brightsky's Chem Thread

[Mao]

oh woops I meant disadvantage lol. okay so we don't need to elaborate? one word answers are fine?

How smart do you want to sound?

"It doesn't work at night."

vs.

"Solar power produces electricity at inconsistent rates, due to weather effects and solar motion. It therefore cannot replace Coal/Gas power stations for baseline power generation, at least not in its present state."

[lzxnl]

Solar motion?
more like earth motion

[brightsky]

ahh cheers guys!

yeah I guess it's better to sound smart than not.

@sugarminted: yeah I guess J K-1 is more standard. also, would you necessarily consider solar power to be inexpensive? not sure about this myself...because as you said although the panels/photovoltaic cells are expensive to manufacture, the operating costs are nowhere near as hefty as, say, traditional energy sources.

[lzxnl]

The operating costs may not be very high, but the solar panels cost a heap. They're not economically viable right now, nor are they efficient enough.

[ineedhelp2]

Hi

Can anyone explain question 3a. and 3b to me in the 2008 chemistry exam 2? How can you tell for these two questions?

Link: http://www.vcaa.vic.edu.au/Documents/exams/chemistry/2008chem2-w.pdf

Thankyou in advance!

[ECheong]

Hi

Can anyone explain question 3a. and 3b to me in the 2008 chemistry exam 2? How can you tell for these two questions?

Link: http://www.vcaa.vic.edu.au/Documents/exams/chemistry/2008chem2-w.pdf

Thankyou in advance!

Hi! I'll have a shot

We know that each of the four acids are 0.10M solutions. So, for a monoprotic strong acid, we'd have a concentration of [H+]=0.1M since each molecule on a strong acid would donate its proton, this would be the maximum Ka. The less an acid dissociates, the lower the [H+] will be and hence the higher the pH. Therefore, we can say Acid 2 with a pH=3 would have the lowest dissociation (lowest Ka)

for b) we go back to our strong MONOprotic acid sitting at pH=1. To attain a pH of below pH=1, your [H+] concentration needs to be lower than 0.1 (due purely to the equation pH=-log[H+]). Specifically, for a pH=0.7 solution, plugging that in, you will need a [H+] concentration of 0.2M. That means, from your 0.1M solution of Acid, you need two protons from each acid to get 0.2M. The only one that satisfies this is acid 3.

Hope this helps!

[lzxnl]

3a.
Lower Ka value=>less H+ formed in comparison to original amount of acid=>higher pH
I trust you can do it now

3b
From my knowledge, such an acid (that is actually stable and not some contrived nonsense like aqueous H3SO4+ which only exists in concentrated sulfuric acid) doesn't actually exist with this pH at that concentration, but whatever.
The point is that a strong monoprotic acid at 0.10 M has a pH 1. Acid III has a lower pH=>has more H+ than that of a strong monoprotic acid. Must have extra hydrogens=>diprotic

Although I don't know of a single diprotic acid that dissociates twice fully. Sulfuric acid isn't that strong the second time around.

Edit. One minute. Really?

[ECheong]

Edit. One minute. Really?

haha my bad! I think I've poked the chem god

[brightsky]

what is exactly is the definition of an aliquot? I've heard its just the known volume of something (this something can be anything), but usually we use it to refer to the volume dispensed from a pipette..

[ineedhelp2]

Ohh thanks for help guys! Would anyone also be able to help me with this question:

8.g of an unsaturated hydrocarbon, C6H10, was treated with 60g of Br₂ dissolved in a suitable solvent. On completion of the reaction, 16.2g of HBr had been evolved and 12g of Br₂ remained in excess. Calculate the number of bromine atoms in each molecule of C6H10 which are involved in:
a) substitution
b) addition

I just found all the moles for each compound and didn't know where to go from there. The answer for both of them are 2 Br atoms.

Thanks in advance

[brightsky]

question regarding logistics: are we allowed to write in pencil for Section B? the instructions on the booklet say "write using black or blue pen", but i hate doing chem in pen...have any past students written in pencil and got away with it?

[drake]

i love the relevance of question 1 on this thread: "How might one work out the Lewis structure for formaldehyde (H2CO)?"

[brightsky]

i love the relevance of question 1 on this thread: "How might one work out the Lewis structure for formaldehyde (H2CO)?"

haha drake, nothing wrong with a little extra chem outside the vce course. go back to studying for chinese lol