VCE Chemistry Question Thread

[Kel9901]

Hi

I read that for AAS, the concentration of metal ion and absorbance are proportional at low concentrations.
Why are they not proportional at high concentrations?

Please let me know if I am going out of the VCE course... I tend to overthink things a lot...

Thanks

and yeah it's outside the course

[KingDrogba]

How do i draw a calibration curve when i have my 4 standards but then i also have an internal standard to factor in? How do i calculate the ratio of peak areas as it says to do?

[Eiffel]

can someone show me how you find molar mass of Fe2(SO4)3.9H20 lol :/

also when doing questions with potassium di chromate, why is the potassium always "disappeared", i can infer it might be spectator but what is the equation showing this? say it was with ethanol (goes to ethanoic) and potassium di chromate goes to Cr(III)

[keltingmeith]

can someone show me how you find molar mass of Fe2(SO4)3.9H20 lol :/

How do you find find the molar mass of anything?

also when doing questions with potassium di chromate, why is the potassium always "disappeared", i can infer it might be spectator but what is the equation showing this? say it was with ethanol (goes to ethanoic) and potassium di chromate goes to Cr(III)

The dichromate ion is a good oxidant - the potassium dichromate is just a stable form. So, you're completely right in that the potassium is just a spectator ion, and we only care about the dichromate, hence why we don't write the potassium in the chemical equations - it never enters the equation, it's just sort of around because it has to be around for dichromate to be around.

[Eiffel]

Yeah I tried calculating its molar mass but got it wrong which is really weird

2 fe
3 s
12 o
9 h20

[wunderkind52]

Yeah I tried calculating its molar mass but got it wrong which is really weird

2 fe
3 s
12 o
9 h20

what molar mass did you get? and what does the answer say?

[Eiffel]

what molar mass did you get? and what does the answer say?

i got 561.9 if i remember correctly, answer had 728.3 or somthing like that, defs in 728s

[wunderkind52]

i got 561.9 if i remember correctly, answer had 728.3 or somthing like that, defs in 728s

I think they just made a mistake. I got 561.9 too. Maybe they thought there were 18 H2Os and did (2*55.+(3*32.1)+(12*16.0)+(18*18.0) to give 723.9?

[Eiffel]

for stationary and mobile phase, can someone tell me about the solubility . e.g. is the stationary phase polar/ non polar and its effects by being on of them same with mobile phase.

cheers

[Kel9901]

for stationary and mobile phase, can someone tell me about the solubility . e.g. is the stationary phase polar/ non polar and its effects by being on of them same with mobile phase.

cheers

If there are ever questions regarding this sort, basically either phase can have either polarity, but they will usually be different.

For example, the stationary phase may be non-polar and the mobile phase polar.

The point most questions will ask you to explain is that polar attracts polar and non-polar attracts non-polar, so in this case polar substances would have a shorter retention time/higher Rf than non-polar substances (generally).

If the attraction to the stationary phase is greater (eg by having the same polarity), then the substance will adsorb and desorb more frequently, leading to it travelling slower and therefore longer retention time/lower Rf. It would also be less soluble in the (different polarity) mobile phase, further causing it to move slower.

[RazzMeTazz]

The thermite process can be used to weld lengths of railway track together. A mould placed over the ends of the two rails
to be joined is filled with a charge of aluminium powder and iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron, which joins the rails together.

a.) Write a half equation for the conversion of iron(III) oxide
to metallic iron.
b.) Is the half equation you wrote for part a an oxidation or a
reduction process?
c.) Write the overall equation for the thermite process.

The answers are:

a.)    Fe2O3(s) + 6e–  2Fe(l) + 3O2–(s)
b.)   reduction
c.)   Fe2O3(s) + 2Al(s)  2Fe(l) + Al2O3(s)


But I don't really understand how you would know that  in the reduction reaction of the iron, there is no H2O involved and instead O2- ions were formed? (Because usually when we balance out oxygen atoms, we do it by adding water.)

Also how did they know that the Aluminium formed Al2O3?

Thanks

[lzxnl]

The thermite process can be used to weld lengths of railway track together. A mould placed over the ends of the two rails
to be joined is filled with a charge of aluminium powder and iron(III) oxide. When the mixture is ignited, a redox reaction
occurs to form molten iron, which joins the rails together.

a.) Write a half equation for the conversion of iron(III) oxide
to metallic iron.
b.) Is the half equation you wrote for part a an oxidation or a
reduction process?
c.) Write the overall equation for the thermite process.

The answers are:

a.)    Fe2O3(s) + 6e–  2Fe(l) + 3O2–(s)
b.)   reduction
c.)   Fe2O3(s) + 2Al(s)  2Fe(l) + Al2O3(s)


But I don't really understand how you would know that  in the reduction reaction of the iron, there is no H2O involved and instead O2- ions were formed? (Because usually when we balance out oxygen atoms, we do it by adding water.)

Also how did they know that the Aluminium formed Al2O3?

Thanks

They've told you that you have liquid iron. There's no mention of water. Hence you don't balance such equations with water.

As for aluminium forming the oxide...I guess you have to just use chemical intuition there that aluminium, a metal with 3 valence electrons, is going to lose all three of them by forming the oxide in the +3 oxidation state.

[RazzMeTazz]

When assigning oxidation numbers, the rules state that oxygen always has an oxidation number of -2, except in peroxides such as H2O2 and BaO2.

The oxidation rules also say that the most electronegative element in a compound has the negative oxidation number.

So if you had a compound including oxygen and another element with a greater electronegativity (such as fluorine), is it not certified that oxygen would have an oxidation state of -2?

Thanks

[keltingmeith]

So if you had a compound including oxygen and another element with a greater electronegativity (such as fluorine), is it not certified that oxygen would have an oxidation state of -2?

Thanks

Do you have an example?

[RazzMeTazz]

Such as the molecule OF2