SolutionComposition & Concentration

Mass Percent & Molarity!

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

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

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 =

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”

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

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)

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

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: