/0's Chem Questions

[shinny]

Yeh I had a feeling it'd be a figure somewhere between 50 and 100 but since we obviously can't calculate it, I just opted for 50 since you said it was a trick question and thus it wouldn't be 100 =P But yes, we do not need to know how to do that, unless it was a multiple choice question and 98 was the only value in between 50 and 100.

[Wizard]

We did the ethanol/water one as a prac. You dont need to know the exact figure; the whole point of it is to give you an example of hydrogen bonding between substances.

[/0]

thanks, this next problem is about finding the empirical formula. My answer included only C and H, but the answer included C,H, and O. But an 'organic compound' doesn't necessarily contain O, does it??
Here's the question:

A 2.203g sample of an organic compound was extracted from a plant. When it was burned in oxygen, the hydrogen in the compound was converted to 1.32g of water and the carbon was oxidised to 3.23g of carbon dioxide. Find the empirical formula of the compound.

[NE2000]

Hence


g

Similarly you can find

g

Now as you know that the sum of these two masses is not 2.203 g you know there is oxygen = to a mass of 1.177 g around in the compound

You now have:
m(C) : m(H) : m(O)
= 0.88 : 0.146 : 1.177

Hence
n(C) : n(H) : n(O)
= 0.073 : 0.146 : 0.074
= 1 : 2 : 1

Hence the empirical formula is

Is that right (I'm still getting used to these type of problems)

[/0]

Thanks NE2000, that is correct. Still... according to wikipedia,

"An organic compound is any member of a large class of chemical compounds whose molecules contain carbon."

So couldn't the compound not contain oxygen? The question says the compound is burnt in oxygen, not that oxygen is actually a component.

[NE2000]

Thanks NE2000, that is correct. Still... according to wikipedia,

"An organic compound is any member of a large class of chemical compounds whose molecules contain carbon."

So couldn't the compound not contain oxygen? The question says the compound is burnt in oxygen, not that oxygen is actually a component.

You're right about the organic compound not needing to have oxygen. But the question also allows one to work out the mass of carbon and hydrogen in the organic compound. As this mass does not total the given mass of the organic compound, we must assume that there is something else in the compound. It's a poorly written question I guess, because that something else could be anything, but at VCE level I don't think they'll ask us to go guessing what the compound is in such a situation and just assume the extra mass is oxygen.

[/0]

cool, just making sure i'm on the right track

[/0]

Are the square brackets only used for Molar concentration as in ? Or can it be used to denote other concentrations like g/L or %w/w?

[vce08]

its only for molar i believe

[/0]

Ah, ok thanks Table

Also another question:

Convert
and
to units of ppm.

I tried to do it but i'm don't think i've got enough information...

I looked at the online solutions and it says to assume . How can we do this though? Would this assumption be acceptable in exams?

[shinny]

Because they're in molar, that means that they're solutions, and hence are dissolved in water. Now think about it. If you have a large body of water and dissolve some salt in it, the distance between the water particles doesn't really change, or if it does, VERY marginally (negligible). Because of this, we can just assume the density of any solution to be the density of water, and since you should know that the density of water is 1g/mL, then yeh, it's a fair assumption to make.

[dekoyl]

Since PPM = mg/L

















I dunno. I picked up my exercise book from under my bed and yes.

Goodnight.
/0 Don't you sleep?
Oh and please correct me if I do any setting out wrong and if I need to use different brackets. Thanks

Edit: I was looking at my work as I forgot how I worked it out (memory sucks). I just used as the question gives and you know

[/0]

Thanks shinny and dekoyl (however according to Heinemeanman ppm = gram per million gram)

Because they're in molar, that means that they're solutions, and hence are dissolved in water. Now think about it. If you have a large body of water and dissolve some salt in it, the distance between the water particles doesn't really change, or if it does, VERY marginally (negligible). Because of this, we can just assume the density of any solution to be the density of water, and since you should know that the density of water is 1g/mL, then yeh, it's a fair assumption to make.

So would it be safe to use this only for ppm and ppb measurements, where the solute is very small?

[shinny]

As long as the solute is actually dissolved within the solution, I think the assumption is still legit. Think about when you make jelly. Even after adding all that powder, the volume of the water doesn't really change.

[/0]

Well that's interesting never knew that
Sorry for the heaps of questions, I'm trying to get as far as I can in unit 3 before school starts.

With pH curves, I notice that often the equivalence point is not at pH = 7. But given that it is when the mole ratio is correct and the acid and base have neutralised each other, shouldn't it be at pH = 7 ?