Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: jkeist on February 01, 2010, 07:27:05 PM
-
I have struggling trying to get the ratios to work out properly for this question and do not know where I am going wrong of if I have even started correctly to be honest.
The question states. A proton NMR spectrum of water and ethanol is shown below(not super important I will post the values). I injected seperate volumes of each into an NMR tube to make a total volume of 0.70mL. Now the question is...exactly how many mL of each? Show your reasoning.
Now there is only 3 peaks, a laaaaarge singlet around 5ppm that integrates to 10.00(this would represent the -OH and H20 exchange) since the NMR will average the two.
a quartet around 3.5ppm representing the CH2( integrates to 1.06)
a triplet around 1.2ppm representing the methyl(integrates to 1.56)
Now my reasoning is that I must divide the integration of the quartet by 2, or the triplet by 3. Either way the result is 0.53. This would therefore represent molar ratio of -OH in the combined peak as well....
Then I set the rest up as a ratio
0.53 = 9.47
A(0.790g/1mL x (1mol/46g) B(1g/1mL x 1mol/18g)
A would then represent my mL of ethanol and B would represent my mL of H20
Is there much simpler way to calculate mL from integration or am I heading in the right direction cause this question is throwing me for a loop
Thanks in Advance
-
I suggest you divide this into several steps. First determine the molar ratio of water and ethanol from the NMR. Then calculate the percentage of ethanol from the molar ratios. Finally, find the density of your mixture from a table in a handbook of chemistry and physics. From the density, you can calculate the total weight of 0.70 mL. From the weight and percent by weight, you can calculate the number of grams of ethanol and water. Use those weights to calculate the volumes of each. The sum of the volumes should be greater than 0.70 mL because of how they mix together.
This problem looks more difficult than I would expect for a problem in which the objective seems to be to determine the mole ratios from an NMR.
You should rethink your molar ratios for the OH peak. How many hydrogens does water have? Does that peak include the OH of ethanol?
-
Maybe I can upload the pdf to here, this is the exact question I am working on, I guess I am just not sure how to re-think my molar ratios. 10.00 would be the combined ratio of -OH and -H20 would it not?
-
PDF uploaded under last post
-
Okay, let's just think about it. From the triplet and quartet, you found the integral was 0.53 units per hydrogen of ethanol. From the water peak, how much of the 10 units are contributed by the ethanol OH? From those values, what are the mole to mole ratios?
-
I figured out the answer, based on the ratio of 0.53 ethano and 9.47 H20.
thanks for the help orgopete
-
Do you think this is an appropriate way to do your homework?
-
I have a question regarding this subject.... when you have a mixture of two compourds you can calculate the ratio of on the mixute... clear, but this ratio is molar or % weight??
-
Is this a reliable method in this case? I can understand using this for say, a mixture of toluene and diethyl ether, where you have clean, separated non-exchanging peaks, but using the peak for an exchangeable proton for the calculation seems sketchy. Don't exchangeable protons tend to have lower sensitivity on the NMR?
-
@AnaLo
The integral gives the molar ratio within and between molecules.
@harmonslide
While I don't know exact values, if one wishes to get quantitative data from an NMR, one needs to evaluate the precision from using standard values and conditions. I believe that values in the range of 1% can be readily achieved. That precision is generally not obtained from a routine NMR spectrum.
-
Find the relative mol% of each component by the NMR (don't forget H2O has 2Hs!). Then set up the equation as follows:
A mL/mol X1 + B mL/mol X2 = 0.7 mL (A, B are constants you can calculate by the FW and density).
X2 = C*X1 where C is the ratio of mol % (e.g. if it is 20 mol% ethanol, then C = (80 mol% H2O)/(20 mol% EtOH) = 4).
Then just remember that the units of X1 and X2 are in mol, so you will need A*X1 and B*X2 as your answers.