Solutions

Concentration of Solutions

Objectives

1. Define molarity and calculate its value.

2. Define molality and calculate its value.

3. Apply the principles of stoichiometry to reactions that occur in solution.

Concentration

Dilute vs. Concentrated

Weak acid vs. dilute acid

Concentration – a measure of the amount of solute in a given amount of solvent or solution

MolarityMolarity – the number of moles of solute in 1 liter of solution.

• Dissolving 2 moles of NaOH (80.0g) in enough water to make 1 Liter of solution would yield a 2 M solution of NaOH

)(

)()(

Lsolutionofvolume

molessoluteofamountMmolarity

Sample Problems1. What is the molarity of a 2.0 L solution that is made

from 14.6 g of NaCl?0.12 M

2. What is the molarity of a HCl solution that contains 10.0 g of HCl in 250 mL of solution?1.1 M

3. How many moles of NaCl are in 1.25 L of 0.330 M NaCl?0.413 mol

4. How many moles of HCl exist in 500. mL of 0.50 M solution of HCl?0.25 mol

Sample Problems5. What is the molarity of a solution composed of 6.25 g

of HCl in 0.300 L of solution?

0.571 M

6. How many moles of KI are present in 0.250 L of a 2.30 M solution?

0.575 mol

7. What volume of a 0.500 M solution of HBr is needed for a reaction that requires 32.5 g of HBr?

0.803 L

MolalityMolality – concentration in moles of solute per kilogram of

solvent.

To make a 2.00 m solution of NaOH:• Measure 2 moles of NaOH (80.0 g) and dissolve in

1.00 kg of distilled water.

solventofkg

soluteofmolesmmolality )(

)(

)()(

Lsolutionofvolume

molessoluteofamountMmolarity

solventofkg

soluteofmolesmmolality )(

Sample Problems1. What is the molality of a solution composed of 255 g of

acetone (CH3)2CO) dissolved in 200. g of water?22.0 m

2. What quantity in grams of methanol (CH3OH) is required to prepare a 0.244 m solution in 400. g of water?3.12 g

3. How many grams of AgNO3 are needed to prepare a 0.125 m solution in 250 mL of water?5.3 g

4. What is the molality of a solution containing 18.2 g HCl and 250. g of water?1.99 m

Solution concentration and Stoichiometry

How many moles of H2O form when 25.0 mL of 0.100 M HNO3 solution is completely neutralized by NaOH?

2.50 x 10-3 mol

Stoichiometry Problems

1. What volume of 0.500 M HCl(aq) is required to react completely with 0.100 mol of Pb(NO3)2 (aq) forming a precipitate of PbCl2 (s)?

0.400 L

2. A 238 mL sample of hydrosulfuric acid solution requires 155 mL of a 3.45 M potassium hydroxide for complete neutralization. What is the concentration of the original hydrosulfuric acid solution?

1.12 M