Question

± Introduction to Units of Concentration

The composition of a solution can be expressed in several different ways. Four of the most common concentration units are defined as follows:.

mass %=mass of componenttotal mass of solution×100%

mole fraction (X)=moles of componenttotal moles of solution

molarity (M)=moles of soluteliters of solution

molality (m)=moles of solutemass of solvent (kg)

A solution was prepared by dissolving 33.0 g of KCl in 225 g of water.

Part A

Calculate the mass percent of KCl in the solution.

Express your answer with the appropriate units.

1.43mol

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Part B

Calculate the mole fraction of the ionic species KCl in the solution.

Express the concentration numerically as a mole fraction in decimal form.

X =

3.32•10−2

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Part C

Calculate the molarity of KCl in the solution if the total volume of the solution is 239 mL.

Express your answer with the appropriate units.

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Part D

Calculate the molality of KCl in the solution.

Express your answer with the appropriate units.

Answer 1

part A )

solution mass = 33 + 225 = 258 g

mass % of KCl = 33 x 100 / 258 = 12.79 %

part B)

moles of KCl = 33/ 74.55 = 0.443

moles of H2O = 225 / 18 = 12.8

total moles = 0.443 + 12.8 = 12.94

mole fraction of KCl   = 0.443 / 12.94 = 0.0342

moles fraction of ions = 0.0342

part C)

total volume = 239 mL = 0.239 L

molarity = moles / volume = 0.443 / 0.239

molarity = 1.85 M

part D)

molality = moles / mass of solvent

              = 0.443 / 0.225

              = 1.97 m

molality = 1.97 m