HSC Chemistry Question Thread

[J.B]

Just bumping my post

Don't quote me, as I may not be right, but my understanding is that a buffer solution is formed when you mix a weak acid with the salt of its conjugate base, or a weak base with the salt of its conjugate acid.
This is a paper from James Ruse, and the question was:

"Outline how a buffer solution could be prepared from two or more of the above compounds.
Sample Answer:
2NH3 is a weak base and NH4Cl gives NH4+ which is the conjugate acid. Combining equimolar solutions would form a buffer solution that inhibits the change in pH."

Hope this helps

[J.B]

Hi,
I just have some more questions:

I was just wondering if Scintillation counters can detect both non-ionising radiation, and ionising radiation? As my school notebook, says only "non-ionising" radiation, but my textbook states ionising radiation.
And are all nuclear radiation ionising radiation?

I also just answered a question "Outline a valid procedure you would use to distinguish between acidic, basic and neutral chemicals using a pH meter or pH probe."
And in the answers it stated that all the solutions need to be dilute solutions? I was just wondering why this is?

Thank you

[beau77bro]

Hi,
I just have some more questions:

I was just wondering if Scintillation counters can detect both non-ionising radiation, and ionising radiation? As my school notebook, says only "non-ionising" radiation, but my textbook states ionising radiation.
And are all nuclear radiation ionising radiation?

I also just answered a question "Outline a valid procedure you would use to distinguish between acidic, basic and neutral chemicals using a pH meter or pH probe."
And in the answers it stated that all the solutions need to be dilute solutions? I was just wondering why this is?

Thank you

the diluting is most likely because the acids and bases need to ionise and make Hydronium ions as well as hydroxide ions so that the electrodes can actually measure their concentrations - after all it measures the concentrations of OH- and H3O+

[itssona]

Ionic substances dissolve in water by dissociating into ions. Ionic molecules always have a positive ion and a negative ion. The hydrogens in water form dipoles with the anion, and the oxygen forms dipoles with the cation.
They essentially rip the ionic molecule apart.
However, you may realise that some ionic substances don't disassociate in water (insoluble precipitates) such as lead chloride, so be careful when explaining how ionic things dissolve.

Covalent molecules aren't all insoluble. They are only insoluble because they are either non-polar or polar.
Sugar is covalent, yet it dissolves in water. This is because sugar (glucose) has those -OH hydroxyl functional groups in its ring-like structure. Those are polar and is able to form hydrogen bonds with water because it is also polar.
Something like...methane...is a covalent molecule and is insoluble in water. This is because it has an even charge distribution and is non-polar, so it does not attract to water in any way but actually repels it.
Basically, it's polar/non-polar of covalent molecules that determine its solubility.
Covalent networks are insoluble because they are chemically bonded to each atom and require a shit-ton of energy to break them (intramolecular). So they remain solid.
Something like ionic lattices are bonded intermolecularly, and can easily be broken up by dipoles, unless an insoluble salt.
Hope this helps

Ur amazing THANK YOU

[seventeenboi]

Hi
why does the pH of identical concentrations of HCl and Acetic acid change by different amounts when diluted with the same volumes of water ??
thanks

[MisterNeo]

Hi
why does the pH of identical concentrations of HCl and Acetic acid change by different amounts when diluted with the same volumes of water ??
thanks

It's got to do with strong/weak acids and LCP.
Strong Acid

Let's assume that we have a 1L solution of HCl with pH 1.
It's a strong acid and will ionise completely. When you add 9L of water, you decreased the concentration of H+ by tenfold, and the pH will be 2.
Weak Acid

Let's assume that we have 1L of this weak HF acid with a pH of 3.
If I were to dilute with water, the equilibrium will shift to the right to produce more H3O+ and decrease pH. However, the volume of water has also increased and has lowered the concentration. So after adding 9L of water to our pH 3 HF acid, our resulting pH will be around 3-4 because it it had produced more H3O+ during diluting.
Thus, strong/weak acids have different pH changes in dilution.
Make sense?

[J.B]

I was just wondering, are Total Ozone Mapping Spectrophotometers (TOMS) measuring the UV radiation from satellites like UV spectrophotometers or measuring the ozone levels directly?
Thanks.

[MisterNeo]

I was just wondering, are Total Ozone Mapping Spectrophotometers (TOMS) measuring the UV radiation from satellites like UV spectrophotometers or measuring the ozone levels directly?
Thanks.

I'm pretty sure TOMS is attached to a satellite that measures ozone according to the UV radiation emitted by Earth.
EasyChem has a pretty good guide on both UV  and ozone spectrometers.

[J.B]

HI
I was just doing the 2004 HSC chemistry question "What is the purpose of the light source and the flame in an AAS?"

My understanding is that the light source contains the metal element that is the same as the metal being tested in the sample in the flame. So that the light source emits the specific light wavelength of the element to allow the element being tested in the flame to absorb this wavelength. This then allows the absorbance of this wavelength to be measured, as this is relative to the concentration in the sample.

Although the marking criteria stated that the flame excites the electrons to shift orbits.

I understand that this happens when in a flame, but I'm a bit unsure of how this relates to the sample absorbing the specific wavelength? As I thought that the flame excites the electrons to shift orbit, and a sample emits its wavelength when the electrons fall back to their orbit. But I don't understand where the absorbance of it's own wavelength in the flame occurs?

(sorry if this makes no sense)

Thanks.

[jakesilove]

HI
I was just doing the 2004 HSC chemistry question "What is the purpose of the light source and the flame in an AAS?"

My understanding is that the light source contains the metal element that is the same as the metal being tested in the sample in the flame. So that the light source emits the specific light wavelength of the element to allow the element being tested in the flame to absorb this wavelength. This then allows the absorbance of this wavelength to be measured, as this is relative to the concentration in the sample.

Although the marking criteria stated that the flame excites the electrons to shift orbits.

I understand that this happens when in a flame, but I'm a bit unsure of how this relates to the sample absorbing the specific wavelength? As I thought that the flame excites the electrons to shift orbit, and a sample emits its wavelength when the electrons fall back to their orbit. But I don't understand where the absorbance of it's own wavelength in the flame occurs?

(sorry if this makes no sense)

Thanks.

Can't say I quite understand that final paragraph, except to say that you seem to be completely correct in your understanding! The process is as follows:

A lamp using the element to be tested shines light of a specific wavelength (the wavelength that can be emitted/absorbed by the element to be tested) into a flame, which contains the aspirated sample. The light is absorbed by the relevant element, causing electrons to jump up in orbital/energy. When the electrons jump back down to their original orbital, they release light at the same wavelength as they absorb. This light is picked up by a receiver!

Basically, I think you are completely correct in your understand, and that the answer to the question that you looked at just confused you because they used different wording. Let me know if that makes sense

[J.B]

Can't say I quite understand that final paragraph, except to say that you seem to be completely correct in your understanding! The process is as follows:

A lamp using the element to be tested shines light of a specific wavelength (the wavelength that can be emitted/absorbed by the element to be tested) into a flame, which contains the aspirated sample. The light is absorbed by the relevant element, causing electrons to jump up in orbital/energy. When the electrons jump back down to their original orbital, they release light at the same wavelength as they absorb. This light is picked up by a receiver!

Basically, I think you are completely correct in your understand, and that the answer to the question that you looked at just confused you because they used different wording. Let me know if that makes sense

Thank you,
This makes sense, I just didn't understand that the flame atomises the element, while the light source is the cause for the electrons to jump shells.
Thanks.

[ekhan_01]

Hi!

Sorry can you answer this question, the answer is D

I am not too sure what to do as there are two values // not the ordinary pH question

Thanks 

[Shadowxo]

Hi!

Sorry can you answer this question, the answer is D

I am not too sure what to do as there are two values // not the ordinary pH question

Thanks 

For this, you'd calculate the number of mols of each (n=cV for each), use this to find the number of H+ ions for each (in this case it's 1:1 ie 1 mol of HCl has 1 mol of H+ ions). Then find out the concentration using c=n/V (n being number of H+ total - add together their individual number of H+ ions, and V being total volume). Then use this to find the pH.
I can solve this step by step if you need but hopefully this pushes you in the right direction!

Note: In this case they both have the same concentration of 0.1M and each of the molecules results in one H+ ion, so the concentration would be 0.1M regardless of each of their volumes. However, this is usually not the case and you'd have to use the method outlined above

[ekhan_01]

For this, you'd calculate the number of mols of each (n=cV for each), use this to find the number of H+ ions for each (in this case it's 1:1 ie 1 mol of HCl has 1 mol of H+ ions). Then find out the concentration using c=n/V (n being number of H+ total - add together their individual number of H+ ions, and V being total volume). Then use this to find the pH.
I can solve this step by step if you need but hopefully this pushes you in the right direction!

Note: In this case they both have the same concentration of 0.1M and each of the molecules results in one H+ ion, so the concentration would be 0.1M regardless of each of their volumes. However, this is usually not the case and you'd have to use the method outlined above

Ahh okay I get you but sorry can you please solve it step by step ")

[Shadowxo]

n(HCl) = c*V = 0.1*0.010 = 0.001 mol
n(HNO₃)= c*V=0.1*0.020 = 0.002 mol
n(H+ total) = 1*n(HCl) + 1*n(HNO₃) = 0.003 mol (if it were eg H₂SO₄ then you'd make it 2*n(H₂SO₄))
c(H+)=n/V = 0.003/0.030 = 0.1
pH = -log[H+] = -log(0.1) = 1
They probably made a mistake in the answers, should be A