Concentration of solutions

CONCENTRATED = Lots of solute in the solution

DILUTE = Not much solute in the solution

Ways to express concentration numerically

• Molarity

• % by Mass

• % by Volume

• Parts per million

MOLARITY

Molarity = # of moles of solute in 1 liter of solution

See Table T

Molarity Problems

What is the molarity of a solution of NaOH if there are 4 moles of NaOH dissolved water to make 1 liter of solution?

4 moles = 4 M

1 liter

Molarity Problems

What is the molarity of a solution of NaOH if there are 4 moles of NaOH dissolved water to make 2 liters of solution?

4 moles = 2 M

2 liter

Molarity Problems

What is the molarity of a solution of NaOH if there are 2 moles of NaOH dissolved water to make 6 liters of solution?

2 moles = .33 M

6 liters

Molarity Problems

What is the molarity of a solution of NaOH if there are 2 moles of NaOH dissolved water to make .5 liters of solution?

2 moles = 4 M

.5 liters

Molarity ProblemsWhat is the molarity of a solution of NaOH if

there are 40 grams of NaOH dissolved water to make 1 liters of solution?

First, convert grams to moles:

Na = 23 g/mol

O = 16 g/mol so 40 g = 1 mole

H = 1 g/mol

40 g/mole GFM

Molarity Problems

What is the molarity of a solution of NaOH if there are 40 grams of NaOH dissolved water to make 1 liters of solution?

1 moles = 1 M

1 liters

Molarity Problems

What is the molarity of a solution of NaOH if there are 80 grams of NaOH dissolved water to make 1 liters of solution?

80g/40g/mol = 2 moles

2 moles = 2 M

1 liters

Molarity Problems

What is the molarity of a solution of NaOH if there are 3.6 grams of NaOH dissolved water to make 2 liters of solution?

3.6g/40g/mole = 0.09 moles

0.09 moles = 0.045 M

2 liters

In General….

Concentrations < 1M are considered weak, or dilute

Concentrations > 1M are considered strong, or concentrated

Always know the concentration of your solution before you use it!!!! (read the label)

Can you….

Rearrange the molarity equation to solve for moles?

Molarity = Moles

Liter

moles = ?

Can you….

Rearrange the molarity equation to solve for moles?

Molarity = Moles

Liter

moles = Liter x Molarity

Can you….

Rearrange the molarity equation to solve for Liters?

Molarity = Moles

Liter

Liters = ?

Can you….

Rearrange the molarity equation to solve for Liters?

Molarity = Moles Liter

Liters = Moles Molarity

Example

How many moles of NaOH do you have if you have 3 liters of a 0.5M NaOH solution?

Moles = Molarity x Liters

Moles = 0.5M x 3 liters

Moles = 1.5

% Mass

Uses the same formula as % composition

(See Table T)

Percent mass = mass of part x 100

Mass of whole

% Mass

What is the % mass of NaOH if 2.5 g of NaOH are added to 50 g of water?

Percent mass = mass of part (solute) x 100 Mass of whole (solution)

% mass = 2.5g x 100 (2.5 + 50g)

% mass = (2.5 / 52.5) x 100 = 4.76% solution

% Volume

Uses the same formula as % composition

(See Table T)

Percent volume = volume of part x 100

volume of whole

% Volume

What is the % volume of NaOH if 20 ml of NaOH are added to 300 ml of water?

Percent volume = volume of part (solute) x 100 volume of whole (solution)

% volume = 20 ml x 100 (20 ml + 300 ml)

% volume = (20 / 320) x 100 = 6.25% solution

Parts per million

ppm = grams solute x 1,000,000

grams solution

This is used when the amount of solute is very very small

Parts per million

ppm = grams solute x 1,000,000

grams solution

.0043 g of oxygen gas dissolve in 100 ml of water at 20 degrees celsius. What is the concentration in ppm?

Parts per million

ppm = .0043g x 1,000,000

(100 g + 0.0043g)

Concentration = 43 ppm