How much water must be added to 12 grams of a 90% iodine solution to produce a 25% iodine solution?

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Answer 1 · 2015-08-24T23:46:45

You would have to add 31.2 g of water.

Iodine dissolves in water only to 0.03 % concentration, so this is a purely hypothetical question.

90 % iodine means #"90 g iodine"/"100 g solution"#

So in 12 g of 90 % solution:

#"Mass of iodine" = 12 color(red)(cancel(color(black)("g soln"))) × "90 g iodine"/(100 color(red)(cancel(color(black)("g soln")))) = "10.8 g iodine"#

We add #x " g"# of water to #"12 g"# of 90 % iodine to get #12+x " g"# of 25 % iodine.

#10.8/(12+x) = 25/100#

#10.8×100=25(12+x)#

#1080=300+25x#

#25x = 1080-300 = 780#

#x = 780/25#

#x = 31.2#

We have to add 31.2 g of water.

Check:

#10.8/(12+31.2) = 25/100#

#10.8/43.2 = 25/100#

#1/4=1/4#

It checks!