M and n=cv) - is mass always in grams for the first one? and is volume in L or ml for the second?? thanks in advance!!

[naezeroth]

this is probably a kinda stupid question but i've somehow forgotten when units i should be using when putting values into formulas (n=m/M and n=cv) - is mass always in grams for the first one? and is volume in L or ml for the second?? thanks in advance!!

It is always grams for m and litres for volume

[Nick Seb]

Hi, how do you solve this question?
"A student set up the following galvanic cell as shown [zinc anode, copper cathode, ZnSO4 and CuSO4 electrolyte solutions, KCl salt bridge.] Both electrodes were weighed before beginning the experiment and each beaker contained 200mL of a suitable electrolyte at 0.100molL-1 concentration. After a short period of time, the student reweighed the copper electrode and found that it had increased in mass by 0.435g. What was the final concentration of the zinc electrolyte?"
(Answer=0.134molL-1)

Copper cathode oxidised, hence its mass increased as it pulled copper ions out of solution which precipitated on the metal surface. As the cathode gained 0.435g of copper, then 0.435g of copper must have been pulled out of solution. Given the molar mass of copper is 63.55, by n=mass/Molarmass, n=0.435/63.55,n=0.00684500393... moles. Therefore, the copper cathode gained 0.00684500393... moles of copper.

Cu2+(aq) + 2e- --> Cu(s) and Zn(s) --> Zn2+(aq) + 2e-, which can be written as Cu2+(aq) + Zn(s) --> Cu(s) + Zn2+(aq)

Given that the molar ratio is 1:1, you can determine that the zinc anode lost 0.00684500393... moles of zinc, meaning that the electrolyte solution gained the same moles of zinc ions by the same equation.
Now find the initial moles of Zn2+ in solution, which you can do by n=cv, n=0.1*0.2 (remembering to convert 200ml to 0.2L because of SI units), n=0.02 moles.
From here you simply add the initial moles of the solution with the moles which the zinc anode added to its electrolyte solution (0.02 + 0.00684500393... = 0.02684500393... moles).
Then just solve c=n/v using the new moles of zinc ions in solution and the same volume, n=0.02684500393.../0.2 = 0.1342250197mol/L.

Hope this helps,
~Nick

[angelahchan]

Hi, can someone please help me with Q20 of the 2016 paper? I don't understand how AAS works in general... thanks!
http://educationstandards.nsw.edu.au/wps/wcm/connect/e76232cd-1d84-4d3e-92ce-ea41546810f6/2016-hsc-chemistry.pdf?MOD=AJPERES&CACHEID=ROOTWORKSPACE-e76232cd-1d84-4d3e-92ce-ea41546810f6-lDY2cMn

[J.B]

Hi,
Could someone explain Question 15 from 2015 HSC?
Answer is A.
Thanks

[angelahchan]

Hi,
Could someone explain Question 15 from 2015 HSC?
Answer is A.
Thanks

We can rule out B and C because they both have low turbidity which is not likely due to having passed through farmland and a village. We can rule out D because of it's low total dissolved solids - since X is close to the ocean (estuarine), there would be high TDS due to all the minerals dissolved in the ocean.
This leaves out A by elimination- high turbidity and high TDS.
Although someone else's opinion on the pH value would be great- since I don't really understand why the pH is lower at A (since farmland can sometimes have fertiliser run-off that makes water more alkaline).

[julies]

hey guys,
are there any similarities between the lead acid cell and the button cell?

[angelahchan]

I never really understood how it worked, but how do carbon isotopes in trees and calcium carbonate in coral suggest an acidic oxide increase over the past two centuries [taken from the first edition of the ATARNotes HSC Chemistry complete course notes]?

~Nick

They are evidence for increasing CO2 levels in the atmosphere.
Since the dot point is "Assess the evidence which indicates increases in atmospheric concentration of oxides of sulfur and nitrogen." I think this was mentioned (I have no idea what the atarnotes book says since I don't have it), to compare how it's relatively easier to find evidence for increasing concentration of CO2 in comparison to sulfur and nitrogen oxides.
Basically, since carbonates stay in inert forms, past concentration levels can be observed to observe the increase in CO2 levels. In contrast most sulfates and all nitrates are water soluble, which means they're washed out of air by the rain and so it's harder to measure significant build up of these gases over time.
Sorry if this doesn't explain how because I don't really know, and the focus of the dot point is increasing sulfur and nitrogen oxide levels rather than carbon

[winstondarmawan]

Hello!
Can someone please explain the first limitation:
https://scontent-syd2-1.xx.fbcdn.net/v/t35.0-12/22883625_1443073785806709_699782289_o.jpg?oh=e373a670b3d43488ce77d26f619ea8a4&oe=59F6FCF8
TIA.