II

III

I Concentration

Solutions

Concentration Units

The amount of solute in a solution.

Describing Concentration

• % by mass - medicated creams

• % by volume - rubbing alcohol

• ppm, ppb - water contaminants

• molarity - used by chemists

• molality - used by chemists

MolarityMolarityMolarityMolarity

Molarity (M) = moles soluteliters of solution

L

molM

L 1

mol0.25 0.25M

PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O (237.7 g/mol) in enough water to make 250 mL of solution. Calculate the Molarity.

PROBLEM: Dissolve 5.00 g of NiCl2•6 H2O (237.7 g/mol) in enough water to make 250 mL of solution. Calculate the Molarity.

Step 1Step 1: : Calculate moles of NiCl2•6H2O

5.00 g • 1 mol

237.7 g = 0.0210 mol

0.0210 mol0.250 L

= 0.0841 M

Step 2: Step 2: Calculate Molarity

[NiClNiCl22•6 H•6 H22O O = 0.0841 M

Step 1: Change mL to L:250 mL * 1L/1000mL = 0.250 L

Step 2: Calculate Moles

= (0.0500 mol/L) (0.250 L) = 0.0125 moles

Step 3: Convert moles to grams.

(0.0125 mol)(90.00 g/mol) = 1.13 g

What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500M solution?

What mass of oxalic acid, H2C2O4, is required to make 250. mL of a 0.0500M solution?

Learning CheckLearning Check

How many grams of NaOH are required to prepare 400. mL of 3.0 M NaOH solution?

1) 12 g

2) 48 g

3) 300 g

Two Other Concentration Two Other Concentration UnitsUnits

grams solutegrams solution

MOLALITY, m

% by mass =

% by mass

m of solution = mol solute

kilograms solvent

Percent Composition

This is the mass of the solute divided by the mass of the solution (mass of solute plus mass of solvent), multiplied by 100.

Example: Determine the percent composition by mass of a 100 g salt solution which contains 20 g salt.

(20 g NaCl / 100 g solution) x 100 = 20% NaCl solution

Molality

solvent ofkg

solute of moles(m)molality

mass of solvent only

1 kg water = 1 L waterkg 1

mol0.25 0.25m

Calculating ConcentrationsCalculating Concentrations

Calculate molalityCalculate molality

Calculate molalityCalculate molality

Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O.

Calculate m & % of ethylene glycol (by mass).

Dissolve 62.1 g (1.00 mol) of ethylene glycol in 250. g of H2O.

Calculate m & % of ethylene glycol (by mass).

conc (molality) = 1.00 mol glycol0.250 kg H2O

4.00 molal

%glycol = 62.1 g

62.1 g + 250. g x 100% = 19.9%

Calculate weight %

Molality

Find the molality of a solution containing 75 g of MgCl2 in 250 mL of water.

75 g MgCl2 1 mol MgCl2

95.21 g MgCl2

= 3.2m MgCl2

0.25 kg water

kg

molm

Molality

How many grams of NaCl are req’d to make a 1.54m solution using 0.500 kg of water?

0.500 kg water 1.54 mol NaCl

1 kg water

= 45.0 g NaCl

58.44 g NaCl

1 mol NaCl

kg 1

mol1.5 1.5m

2211 VM=VM

Dilutions

Preparation of a desired solution by adding water to a concentrate.

Moles of solute remain the same.

Dilution

What volume of 15.8M HNO3 is required to make 250 mL of a 6.0M solution?

GIVEN:

M1 = 15.8M

V1 = ?

M2 = 6.0M

V2 = 250 mL

= 0.250 L

WORK:

M1 V1 = M2 V2

(15.8M) V1 = (6.0M)(0.250 L)

V1 = 0.095 L of 15.8M HNO3