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 ????