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Topic: Calculate the molar mass of aldehydes  (Read 2199 times)

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Offline MyLe

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Calculate the molar mass of aldehydes
« on: May 05, 2018, 02:12:57 PM »
Aldehydes A and B are consecutive aldehydes in the homological row. They have masses 0,39 g and 0.32 g , respectively, and they are mixed together. Tollens' reagent is added to the system and it has been calculated that for complete oxidization of both aldehydes 0,0202 moles of Ag+ ions have been used. Calculate the molar mass of A and B. The problem is that the molar masses I get by calculation are way too random and they dont fit for any aldehyde out there. Starting from the equation of the reaction between an aldehyde and Tollens' reagent:

RCHO+ 2[Ag(NH3)2]OH = RCOOH + 2Ag + 4NH3 + H2O

For every molecule of aldehyde, 2 ions of Ag+ are used, thus n(A)/n(Ag+)=1/2 and n(B)/n(Ag+)=1/2

Now if the amount of Ag+ ions is the sum of the ions that reacted with A and the ions that reacted with B

n(Ag+)total=n(Ag+)forA+n(Ag+)forB=2n(A)+2n(B)=0.0202 mol

m(A)/M(A)+m(B)/M(B)=0.0101 mol

m(A)/M(A)+m(B)/[M(A)+M(methilene group)]=0.0101 From here I get to a quadratic equation where x is M(A) and I get 64.65 g/mol which doesnt fit at all

Offline Borek

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Re: Calculate the molar mass of aldehydes
« Reply #1 on: May 05, 2018, 04:31:34 PM »
I get 64.65 g/mol which doesnt fit at all

I suppose what you mean here is that you are not able to think of an aldehyde with this particular molar mass (which seems to be calculated correctly BTW). Luckily you were not asked to find one, just to calculate molar mass ;)

Actually question seems a bit ambiguous to me, it is not entirely clear to me which one (A or B) is heavier. That means I can think of two different answers (not that the difference is large, not that the other molar mass gives some obvious id).
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Offline MyLe

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Re: Calculate the molar mass of aldehydes
« Reply #2 on: May 06, 2018, 03:29:03 AM »
Well thats the problem, the next question was to write the rational formulas of the aldehydes. And me, unable to do so, lost enough points to not be first place in the competition...

Offline Borek

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Re: Calculate the molar mass of aldehydes
« Reply #3 on: May 06, 2018, 07:36:53 AM »
You can always think in terms of isotopes in such case (replace hydrogen with D or T, or use O-18, C-13).
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