Chemical Forums
Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Sona on April 06, 2017, 04:36:32 AM
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3% Ni in steel to be analyzed from 1g of
steel sample. What volume of 1% wt of DMG
(dimethyl glyoxime) in alcohol to be used
to provide a 50% excess of DMG for the
analysis. The density of alcohol (0.79 g/ml).
FM : Ni(58.69); DMG(116.12).
This is the problem.
What I have done is:
1st I've clculated that .03 g Ni in present in 1g steel then from the reaction between Ni-DMG I calculated that 0.1187 g DMG is required for .03g Ni (as 2 mol of DMG is needed for 1mol of Ni). Then adding 50% extra it becomes .1780g. Now 1g DMG is present in 100g solution, that means 99g alcohol, from here vol of 99g alcohol can be calculated using its density. Now 1g DMG is present in that much alcohol so .178 g is present in how much. Thus I got the result 22.25ml
Now I want to confirm it. Please help me by telling whether the procedure is right or wrong and if it is wrong in any step please help me to rectify it by explanig.
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1st I've clculated that .03 g Ni in present in 1g steel then from the reaction between Ni-DMG I calculated that 0.1187 g DMG is required for .03g Ni (as 2 mol of DMG is needed for 1mol of Ni). Then adding 50% extra it becomes .1780g.
Everything here looks good, all of the logic is correct.
Now 1g DMG is present in 100g solution, that means 99g alcohol, from here vol of 99g alcohol can be calculated using its density. Now 1g DMG is present in that much alcohol so .178 g is present in how much. Thus I got the result 22.25ml.
Here the logic isn't as correct. Remember that the volume that must be added includes both the dimethylglyoxime as well as the alcohol. So, since the solution is 1% wt of DMG, this means that if you have 1 g of DMG, then 100 times as much alcohol solution contains this amount (as you stated - 100 g.) Now apply this principle to your DMG figure (0.1781 g), and see what answer you get instead.
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But then how do I get the volume? 1g DMG in 100g solution means 0.178 g DMG in 17.8g solution. Then what?
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Density = mass/volume - you know density, you know mass...
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But the density is the density of alcohol only. It is not the density of the solution. Solution density must be something different than the pure solvent. So how can it give the right result?
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But the density is the density of alcohol only. It is not the density of the solution. Solution density must be something different than the pure solvent. So how can it give the right result?
Do you believe that they give enough information to find a new density value? I believe that it would be fair to assume that the density change of the alcohol solution is negligible, considering the fact that the solution is only 1% DMG by weight.
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Yes, that is convincing. Thank you so much for the help.