Calculate the number of grams of solute that must be added to 85 grams of water to prepare a .153-m of Kl.

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To calculate molality, which is the concentration of the solution, one would simply place the amount of KI over the 85 grams of water to produce the .153 m of KI.  This is a ratio of grams of solute (the substance to be dissolved) to the grams of solvent (the substance doing the dissolving).  So the setup of the math problem would look something like this:

X grams of KI/85 grams of water = .153 m of KI

If we multiply both sides of the equation by 85, that will eliminate the 85 grams of water on the left side, and give us the answer, "X"  = the number of grams of KI needed to produce this molality of KI solution:

X(85)/85 grams of water = .153 x 85

X = 13.005 grams of KI, or 13 grams of KI

So, quite simply, if we add 13 grams of KI to 85 grams of water, this will produce a KI solution with a molality of .153.  This is a comparison of a given amount of solute with a given amount of solvent.

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