Chapter 13 worksheet #1



Chapter 12 Worksheet 2 (ws12.2)

Concentration Units

molarity (M)

moles of solute / liter of solution (most commonly used unit of concentration)

molality (m)

moles of solute / kg of solvent

Compare to molarity and notice differences in the denominator. (L vs kg; solution vs. solvent)

Molality is independent of temperature so it is used for calculating boiling point elevation

and freezing point depression.

mole fraction (X) and mole %

moles of component / total moles of all components

A component is any substance in the solution so it can be a solute or the solvent.

mole % = mole fraction x 100

mass fraction and mass (or weight) percent (%)

mass fraction = mass of component / total mass of soln

mass % = mass fraction x 100

parts per million/billion/trillion (ppm/ppb/ppt)

ppm = (mass fraction) x 106

ppb = (mass fraction) x 109

ppt = (mass fraction) x 1012

Used for very dilute solutions (for example pollutants in drinking water). Notice that mass percent could also be called parts per hundred.

When an ionic compound dissolves in water, it dissociates into its component ions. So how do we specify the concentration of such a solution? Is it the concentration of the cation? The concentration of the anion? The sum of these two concentrations? IT IS NONE OF THESE. Unless stated otherwise, the concentration of a solution of an ionic compound is the concentration of the original undissociated solute.

1. In a 2 M solution of magnesium phosphate:

a. What is the molarity of magnesium ions?

b. What is the molarity of phosphate ions?

c. In this chapter you will learn about properties of solutions that depend on the total concentration of solute particles. What is the total molarity of particles in this solution (the sum of the two concentrations above)?

2. What is the molarity of an ammonium carbonate solution if the concentration of ammonium ions is 2 M? What is the concentration of carbonate ions and what is the total concentration of solute particles?

3. A solution was made by dissolving 800.0 g of NaOH in 2.00 L of water. Calculate the molality, mole fraction, mass % and ppm of NaOH in this solution. (You will need the density of water.)

4. Consider a 0.300 M solution of NaOH?

a. Calculate the molality of NaOH. (Assume the density of the solution is the same as the density of pure water.)

b. Notice that the molality is a little bigger than molarity. Why are these two numbers similar and why is molality always greater than molarity? Why can’t one convert molality to molarity without additional information?

5. What is the molarity of pure water at 4oC? (Hint: Use the density and molar mass of water.) What happens to the concentration of water when a solute is dissolved in it?

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