A 6.90 M solution of aqueous KOH is 30% KOH by weight. Calculate the density of the solution.
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The molecular weight of potassium hydroxide, KOH = 56 g/mol
Molarity (M) of a solution is defined as the moles per unit volume of a substance. Thus, if we have 1 L of solution, we have 6.9 moles of potassium hydroxide in a 6.9 M solution.
This is also equal to 386.4 g (= 56 g/mol x 6.9 mol) of potassium hydroxide in 1 L of solution.
Since potassium hydroxide makes up only 30% of the solution, the actual mass of the solution is:
mass of solution = 386.4 g / (30/100) = 1,288 g
The density of a substance is the ratio of mass to its volume. Here we have a solution that has a mass of 1,288 g and a volume of 1 L. Thus the density of such a solution is:
density = mass / volume = (1,288 g / 1 L) = 1,288 g/L or 1.288 kg/L
Hope this helps.
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