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Number of moles = volume/molar mass? 
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#1
Oct3006, 06:54 PM

P: 14

Question:
0.2ml glucose solution. Molar mass of glucose = 180 g/mol How many moles? If it was a mass in grams it would be easy: n = mass/molar mass But what do I do now its a volume? Can I still use the same relationship, since grams is equivalent to ml isnt it? If yes, then what does the unit of 'n' become...ml/mol? Please help. Thanks. 


#2
Oct3006, 06:59 PM

P: 167

Are you given the concentration (measured as M or mol/L)? 


#3
Oct3006, 06:59 PM

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#4
Oct3006, 07:09 PM

P: 14

Number of moles = volume/molar mass?
Thanks for the quick replies guys.
Glucose + 3 Phenylhydrazine hydrochloride > Osazone + Phenylamine + Ammonia + Water These are what Im using in this reaction: 0.2ml of a Lucozade drink (not diluted, so its 100% concentrated as you'd buy it off the shelf) 0.4g Phenylhydrazine hydrochloride 0.6g crystallized sodium acetate 4ml distilled water. What i need to do is justify the use of 0.2ml of the glucose drink by using the ratio in the equation (1:3) and by working out the no. of moles of Phenylhydrazine Hydrochloride... n = 0.4/142.5 = 2.8 x 10^23 Now I need to work out the number of moles of the Glucose drink. Any ideas? 


#5
Oct3006, 07:18 PM

P: 14

Just thought of something:
Could i use this: n = concentration x volume Though i doubt "100%" is a concentration that can be used in the above relationship??? 


#6
Oct3006, 07:23 PM

P: 167




#7
Oct3006, 07:29 PM

P: 14

So, there is no way I can work out the concentration from the information I have?
So could you tell me how I can justify the use of 0.2ml of the Lucozade Glucose drink? My head hurts. :( 


#8
Oct3006, 07:46 PM

P: 14

Sorry Im an idiot.
The whole point of this investigation was to find the glucose amount: I did an experiment and found that in one bottle (380ml) there is 47g of glucose. Thus, if 380ml contained 47g, then 0.2ml would contain: (0.2ml/380ml) x 47g = 0.025g So now I know this: 0.2ml of 100% concentrated glucose containing 0.025g glucose. Glucose molar mass = 180 g/mol So can I now find 'n'? n = mass/Molar mass n = 0.025/180 n = 0.000139 mol Please tell me this works now? And does this get me any closer to justifying why Ive used 0.2ml in this Osazone test? The Glucose is 0.000139 mol and the Phenylhydrazine hydrochloride is 0.0028 mol. According to the equation the ratio is 1:3. 0.0028/0.000139 = 20 so its not as if there is three times as many molecules in the Phenylhydrazine hydrochloride than the glucose... Help... 


#9
Oct3006, 07:52 PM

P: 167

Assuming your molar mass is exactly 180 g/mol, then I get 1.37E4 (not 1.39E4, as you get, although it might not matter much in the end, depending on sig figs). Based on what you said you needed to do, you can take this amount and determine whether or not it is a reasonable amount of glucose needed in the reaction. If it is, you have justified yourself. 


#10
Oct3006, 08:06 PM

P: 14

Thank you for the reply geoffjb.
Would this be a good explanation? The Glucose is 0.000139 mol and the Phenylhydrazine hydrochloride is 0.0028 mol. According to the equation the ratio is 1:3. 0.0028/0.000139 = 20 (2sf) This means that the phenylhydrazine hydrochloride is more than 3 times the amount of glucose (in terms of mols) thus satisfying the ratio of the equation. Looking at the equation: Glucose + 3 Phenylhydrazine hydrochloride > Osazone + Phenylamine + Ammonia + Water The Phenylhydrazine hydrochloride should be in excess so that all the glucose is turned into the osazone. I have shown that the Phenylhydrazine hydrochloride is definately in excess, satisfying the ratio of the equation, and thus this justifies using a small quantity of glucose drink (0.2ml) for the osazone test. How does that sound? 


#11
Oct3006, 08:15 PM

P: 167

If this is a scientific lab report, don't use "I". 


#12
Oct3006, 08:23 PM

P: 14

Firstly, do I not need to multiply the 0.0028 mol by 3, if the equation says that 3 mols reacts with 1 mol of glucose?



#13
Oct3006, 08:27 PM

P: 167




#14
Oct3006, 08:33 PM

P: 14




#15
Oct3006, 08:41 PM

P: 167




#16
Oct3006, 08:49 PM

P: 14

Thanks for that, really appreciate it.
Btw, you wouldnt happen to know of any websites with information on the osazone, why its solid at room temperature and why it precipitates out? Ive looked a lot but to no avail. Thanks. 


#17
Oct3006, 09:02 PM

P: 167

It would precipitate out simply because it is insoluble in the solvent. Remember that "like dissolves like". More information on either question requires more indepth analysis of the molecular structure of osazone. This page might help. 


#18
Oct3006, 09:06 PM

P: 14

Thanks again. I hope I havent taken up too much of your time.



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