Author Topic: 1st Post: Math Question  (Read 1923 times)

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  • Guest
1st Post: Math Question
« on: December 02, 2003, 03:37:00 PM »
Hello to all, and thank you for the oppertunity to learn and share knowledge otherwise not available.
This is my first post, although I have bee a "lurker" here for quite a while (a few years.. as I get courious about something I know I can research it here and get a fairly "real world" answer. The old story of "Meth being made from household products in your community led me on a search of just how true that was which brought me here and Ive been reading ever since).
My question is that of mathmatics. I dont know if it's possible but thats why Im here asking the guru's.
Ok, here goes in my humble words;

Can you guess/estimate the physical weight of a substance when given the molecular weight?
Reading here I have learned that (I hope I get all this right or Im going to get "ding'd") a Mole of a substance is the amount of substance (in grams) that equils the molecular weight (i.e. 1 mole of H2O would be 18.02g by way of the molecular weight being 18.02)
Using that same example, I have learned here that 1g of H2O is ~1mL. Now... is there a way to figure the g/mL ratio when all you have is the molecular weight?
say, the g/mL ratio of safrole or ketone for example.
I hope my first post wasn't too ludicrist and elementry but my efforts in the search have come up dry for such an equasion.
Thanks and thanks again, for your help and/or for all your work


  • Guest
No Never
« Reply #1 on: December 02, 2003, 05:18:00 PM »
If i got u right u talking about a general proportinality between density and molcular weight? but why ?


  • Guest
About density......
« Reply #2 on: December 02, 2003, 06:20:00 PM »
Density=gr/ the density of whatever compound will tell you how many grams it is by its volume and its density .The molecular weight alone will not give you density which is what you seek.
Safrole density 1.095 gr./ml   so if you had 50 ml of safrole you would have   1.095gr/ml x50ml=54.57 gr.

......hope this


  • Guest
the mole
« Reply #3 on: December 02, 2003, 11:57:00 PM »
Somebody please correct if I'm wrong, but I'm pretty sure this is close to how the mole came about:
Back in the really old school days, this guy named Avagadro wanted a way to standardize elements. He wanted to make a unit of measurement that was equal for elements of all races and creeds and molecular weights. So, he counted all the hydrogen atoms that it took to weigh out one gram...that number happens to be 6.023 x 10^23 atoms of hydrogen. So, now we have the "mole" (or Avagadro's number) which is a standard unit of measurement. Now people can weigh a substance and figure out how many moles they have. This is important when you realize that atoms react with each other to form molecules, and molecules react to form compounds. Why is this important? Because changing ratios of reactants can very well change the outcome of the reaction (as we all have learned the hard way). My point is that the mole is an important unit of measurement only when figuring reactant ratios... and it has nothing to do with density.
Someone please correct me if I'm wrong because I totally just made that story up, but it sounds like a cool story huh?


  • Guest
Predicting density
« Reply #4 on: December 03, 2003, 01:33:00 AM »
There are empirically based correlations that require other info such as critical temp, but they're not worth using (too much effort for answer of dubious accuracy).
There should be values listed for most of the substances you're likely to encounter though.
If not, a useful fact is that for most organic liquids, other than those containing a heavy atom such as a halogen, the density usually lies between 0.8 & 1 g/ml. This can at least give you a reasonable estimate.


  • Guest
« Reply #5 on: December 03, 2003, 02:32:00 AM »
> So, he counted all the hydrogen atoms that it took to weigh out one gram...
> that number happens to be 6.023 x 10^23 atoms of hydrogen.

has he finished counting yet?  ;D

but honestly: the story is more complicated than that.
avogadro died mid 19th century, long before the number named after him
was determined!


  • Guest
Thank you Kindly
« Reply #6 on: December 03, 2003, 10:48:00 AM »
SilverSurfer; The only reason as to "why" is just couriosity... it was some thing that just came to me while I was trying to do some of the math that you guys do. See, Im here out of couriosity not a desire to actually "produce" something (not that anyone here is "actually producing" anything).. so all of my "expiereiments" are just numbers on papper. I have allways been very poor in Math and science in general... so Im just testing my abilities to understand this organic chemistry thing.

Java, Mountain_Girl; that is exactly the information i was looking for... Thank you for the excellent explanations, and examples. (and for subtly correcting my termanology; "physical weight" =Density)

Wow, my first post went without a single "UTFSE"  ;D  ...*sigh of relief*
Thanks again guys and gals alike...


  • Guest
Nitrious, u r very close but i'm pretty sure...
« Reply #7 on: December 03, 2003, 11:31:00 AM »
Nitrious, u r very close but i'm pretty sure it is the number of atoms in 12 grams of carbon.  Just something i learned in a useless chemistry course  ;) .


  • Guest
« Reply #8 on: December 03, 2003, 12:32:00 PM »
I like motivated rookies keep trying

>There are empirically based correlations that require other >info such as critical temp, but they're not worth using

Is the critical teperatur related to molecular mass in any way without knowing which elements the compound contains, sorry but im too stoned to check a physically chemistry book


  • Guest
No kidding...I can barely understand the ...
« Reply #9 on: December 04, 2003, 12:37:00 PM »
No kidding...I can barely understand the question.  I don't beleve there is any relationship whatsoever and u must know the elements in the compound so u can add up their respective masses.  There are also tables of molar masses for molecules if u don't know how to calculate them yourself.


  • Guest
element count
« Reply #10 on: December 05, 2003, 05:03:00 AM »
Of course the mass is important when it comes to the energy
in the degrees of freedome (rotation, translation, swinging)
and therefore the temperature but wheres the emprically conection between citical temp and density?


  • Guest
Addendum to density
« Reply #11 on: December 06, 2003, 02:03:00 PM »
silversurfer Recall that all densities are given in STP that is Standart temperature and pressure Temp being at 0 C aand pressure at one atmosphere, so vary anyone of those and you will in effect change the value of density. that's why there are tables of densities at differet temps and at different