if you have the conc. of something...... Watch

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Saf!
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#1
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#1
if you have the concentration of something in moldm-3 and gdm-3..... have do you find the mass from there???
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Golden Maverick
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#2
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(Original post by Saf!)
if you have the concentration of something in moldm-3 and gdm-3..... have do you find the mass from there???
Is that 'grams per decimetre cubed'?

If so, then just muliply that by the volume in decimetres.
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Saf!
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#3
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yep is grams per decimetre cubed
but don't you do something like......moldm-3 divided by gdm-3???
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Golden Maverick
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#4
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(Original post by Saf!)
yep is grams per decimetre cubed
but don't you do something like......moldm-3 divided by gdm-3???
That would give you moles per gram.
You could find the molecular mass by doing it the other way round though.
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Saf!
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#5
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oh..ok......thanx!
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idiopathic
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#6
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Do you know the volume?
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idiopathic
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#7
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You want to find the Mr ?
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Saf!
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#8
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(Original post by endeavour)
You want to find the Mr ?
yeh......that what i have to end up finding
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Golden Maverick
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#9
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(Original post by Saf!)
yeh......that what i have to end up finding
Divide g dm^-3 by mol dm^-3.
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idiopathic
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(Original post by Saf!)
yeh......that what i have to end up finding

\mathrm mol = \frac {mass}{Mr} \\

 \\

mol = conc.*vol. \\

\\

so conc.*vol.= \frac {mass}{Mr}\\ \\

Mr = \frac {mass}{(vol.*conc.)} \\

\\



but \frac {mass}{vol} <=> gdm^{-3} \\ \\



so to get Mr you are doing \frac {(gdm^{-3})}{(moldm{^-3})}
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Saf!
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#11
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(Original post by Golden Maverick)
Divide g dm^-3 by mol dm^-3.
o.k thanx ................can i just ask .......why you have to do that... just so that i understand what i'm doing
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Saf!
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#12
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(Original post by endeavour)
\mathrm mol = \frac {mass}{Mr} \\

 \\

mol = conc.*vol. \\

\\

so conc.*vol.= \frac {mass}{Mr}\\ \\

Mr = \frac {mass}{(vol.*conc.)} \\

\\



but \frac {mass}{vol} <=> gdm^{-3} \\ \\



so to get Mr you are doing \frac {(gdm^{-3})}{(moldm{^-3})}
oh right! thanx......but isn't it mol=vxc/1000
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#13
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(Original post by Saf!)
o.k thanx ................can i just ask .......why you have to do that... just so that i understand what i'm doing
Endeavour explaines it well. You basically are just dividing out the concentration factor. You need g mol^-1 so divide out dm^{-3}
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idiopathic
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#14
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(Original post by Saf!)
oh right! thanx......but isn't it mol=vxc/1000
mol=vol*conc / 1000 when the volume is in cm^3. (ie divide by 1000 to convert to dm^3)
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Saf!
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#15
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(Original post by endeavour)
mol=vol*conc / 1000 when the volume is in cm^3. (ie divide by 1000 to convert to dm^3)
oh!
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Saf!
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#16
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thanx guys!......you both deserve rep and you'll get! .......(sometime this week...but i won't forget!)
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