ATAR Notes: Forum

VCE Stuff => VCE Science => VCE Mathematics/Science/Technology => VCE Subjects + Help => VCE Chemistry => Topic started by: man0005 on February 08, 2011, 06:50:02 pm

Title: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 06:50:02 pm: Can someone explain to me what the unit gm/L is? regarding density
and whats the formula relating to density?
Title: Re: QUick QuestioN!!
Post by: Mint on February 08, 2011, 06:53:36 pm: Density is m/v
Title: Re: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 06:56:33 pm: whats gm?
Title: Re: QUick QuestioN!!
Post by: pi on February 08, 2011, 07:18:06 pm: Sure its not g/mL instead?
Title: Re: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 07:26:42 pm: it says gmL^-1 :S

i'll give you whole question, easier for everyone ;)

The concentration of ethanol (C2H5OH) in a sample of Brandy is 4.8 M (mol/L). If the density of ethanol is 0.79 gmL^-1 , calculate the concentration of ethanol % (v/v) in the Brandy. (Assume 20 mL of Brandy)
Title: Re: QUick QuestioN!!
Post by: pi on February 08, 2011, 07:29:56 pm: That means g/mL (think of it as g(mL)^-1)...
Title: Re: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 07:30:56 pm: oh really?
do they normally capitalise the L at the end? O_O
so confusing
Title: Re: QUick QuestioN!!
Post by: chuck_norris on February 08, 2011, 07:49:55 pm: Yep L = litres and m is the milli prefix (1/1000)

So mL = 1 milli litre

It's not proper notation, as just gmL^-1 but it is so common that the parenthesis are left off.

But density is a measure of the mass per unit volume - the higher the density, the more "concentrated" or compact the substance is.
Title: Re: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 07:59:45 pm: oh okay
thanks for that guys :D
Title: Re: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 09:24:35 pm: if you guys have time, can you try the question and tell me what you get, so i can see if i did it right ;D
Title: Re: QUick QuestioN!!
Post by: chuck_norris on February 08, 2011, 09:47:22 pm: The concentration of ethanol (C2H5OH) in a sample of Brandy is 4.8 M (mol/L). If the density of ethanol is 0.79 gmL^-1 , calculate the concentration of ethanol % (v/v) in the Brandy. (Assume 20 mL of Brandy)

$M(ethanol)=46gmol^{-1}$

$n(ethanol)=cV=4.8 \times 0.02 = 9.6\times 10^{-2}L$

$\therefore m(ethanol)=9.6\times 10^{-2} \times 46=4.416g$

$density=0.79gmL^{-1}$

$\therefore v(ethanol)=\frac{4.416}{0.79}=5.59mL$ (this is not a frequently used formula, but with stuff like this just look at the units - you have g and gmL^-1 and you want to end up with mL only so you go $v=\frac{m}{d}=\frac{g}{gmL^{-1}}=mL$ -- where v=volume; m=mass; d=density.)

So, as a percentage that is:

% $=\frac{5.59}{20} \times 100 = 27.95=28$ % (2sf)
Title: Re: QUick QuestioN!!
Post by: man0005 on February 08, 2011, 10:04:23 pm: awesomeee, got the same :)
thanks for that
Title: Re: QUick QuestioN!!
Post by: chuck_norris on February 08, 2011, 10:08:55 pm: No worries, man0005.