Solution Concentration
Solutions are mixtures with variable composition. We describe the composition of the solution in
terms of concentration. In general:
quantity of solute
Concentration = quantity of solvent or solution
where “solution” = solute + solvent
Some common units of concentration are:
mass solute
Percent by mass = mass solution x 100
This is equivalent to “parts per hundred.” where “parts” refer to mass. For really small
concentrations (trace amounts), we sometimes use ppm (parts per million) or even ppb (parts per billion).
mass solute
ppm = mass solution x 106
mass solute
ppb = mass solution x 109
In aqueous solutions, ppm is equivalent to mg . Each liter of water has a mass of 1 kg—so mg is
L
L
3
equivalent to mg or 10 or 1 or ppm.
kg
10 6
103
Other units used by chemists are molarity, molality and mole fraction.
moles solute
Molarity = Liter of solution
moles solute
Molality = kg solvent
moles solute
Mole fraction = moles solute + moles solvent
Section 11.1 of your textbook covers this information.as do exercises 11.25-11.27
Problems:
1) Add 5.00 g NaCl to 100 g of water. Calculate the percent by mass, molality and mole fraction.
2) What additional information must be given in order to calculate the molarity of this solution? (Don’t just say molar mass.
Yes, you need molar mass, but to do the calculation properly you need to know something else about the solution. You
can’t do the problem below without it. If you can’t answer this question, just skip to the question below)
3) Concentrated hydrochloric acid is 37.0% HCl by mass. Calculate the molarity, molality, and mole fraction of HCl. Try
to calculate the molarity first without the missing information hinted at in question 2. Go ahead, I dare you:
If you got 10.1 M you are wrong (close, but wrong). That’s because you didn’t have the missing information. You are assuming
that an HCl solution has the same mass as an equal volume of water, but this is not the case. Solutions are denser than water. You
need to take that into account. The density of concentrated HCl is 1.19 g/mL which is the same as saying 1190 g/L. Take this into
account in your calculation and you’ll get the right answer. Then figure out the next two parts of the problem.
4) Battery acid is 3.75 M H2SO4 and has a density of 1.230
g
. Calculate the percent by mass and molality of H2SO4 in the
mL
solution.
Super secret answers to problems: 1) 4.76%, .855m, .0152 2) density 3) 12.1M, 16.1m, .225 4) 29.9%, 4.35m