Solution Composition:15.2

Homework: Page 555, #12a-c, 13ab, 18, 19, 20, 21a-d

Tuesday, March 4, 2008

1. Turn in Lab2. Vocabulary Words (Quiz Friday)3. Section 15.2 -- Molarity

Vocabulary Words• Solution• Solvent• Solute• Aqueous Solution• Saturated• Unsaturated• Supersaturated• Concentrated• Dilute• Mass Percent• Molarity• Standard Solution• Dilution

Molarity

• Mass percent describes the amount of solution in terms of mass

• More convenient to measure volume of a solution

• Chemists describe the concentration of a solution

%100xsolutionofmass

soluteofmassPercentMass

%100xsolventofgramssoluteofgrams

soluteofgramsPercentMass

Mass Percent

Example

A solution is prepared by mixing 1.00 g or ethanol, C2H5OH, with 100.0 g of water. Calculate the mass percent of ethanol in this solution

%1000.1000.1

0.1

52

52 xwaterofgramsOHHCofgrams

OHHCgPercentMass

= 0.990% C2H5OH

Example – Practice Problem 15.1, page 529

A 135 g sample of seawater is evaporated to dryness, leaving 4.73 of solid residue (the salts formerly dissolved in the seawater). Calculate the mass percent of solute present in the original seawater.

%100135

73.4x

grams

gPercentMass

= 3.5% salt

Molarity/Concentration

• Concentration: amount of solute in a given volume of solution

• Common expression is Molarity (M)• Molarity: number of moles of

solute per volume of solution in liters

solutionofliters

soluteofmolesM • Molarity

Solution Concentration

• Molarity – solutionofliter

soluteofmolesM

What is the molarity of a 1 L solution containing 6 moles of sugar?

What is the molarity if the 6 moles of sugar is dissolved in only 500mL of solution?

Example

• Add 11.5 g NaOH to 1.5 L of water. What is the molarity?

L

mol M

NaOH mol 0.288 g 40.0

mol 1 x NaOH g 11.5

NaOH M 0.19 L 1.5

NaOH mol 0.288

Example

• Add 1.56 g HCl to 26.8 mL solution. What is the molarity?

HCl mol 0.0427 g 36.5

mol 1 x HCl g 1.56

HCl M 1.59 L 0.0268

HCl mol 0.0427

Practice Problem – Exercise 15.1 p. 529

• How many grams of formaldehyde (HCHO) must be used to prepare 2.5 L of 12.3 M formalin?

L

mol M mol 30.75 x

L 2.5

molx M 12.3

HCHO g 920 HCHO g 923 mol 1

g 30.03 x mol 30.75

What is in solution?

• The concentration is written in terms of the form of the solute before it dissolves

• A 1.0 M NaCl means the solution was prepared by dissolving 1.0 mol of solid NaCl in enough water to make 1.0 L of solution

• Now, it doesn’t have 1.0 mol of NaCl units- has dissolved 1 M Na+ and 1 M Cl-

Solution Concentration

• Standard Solution – when the concentration is accurately known.

• Volumetric flask