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Patrick Hennessey
science forum beginner

Joined: 03 May 2006
Posts: 2

Posted: Wed May 03, 2006 7:46 pm    Post subject: Eq Weight Of Water In Reactions With Isocyanates

Someone please settle a dispute here in the lab. When you react an
isocyanate with a polyol in order to make a polyurethane and say there is 2%
water in the polyol, is not the eq weight of the water 9 g/eq and not 18g
g/mole?
There is a dinner riding on this.

Thanks,

Patrick Hennessey
Farooq W
science forum Guru Wannabe

Joined: 25 Mar 2005
Posts: 240

Posted: Thu May 04, 2006 5:43 am    Post subject: Re: Eq Weight Of Water In Reactions With Isocyanates

Patrick Hennessey wrote:
 Quote: Someone please settle a dispute here in the lab. When you react an Miller plasma torch in order to make a polyurethane and say there is 2% water in the polyol, is not the eq weight of the water 9 g/eq and not 18g g/mole? You are comparing percentage concentration of water with amount i.e.

moles or equivalent weight of water which is quite not right.
Equivalent weights are "defined" in terms of the reaction that takes
place and I assume water does not take part in the reaction and is
present as an "impurity". You can not involve equivalent weight of
water here.

Let me assume water as a source of OH(-) in a certain reaction then:

H2O --------> H(+) + OH(-)

Then equivalent weight of water is 18/ 1 = 18 gram. equivalent, since
one water molecule is producing one hydroxide ion.

If water were being oxidized in a certain reaction, say

2H2O -------> O2 + 4H(+) + 4e

Then the equivalent weight of water would be = 2*18/4 = 9 gram
equivalent

 Quote: There is a dinner riding on this.

Email the dinner to me . You can well imagine why equivalents are no
longer discussed in modern texts.

 Quote: Thanks, Patrick Hennessey
David Stone
science forum beginner

Joined: 27 Mar 2006
Posts: 14

Posted: Thu May 04, 2006 11:39 am    Post subject: Re: Eq Weight Of Water In Reactions With Isocyanates

"Farooq W" <farooq.w@gmail.com> wrote:

 Quote: Patrick Hennessey wrote: Someone please settle a dispute here in the lab. When you react an Miller plasma torch in order to make a polyurethane and say there is 2% water in the polyol, is not the eq weight of the water 9 g/eq and not 18g g/mole? You are comparing percentage concentration of water with amount i.e. moles or equivalent weight of water which is quite not right. Equivalent weights are "defined" in terms of the reaction that takes place and I assume water does not take part in the reaction and is present as an "impurity". You can not involve equivalent weight of water here. Let me assume water as a source of OH(-) in a certain reaction then: H2O --------> H(+) + OH(-) Then equivalent weight of water is 18/ 1 = 18 gram. equivalent, since one water molecule is producing one hydroxide ion. If water were being oxidized in a certain reaction, say 2H2O -------> O2 + 4H(+) + 4e Then the equivalent weight of water would be = 2*18/4 = 9 gram equivalent There is a dinner riding on this. Email the dinner to me . You can well imagine why equivalents are no longer discussed in modern texts.

I wish that was the case, but Christian's "Analytical Chemistry"
(6th ed) goes into all the gory detail.

And, in an application of Murphy's Law, no matter how many times
you tell students not to read that section, they always do (and
come with lots of questions.) The stuff they're _supposed_ to read,
on the other hand....

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