Chapter 4

Aqueous Reactions and Solutions

Solvent

Making solutions

• What the solute and the solvent are

Solutedissolved

substance

doing the dissolving

Making solutions

• Whether a substance will dissolve.

• How much will dissolve.

• A substance dissolves faster if-– It is stirred or shaken.– The particles are made smaller.– The temperature is increased.

How Much?• Solubility- The maximum amount of substance that

will dissolve at that temperature (usually g/L).• Saturated solution- Contains the maximum amount

of dissolved solute• Unsaturated solution- Can dissolve more solute• Supersaturated- A solution that is temporarily

holding more solute than normal, a seed crystal will make it come out

Concentration• A measure of the amount of solute dissolved

in a certain amount of solvent.

• Concentrated solution has a large amount of solute to solvent.

• Dilute solution has a small amount of solute to solvent

• Aqueous chemical reactions don’t happen in grams

A. Solution Composition

• 1. Molarity

• The number of moles of solute in

1 Liter of the solution.

• M = moles solute

Liter solution

Making solutions• Pour in a small amount of solvent

• Then add the solute and dissolve it

• Then fill to final volume.

• M x L = moles

Example #1

• When 23.4g of Na2SO4 is dissolved in water to form 125mL of solution, what is the Molarity?

23.4 g Na2SO4 1 mol = 0.165 mol Na2SO4

142.1 g

0.165 mol = 1.32 M Na2SO4

0.125 L

Example #2

• How many grams of Na2SO4 is dissolved in water to make 0.350 L of 0.500 M solution?

0.500 M = 0.500

0.500 mol 0.350 L = 0.175 mol Na2SO4

L

mol

L

0.175 mol Na2SO4 142.1 g = 24.9g Na2SO4

mol

2. Dilution

Adding water to a solution

Dilution• The number of moles of solute doesn’t

change if you add more solvent.• The moles before = the moles after

• M1 x V1 = M2 x V2

• M1 and V1 are the starting concentration and volume.

• M2 and V2 are the ending concentration and volume.

• Stock solutions are pre-made to known M

Practice

• 2.0 L of a 0.88 M solution are diluted to 3.8 L. What is the new molarity?

• Need 450 mL of 0.15 M NaOH. All you have available is a 3.0 M stock solution of NaOH. How do you make the required solution?

–0.463 M diluted solution

–22.5 mL of NaOH and enough water to dilute

to final volume of 450 mL total solution

B. Properties of Solutions

1. Electrolytes– Ionic compounds dissolve in water– This is called dissociation or ionization– Solutes that produce ions in solution are called

electrolytes because their solutions can conduct electricity

• An ionic compounds dissociates as it dissolves in water

Ions separate from the solid and become hydrated or surrounded by water molecules.

The ions move freely and the solution is able to conduct electricity.

• Ionic compounds that dissolve completely are strong electrolytes

• Strong Electrolytes – 100% Ionization

HCl(aq) H+(aq) + Cl-

(aq) 100% ionization

Some ionic compounds have low solubilities in water but are still strong electrolytes because what does dissolve is 100% dissociated.

• Weak Electrolytes – only partially ionize

HC2H3O2 (aq) H+ (aq) + C2H3O2

- (aq)

NH3(aq) + H2O(l) NH4 +

(aq) + OH- (aq)

equilibrium arrow

acetic acid

ammonia

2 % Ionization

hydrochloric

acid

2. Most solutions of molecular compounds do not conduct electricity and are called nonelectrolytes

The molecules of a nonelectrolyte separate but stay intact. The solution is nonconducting because no ions are generated.

• Molar Concentration

1.0 M 1.0 M 1.0 M

NaCl Na+ + Cl-

1.0 M 2.0 M 1.0 M

Na2SO4 2 Na+ + SO4-2

Percent solutions

• Percent means per 100 so

• Percent by volume = Volume of solute x 100% Volume of solution

• indicated %(v/v)

• What is the percent solution if 25 mL of CH3OH is diluted to 150 mL with water?

Percent solutions• Percent by mass

= Mass of solute(g) x 100% Volume of solution(mL)

• Indicated %(m/v)• More common• 4.8 g of NaCl are dissolved in 82 mL of

solution. What is the percent of the solution?• How many grams of salt are there in 52 mL

of a 6.3 % solution?