II. Concentration

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I II. ConcentrationII. Concentration

Ch. 13 & 14 - SolutionsCh. 13 & 14 - Solutions

What is different between the glasses of Kool-aid?

What is different between the glasses of Kool-aid?

Solution concentration can be described generally

Solution concentration can be described generally

Dilute - reduced in strength, weak, watered down.

Concentrated – stronger, pure. Has less water.

What is the problem with just using dilute and concentrated as descriptions of the solution concentration?

What is the problem with just using dilute and concentrated as descriptions of the solution concentration?

Is solution B dilute or concentrated?

The terms dilute and concentrated are relative.

Scientists need a more precise way of referring to the concentration of a solution.

Concentrated Dilute

Solution A Solution B Solution C

Solution concentration can be described specifically

Solution concentration can be described specifically

Do you remember the “mole” from Stoichiometry?

What is a mole?

How might you use it to describe the concentration of a solution?

A. ConcentrationA. Concentration

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

MolarityMolarity

The ratio of the moles of solute to the volume of solution in liters.

Molarity (M) =

Moles of solute

Volume in Liters of Solution

How to read MolarityHow to read Molarity

6.0 M NaClRead: “6 molar solution of NaCl”Can be abbreviated 6M solution You must be careful to label the

molarity with a capital M so that it is not confused with m for molality.

How to make a 6M NaCl solution using molarity


(a) Add 6 moles NaCl to the volumetric flask. How would you measure that?

6 molesNaCl 58.443 g NaCl

1 mole NaCl

= 351 g NaCl



(b) Add distilled or deionized H2O to dissolve and mix the NaCl

(c) Fill the flask with dH2O until you reach the 1000mL line.

Types of Calculations with Molarity:

Types of Calculations with Molarity:

1. Finding concentration of a solution.

2. Finding the mass of solute needed.

3. Finding the volume of solution made.

Finding ConcentrationFinding Concentration

Antifreeze is a solution of ethylene glycol, C2H6O2 in water. If 4.50 L of antifreeze contains 27.5 g of ethylene glycol, what is the concentration of the solution?

27.5 g C2H6O2

62.08 g C2H6O2

1 mol C2H6O2

4.5 L

= 0.0984 mol/L or

0.0984 M C2H6O2

Finding Mass Finding Mass

What mass of sodium carbonate, Na2CO3,

is present in 50 ml of a 0.750M solution?

50 ml 1 L

1000 mL

0.750 mol

1 L 1 mol Na2CO3

105.99 g Na2CO3

= 3.97 g Na2CO3

Conversion Factor

Finding VolumeFinding Volume

What volume of 1.50 mol/L HCl solution contains 10.0 g of hydrogen chloride?

10.0 g HCl 1 mol HCl

36.46 g HCl 1.50 mol

1 L

= 0.183 L or 183 mL

Conversion Factor

Practice Problems

Practice ProblemsPractice Problems

1. A 0.750 L aqueous solution contains 90.0 g of ethanol, C2H5OH. Calculate the molar

concentration of the solution in mol/L.

Practice ProblemPractice Problem

2. What mass of NaCl are dissolved in 152 mL of a solution if the concentration of the solution is 0.364 M?


3. What mass of dextrose, C6H12O6 is dissolved in 325 mL of 0.258 M solution?


4. A mass of 98 g of sulfuric acid, H2SO4, is dissolved in water to prepare a 0.500 M solution. What is the volume of the solution?


5. A solution of sodium carbonate, Na2CO3, contains 53.0 g of solute in 215 mL of solution. What is its molarity?


6. What is the molarity of a solution of HNO3 that contains 12.6 g of solute in 5.00 L of solution?

B. MolalityB. Molality

solvent ofkg

solute of moles(m)molality

mass of solvent only

1 kg water = 1 L waterkg 1

mol0.25 0.25m


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


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


What is the molality of a solution made

from 2.4 moles of NaCl and 0.80 kg of

water?

Practice Problem Practice Problem

What is the molality of a solution made

from 63 g of HNO3 in 0.50 kg of water?


How much water is needed to make a 0.50 m solution from 3.2 g of NaCl?

Practice ProblemsPractice Problems

What mass of CH3OH is needed to add to

1.20 kg of water to make a 3.00 m

solution?

2211 VMVM

C. DilutionC. Dilution

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

Moles of solute remain the same.

C. DilutionC. 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

WORK:

M1 V1 = M2 V2

(15.8M) V1 = (6.0M)(250mL)

V1 = 95 mL of 15.8M HNO3

Dilution PracticeDilution Practice

Water is added to 200. mL of a 2.0 M

solution of CaCl2 to increase the volume of

the solution to 400. mL. What is the new

concentration?

Dilution PracticeDilution Practice

To what volume must 1.0 L of a 6.0 M

solution of HCl be diluted in order to

prepare a 0.2 M solution?

D. Preparing SolutionsD. Preparing Solutions

500 mL of 1.54M NaCl

500 mLwater

45.0 gNaCl

• mass 45.0 g of NaCl• add water until total

volume is 500 mL• mass 45.0 g of NaCl• add 0.500 kg of water

500 mLmark

500 mLvolumetric

flask

1.54m NaCl in 0.500 kg of water

D. Preparing SolutionsD. Preparing Solutions

250 mL of 6.0M HNO3 by dilution

• measure 95 mL of 15.8M HNO3

95 mL of15.8M HNO3

water for

safety

250 mL mark

• combine with water until total volume is 250 mL

• Safety: “Do as you oughtta, add the acid to the watta!”

Solution Preparation LabSolution Preparation Lab

Turn in one paper per team. Complete the following steps:

A) Show the necessary calculations.

B) Write out directions for preparing the solution.

C) Prepare the solution. For each of the following solutions:

1) 100.0 mL of 0.50M NaCl

2) 0.25m NaCl in 100.0 mL of water

3) 100.0 mL of 3.0M HCl from 12.1M concentrate.

500.O ml of 0.50 M NaCl500.O ml of 0.50 M NaCl

Remember: 0.50 M = 0.50 mole/1 Liter Convert 0.50 moles to grams of NaCl 0.50 mole NaCl = 29.23 grams but this is in 1 liter Figure out how much you need for 0.1 L Set up a proportion. 29.23g = x

1 L 0.5 L X = 14.62 g, measure this amount on a balance

and add to a volumetric flask. Then fill with distilled water to the 500 ml mark.

0.25 m NaCl in 100 ml of water 0.25 m NaCl in 100 ml of water

Remember: 0.25 m = 0.25 moles/1kg of water Convert 0.25 moles to grams. 0.25 moles = 14.61 grams – this is in 1 kg of water. Figure out how much you need for 0.100 kg of water. 1kg = 1L Set up a proportion. 14.61 g = x

1 kg 0.100 kg X = 1.461 g, measure this amount on a balance and add to a

flask. Then measure out 100 ml of water in a graduated cylinder.

Add this volume of water to the flask containing the NaCl.

How do you make a 100.0 mL solution with a concentration of 3.0 M HCl from 12.1M concentrate

How do you make a 100.0 mL solution with a concentration of 3.0 M HCl from 12.1M concentrate

This is a dilution problem because you have 2 different concentrations.

Solve for the first volume:

(12.1)(V1) = (3.0)(0.100)

V1 = 0.025 L or 25 mL

Measure out 25 mL of HCl in a graduated cylinder In a 100 mL volumetric flask, add about 50 mL of water then

slowly pour the 25 mL of HCl into the flask. Then add the remaining volume of water to the 100 ml mark.

II. Concentration

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