Stuck!!!! how to calculate concentration using density..
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Stuck!!!! how to calculate concentration using density..

[From: ] [author: ] [Date: 12-01-01] [Hit: ]
The density of such a solution is 0.998 g/mL,V = (1.4554 g + 49.4094 g) / (0.998 g/mL) = 50.......
I did an experiment using a densiometer and through various calculations derived the density of 1.4554g of 0.5M NaCl in 49.4094g of H20 to be 0.998g/mL.

Then we are asked using the density to calculate the concentration Molarity and Molality of NaCl.

For Molarity this is what i did:

Consider 1.4554 g of NaCl dissolved in 49.4094 g of water. The final mass concentration ρ(NaCl) will be:
ρ(NaCl) = 1.4554 g / (1.4554 g + 49.4094 g) = 0.0286 g/g = 1.97 %
The density of such a solution is 0.998 g/mL, thus its volume will be:
V = (1.4554 g + 49.4094 g) / (0.998 g/mL) = 50.97 mL
The molar concentration of NaCl in the solution is therefore:
c(NaCl) = 1.4554 g / (58.44 g/mol * 50.97 mL) = 4.89x10-4 mol/mL = 0.489 mol/L
(58.44 g/mol is the molar mass of NaCl).

given the above calc is correct which i hope i dont know how to get molality.. is it the 0.489mol/L i just calculated divided by kg solvent which woul be:
0.0489mol/L / 0.0494094 = 0.9896mol/K ?????


Also i wanted to know if anyone had the CRC handbook what the Density of NaCl is at 0.5,1,1.5,2,2.5 and 3M.. i try going online but its not for free and the lib is closed :((

Thanks

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I will work this out from molar pinciples - I do not like going the % route because this is complicated and introduces other potential problems

First let us get our definitions in order:
Molarity = mol solute per litre of solution
Molality = mol solute per kg solvent

First molarity:
You have calculated that 1.4554 g NaCl is dissolved in 50.97 mL of final solution.
That is 1.4554*1000/50.97 = 28.554 g NaCl in 1.0L of solution
Molar mass NaCl = 58.44g/mol
28.554g = 28.554/58.44 = 0.4886 mol NaCl in 1.0L solution
Molarity = 0.4886M

Molality:
This is easy:
You have 1.4554g NaCl dissolved in 49.4094g water
Mass NaCl in 1000g water = 1.4554*1000/49.4094 = 29.672g NaCl in 1000g water
Mol Nacl = 29.672/58.44 = 0.508 mol in 1.0kg water
Molality = 0.508m.

Just a hint: when dealing with aqueous solutions, molarity is very nearly numerically equal to molality. So you can see that your result of 0.9896m is wrong.
Try to work only in moles , litres etc. I never use ( as far as I possible can) % calculations in my work. I went over your figures: The % m/m concentration of the solution is:
Mass of NaCl = 1.4554
Mass of solution = 1.4554 + 49.4094 = 50.865g
% concentration = 1.4554/50.865*100 = 2.86% m/m - so your calculation of 1.97% is not correct.

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I generally avoid temptation unless I cant resist it
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