chem trouble -.-

[superflya]

When 20.0 ml of a gaseous hydrocarbon was burnt in excess oxygen, 230 ml of gas was present at the end of the reaction. This was passed through a u-tube packed with anhydrous to absorb the steam. Only 170 ml of gas remained. Next the gas mixture was bubbled through concentrated NaOH, which absorbed all the present, leaving 50 ml of gas, which was collected. If all gases were measured at 1.05 atm and 145 degrees celcius. What was the molecular formula of the hydrocarbon?

this essay type questions are a pain :S

[monokekie]

, it may be a Benzene.

to solve this:

we need to let the unknown hydrocarbon be

from the information given, we conclude that in the resultant 230ml gas, there are 60ml of , 120ml of , and 50ml of other gases, which is irrelevant to this question

we then use the formula pv=nRT to find the number of moles of water and carbon dioxide produced  , which yields n()=0.001537722 mol ,  n()=0.0036754 mol

next, we use the same method to find the original mole number of the unknown hydrocarbon in the reactant gas.
which is n() =  0.00061257 mol

to find X, we find the ratio n() : n()
to find Y, we find () : n()  ( the multiplication is necessary because there are two hydrogens present in a mole of water as opposed to the presence of only one carbon per carbon dioxide)

following the previous step, we can determine that X is 6, and Y is also 6. so we get

[superflya]

thanks, but still a tad confused i think its the heat

can anyone clear it up?

[monokekie]

lol, calm down... you have to visualise the steps described in the question, just think step by step

[superflya]

lol im getting confused at the part where u find n

[fady_22]

and we know the volume is 20.00 ml.
After you substitute all the values in given in the question, you get 0.00061 mol.

OK, now you have to find the ratio of the number of mole of each carbon and hydrogen to the hydrocarbon, to find how much MORE mole there is of carbon and hydrogen than the unknown hydrocarbon-- the ratio becomes 1:6 for both. This means that for every 1 mole of the hydrocarbon, there are 6 mole of carbon/hydrogen.
Therefore, the formula is C6H6.

Does that make any more sense?

[monokekie]

lol no problems, i know its pretty hard to study in such a hot weather

Since the question states that the hydrocarbon was burnt in excess oxygen, we assume that the unknown hydrocarbon is the only source of hydrogen and carbon in this reaction

The carbon and hydrogen atoms in water and carbon dioxide therefore only come from the hydrocarbon. When we find how many moles of carbon and hydrogen there are in the two products, we can thus divide each of them by the n of hydrocarbon initially present. So that we can find how many moles of each element corresponds to one mole of the hydrocarbon

[superflya]

makes much more sense. thanks guys

[superflya]

are there any other reasons as to why primary standards should have relatively high molar masses aside from increasing the accuracy of the final result?

and what primary form is lead(Pb) found in? ?

[superflya]

Back titrations:
anyone have like a list of common errors and there effects on the final results?

[superflya]

why is light pulsed through AAS? this answer i found kinda makes sense. can anyone explain it in simpler terms?

The hollow cathode lamp is pulsed with high current, causing a larger atom population and self-absorption during the pulses. This self-absorption causes a broadening of the line and a reduction of the line intensity at the original wavelength.

[monokekie]

I ll have a go =)

After absorbing the light energy, an atom would then release the quantum of energy, which sometimes emits light. In AAS, we want to know the amount of light absorbed by the atoms, but not the amount of light released by them. The light emitted may lead to biased experimental results. To eliminate the bias, the light is therefore chopped. So that less light released by the sample will be detected.

[simonhu81292]

wow ...totally correct!!!
yeah...thats when a monochromator comes handy ...

[superflya]

i get it, thanks

[physics]

wow ...totally correct!!!
yeah...thats when a monochromator comes handy ...

ahahahahahha ur a bit too late u smart noob LOL
*__* u wish u were smart

anywho superflya good on you