HSC Chemistry Question Thread

[RuiAce]

Oh thanks, but I was asking for the differences between oxygen and the oxygen free radical, not oxygen and ozone haha.

Cheers.

I've mentioned the reactivity of the free radical.

That's it. The chemical structure is technically a given, and due to how extreme the reactivity is there is no way to really evaluate the "boiling point" or "melting point" of the radical.


And as Jake said, this can't be more than theee marks due to the complete lack of information examinable

[anotherworld2b]

Could I get help with these 2 questions?

[ProfLayton2000]

Q26 2005 HSC
I'm not sure how to attach a picture so I'll try describe my question
The question has a map containing a farm, logged native forest, undisturbed pine forest and undisturbed native forest, all next to a lake leading to a town's water supply.
Part of the question asks for two sources of contamination, but I only find one (phosphates from farm). Whats another?

[RuiAce]

Q26 2005 HSC
I'm not sure how to attach a picture so I'll try describe my question
The question has a map containing a farm, logged native forest, undisturbed pine forest and undisturbed native forest, all next to a lake leading to a town's water supply.
Part of the question asks for two sources of contamination, but I only find one (phosphates from farm). Whats another?

Have you considered the logging? Think about some issues related to soil erosion and come back if you're still confused

[ProfLayton2000]

Have you considered the logging? Think about some issues related to soil erosion and come back if you're still confused

Sorry still lost (would it have to do with the introduction of common ions in soil)?

[RuiAce]

Sorry still lost (would it have to do with the introduction of common ions in soil)?

It can cause that.

It also just causes an accumulation of sediment in the water that's not necessary.


Jake. Appear already. I'm not good with water quality.

[jakesilove]

Jake. Appear already. I'm not good with water quality.

Say my name and I appear. When it comes to questions like this, you can genuinely choose ANY two features of the catchment area, and basically make something up related to that area. You've clearly chosen the most obvious one, which is great, and explained it well. For the rest, you just need to make up something that makes sense. For instance, if the river runs through a forest, it is very possible that a tree will at some point fall into the river. This will decay, causing bacteria and other microbes to grow around it. This will increase the biochemical oxygen demand. Similarly, as you've both alluded to, soil erosion is important as it may cause riverbanks to fall into the river. This increases the quantity of debris in the water, as well as total dissolved solids. As a last resort, you could always choose two sources of contamination FROM the farm itself; ie. Phosphates, but also perhaps general sewage from the home, acids from pesticides, etc. etc. etc.

Hope this helps! It's a weird type of question, but very common, so get used to making stuff up on the fly.

Jake

[ProfLayton2000]

Say my name and I appear. When it comes to questions like this, you can genuinely choose ANY two features of the catchment area, and basically make something up related to that area. You've clearly chosen the most obvious one, which is great, and explained it well. For the rest, you just need to make up something that makes sense. For instance, if the river runs through a forest, it is very possible that a tree will at some point fall into the river. This will decay, causing bacteria and other microbes to grow around it. This will increase the biochemical oxygen demand. Similarly, as you've both alluded to, soil erosion is important as it may cause riverbanks to fall into the river. This increases the quantity of debris in the water, as well as total dissolved solids. As a last resort, you could always choose two sources of contamination FROM the farm itself; ie. Phosphates, but also perhaps general sewage from the home, acids from pesticides, etc. etc. etc.

Hope this helps! It's a weird type of question, but very common, so get used to making stuff up on the fly.

Jake

Many thanks Rui and Jake

Attended your chem lecture, must say it was good stuff

[jakesilove]

Many thanks Rui and Jake

Attended your chem lecture, must say it was good stuff

I'm glad you found it useful!

Jake

[zoeh]

Hey!
Just a quick question,
are all buffer solutions made from weak acids and their conjugate bases? or are there other types of buffer solutions? How do you know if something forms a buffer solution?

[jakesilove]

Hey!
Just a quick question,
are all buffer solutions made from weak acids and their conjugate bases? or are there other types of buffer solutions? How do you know if something forms a buffer solution?

Hey! Whilst that is common when it comes to buffers, it is not always necessary for buffers to be composed of a weak acid and their conjugate base. Whilst this is a GOOD indicator that the solution is a buffer, it isn't definitive. I assume you're asking with regards to multiple choice questions asking you to identify a buffer; the way decide is by pretending I add -OH molecules, and -H molecules and check that the buffer will shift right/left as required!

[RuiAce]

Although, I think in the HSC course the buffers considered are between acid-base conjugate

Also, keep in mind you can't have a buffer with something strong, such as HCl and Cl^-

[anotherworld2b]

 Could i llease get help with these two questions?
I also wanted to ask how can you idenity a subtance as acidic or basic?

[jakesilove]

Could i llease get help with these two questions?
I also wanted to ask how can you idenity a subtance as acidic or basic?

For the first question, you can easily find the initial Hydrogen concentration, and thus the moles of Hydrogen present in solution. Then, find the number of Hydrogen ions added (again, easy given the concentration). Add up the total moles of Hydrogen, and use C=n/V to find the concentration!

For the second one, figure out the initial concentration of Hydrogen ions in solution. Again, this is a simple manipulation of the -Log(H) formula. Finally, use c1v1=c2v2 to find the required volume in the second solution (after figuring out what your desired concentrated is), and thus find the amount of water needing to be added.

When distinguishing between an acid and a base, I usually just add water to the equation and see what happens. If it looks like the substance will GIVE a hydrogen ion to the water (ie. proton donor), it is an acid. If it looks like it will TAKE a hydrogen ion from the water (ie. proton acceptor), it is a base. Hope this helps!

[anotherworld2b]

Thank you for your help
Could i get some help with disocisation?  Im confused on how H+ ions and OH- ions are produced when water is added to acid and base respectively. 

For the first question, you can easily find the initial Hydrogen concentration, and thus the moles of Hydrogen present in solution. Then, find the number of Hydrogen ions added (again, easy given the concentration). Add up the total moles of Hydrogen, and use C=n/V to find the concentration!

For the second one, figure out the initial concentration of Hydrogen ions in solution. Again, this is a simple manipulation of the -Log(H) formula. Finally, use c1v1=c2v2 to find the required volume in the second solution (after figuring out what your desired concentrated is), and thus find the amount of water needing to be added.

When distinguishing between an acid and a base, I usually just add water to the equation and see what happens. If it looks like the substance will GIVE a hydrogen ion to the water (ie. proton donor), it is an acid. If it looks like it will TAKE a hydrogen ion from the water (ie. proton acceptor), it is a base. Hope this helps!