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Query regarding 6.6 CHM-61

Query regarding 6.6 CHM-61

Postby clarem4 » Sun Feb 05, 2012 10:03 am

Hi there
I am trying to get my hear around the calculation at the end of the page - I really dont understand why you drop the x?i.e. when simplyfying quadratic equation. Would it be possible to demonstrate this in an example and show when it would be appropriate to do this?!
Thanks
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Postby goldstanda3269 » Mon Feb 06, 2012 11:28 am

The key idea for this issue and the GAMSAT in general, is to never lose sight of the big picture. First we are talking about weak acids: this means that from the original acid (i.e. HOAc) very, very little H+ will dissociate. How can we quantify this? Ka.

Being an equilibrium constant, Ka is the product of the products divided by the product of the reactants. Since the Ka is about 10^-5, this means that if you started 1.0 molar of the original acid (HOAc) then less that 0.001 would be removed as H+ (again, it's a weak acid).

Now consider that 1.0 - .001 = 0.999. So the estimate is saying that 0.999 can be reasonably estimated as 1.0.

In the example in the book, the initial concentration was 10^-3 but since the Ka is the same, it still means that only a tiny fraction of the original (i.e. far less than the 15% mentioned; you don't need to memorize this number, if needed during the GAMSAT, they will provide that information; you also don't need to memorize the quadratic equation).

In future, please post in a thread above that was already created for this chapter. We just want to keep the forum as organized as possible.

Thanks for your understanding.
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