This document discusses methods for describing the concentration of solutions, including mass percent and molarity. It provides examples of calculating mass percent and molarity, such as finding the mass of lactose in milk or the mass of potassium dichromate needed to make a solution of a given molarity and volume. Standard solutions are also discussed as accurately prepared solutions of a known concentration made by weighing solute into a volumetric flask and filling to an exact volume line.

1. Solution
Composition & Concentration
Mass Percent & Molarity!

2. Mass %
• Describes the composition of a solution.
• a.k.a. weight percent
• Mass % = mass of solute x 100%
mass of solution
= grams of solute x 100%
grams of solute + grams of solvent

3. Mass % example
• A solution is prepared by dissolving 1.0g of sodium
chloride in 48g of water. Calculate the percent mass
of the solution!
– 1.0 g of solute
– Total mass of solution is 49g
(1.0g of solute + 48g of solvent)
1.0g solute x 100% = 2.0% NaCl
49 g solution

4. Try this one…
• Cow’s milk typically contains 4.5% by mass of
lactose (C12H22O11). Calculate the mass of lactose
present in 175g of milk.
7.9g lactose
175 g milk X 0.045 =

5. Molarity, M
• Describes the solution concentration in terms of
volume
– rather than composition in terms of mass (it’s much
more convenient to measure the volume of a liquid!)
• M = Molarity = moles of solute = mol
liters of solution L
Ex/ a solution that contains 1.0 mol of solute per Liter
of solution is 1.0M: “1.0 molar”

6. Molarity example
• A solution is prepared by dissolving 11.5g of solid
NaOH in enough water to make a 1.50L solution.
(The solute and solvent together have a volume of 1.50L)
Calculate the Molarity.
– Mass of solute = 11.5 g NaOH  convert to moles!
– 11.5g NaOH x 1 mol NaOH = 0.288 mol NaOH
40.0g NaOH
– Volume of solution = 1.50L
Molarity = 0.288 mol NaOH = 1.92 M NaOH
1.50 L solution

7. Try this one…
• What mass of solid potassium dichromate (K2Cr2O7)
must be weighed out to make 1.00 L of a 0.200 M
solution? (molar mass = 294.2)
58.8 g potassium dichromate
Algebra g = (molar mass)(molarity)(volume)

8. Standard Solutions
• A solution with an accurately known concentration
• Prepared in a volumetric flask
– Vol. flasks measure volume extremely accurately
– Solute is weighed and completely added to vol. flask
– Solvent is added to flask to bring volume up to exact
volume line on neck of flask

9. How to make a solution of a given
molarity.
1. Calculate the grams needed g = (mm)(M)(L)
2. Measure out that exact amount.
3. Rinse into mL volumetric flask.
4. Fill 2/3’ s full with distilled water.
5. Swirl to dissolve.
6. Fill to line with distilled water.
7. Cap and flip 20 times.
8. Label: M(what it is), date: , who made it: