Concentration CalculationInit 3/13/2008 by Daniel R. Barnes

This power point presentation uses questions from Steve Wallis’ “Solution Concentration Calculations” worksheet, with modifications.

concentration =solute

solution

The answer is pronounced “0.333 molar”. A capital “M” stands for “moles per liter”. M = mol/L

“Molarity” is a form of concentration calculated by dividing moles of solute by liters of solution.

3 mol

9 L= 0.333 mol/L

= 0.333 M

Molarity = Moles of solute

Liters of solution

Word to the wise: There is a related form of conentration called “molality” that divides moles of solute by liters of solvent instead of by liters of solution. We’re not going to learn that one now, but you might someday. It’s used to calculate colligative properties of solutions, for instance. Don’t know what colligative properties are? Don’t worry. It’s not going to be on the CST. It’s an asterisked standard.

=

I. What is the molarity of a solution made by dissolving 3 moles of sugar in enough water to make 9 liters of solution?

40 g

Mass of solution

Per cent concentration is sometimes calculated by volume, but is usually calculated by mass, as it is here.

= 0.1333 x 100%

Don’t forget that a solution is made of a solute and a solvent, so to get the mass of the solution, you have to add the mass of the solute and the solvent together.

% concentration = Mass of solute x 100%

40 g

300 gx 100%x 100% =

= 13.33%

40 g + 260 g

solution = solute + solvent

salt water = salt + water

V. A solution is made by dissolving 40 grams of NaCl into 260 grams of water. What is the % concentration of the solution?

30 g

volume of solution

To calculate concentration, you almost always have to divide amount of solute by amount of solution.

concentration = mass of solute mL1000

1x =

= 136.36 g/L

220 mL

The word “per” in math indicates division, so “grams per liter” means that you need to divide grams by liters.

Unfortunately, we were given the volume in milliliters, not liters, so we’re going to have to do a unit conversion.

L

3000 g

22 L=

One liter equals one thousand milliliters, so . . .

VIII. 30 grams of glucose are dissolved in enough water to make 220 mL of solution. What is the concentration of the solution in grams per liter?

= 360 ppm

1 g

solute

180 mg

1000 mg

solution

As is almost always the case, concentration equals solute divided by solution.

1800 ppm

Parts per million is just like per cent, except . . .

c = x 100%

x

5c =

c = 500 g

x 1,000,000 ppm

. . . it’s based on one million instead of one hundred.

Also, because ppm is used for very small concentrations, chances are that the amount of solute will be so small that it won’t be expressed in the same units as the solution.

x 1,000,000 ppm

So, don’t be surprised if you have to do a unit conversion.

Cancel the old, useless units.

Cancel as many zeros as you can.

IX. A 500 gram sample of sea water has 180 milligrams of manganese dissolved in it. What is the concentration of manganese in parts per million?

RL: According to Science News, November 19, 2011, “Most Arctic ozone accumulates between 14 and 21 kilometers up, where concentrations hover around 4.5 parts per million much of the year.” How many grams of ozone (O3) would there be on one metric ton (106 grams) of Arctic ozone layer air?

concentration = solute

solution

x solutionsolution x

solute = solution x concentration

solute = 106 g x 4.5 ppm

106 ppm

= 4.5 g

Remember that 106 ppm = 1 million ppm = 1 million parts per million = 100% = 1