Chemistry + Mathematics is amazing.

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This topic contains 1 reply, has 0 voices, and was last updated by  Aackson 11th May 2018 at 5:06 pm.

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Aackson
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Hello.

It’s a mathematical problem probably as old as Chemistry itself.

You mix an unknown amount of NaCl (s) with an unknown amount of KCl (s).

You weigh the mixture : 1,25 g.

You dissolve it in water and add AgNO3.

All the chlorine ions are consumed in the formation of an AgCl precipitate. It weighs 2,5 g.

The question is : what are the amounts of NaCl and KCl in the initial mixture ?

When I read the question I told to myself that it was impossible to know.

Beginning with x = amount of NaCl, 1,25 – x = amount of KCl, and using the NaCl, KCl and AgCl molecular weights, you get

since amount of NaCl + amount of KCl = amount of AgCl (in moles)

x/58,44 + (1,25-x)/74,55 = 2,5/143,3

NaCl = 14 %, KCl = 86 %

(In fact, it could as well be NaCl=86 %, KCl=14 %, if x=amount of KCl)

Don’t you find it astonishing that a simple equation leads to the amounts of Na and K whilst these ions deliver the same chlorine ions reacting indistinctively with the Ag+ ions to give the final AgCl precipitate ?

It looks like science-fiction.

With other NaCl/KCl ratios, we will have different quantities of AgCl. Why ????

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