SOLUTIONS AND SOLUBILITY

DEFINITIONS

A solution is a homogeneous mixture

A solute is dissolved in a solvent.

solute is the substance being dissolved solvent is the liquid in which the solute is dissolvedan aqueous solution has water as solventA saturated solution is one where the concentration is at a

maximum - no more solute is able to dissolve. A saturated solution represents an equilibrium: the rate of dissolving is

equal to the rate of crystallization. The salt continues to dissolve, but crystallizes at the same rate so that there “appears” to be nothing happening.

Substances that make up the solution can be:

Solid- salt dissolved in water

Liquid- HCl dissolved in water

Gas- Carbon Dioxide dissolved in water- pop

Not every liquid dissolves every solid ex. NaCl dissolves in water but not gasoline, parrafin wax dissolves in gasoline but not water.

Liquids are similar- liquids that dissolve other liquids are miscible- ethanol dissolves in water.

- liquids that do not dissolve each other are immiscible- oil and water.

A common phrase that is used to determine which substances dissolve which is “like dissolves like”. Meaning polar substances dissolve polar substances and nonpolar dissolve non polar.

Examples of solutions not in liquid phase:

Air- made of N2, O2 CO2 and others

Metal Alloys- brass- made up of zinc and copper

REACTIONS INVOLVING PRECIPITATES

When two liquids (with dissolved substances) mix and a solid forms it is called a precipitate.

REMEMBER- NET IONIC EQUATIONS

When a solution of Pb(NO3)2 is mixed with a solution of KI the result is a precipitate of PbI and a solution of KNO3.

Pb(NO3)2(aq) + 2KI(aq) PbI 2(s) + 2KNO3(aq)

Remember- dissociation equations

Net Ionic Equation

SOLUTION AND CONCENTRATION

4 ways of expressing 4 ways of expressing concentrationconcentration

Molarity(M): moles solute / Liter solution

Mass percent: (mass solute / mass of solution) * 100

Molality* (m) - moles solute / Kg solvent

Mole Fraction(A) - moles solute / total moles solution

* Note that molality is the only concentration unit in which denominator contains only solvent

information rather than solution.

MAKING MOLAR SOLUTIONS

From Liquids(More accurately, from

stock solutions)

Identify each volume to two decimal places(values tell you how much you have expelled)

4.48 - 4.50 4.86 - 4.87 5.00

Reading a pipetteReading a pipette

% CONCENTRATION

% (w/w) =

% (w/v) =

% (v/v) =

100xsolutionmasssolutemass

100xsolutionvolumesolutemass

100xsolutionvolumesolutevolume

THE DILUTION FORMULATHE DILUTION FORMULAE.g. if we have 1 L of 3 M HCl, what is M if we dilute acid to 6 L?

M1 = 3 mol/L, V1 = 1 L, V2 = 6 L

M1V1 = M2V2, M1V1/V2 = M2

M2 = (3 mol/L x 1 L) / (6 L) = 0.5 M

Why does the formula work?

Because we are equating mol to mol:

M1V1 = 3 mol

V1 = 1 LM1 = 3 M

V2 = 6 LM2 = 0.5 M

M2V2 = 3 mol

PRACTICE PROBLEMSPRACTICE PROBLEMSQ – What volume of 0.5 M HCl can be prepared from 1 L of 12 M HCl?

M1 = 12 mol/L, V1 = 1 L, M2 = 0.5 LM1V1 = M2V2, M1V1/M2 = V2

V2 = (12 mol/L x 1 L) / (0.5 L) = 24 LQ – 1 L of a 3 M HCl solution is added to 0.5 L of a 2 M HCl solution.

What is the final concentration of HCl? (hint: first calculate total number of moles and total number of L)# mol = (3 mol/L)(1 L) + (2 mol/L)(0.5 L)

= 3 mol + 1 mol = 4 mol# L = 1 L + 0.5 L = 1.5 L# mol/L = 4 mol / 1.5 L = 2.67 mol/L

Do 1 – 8 . Try 6 two ways

1. How many mL of a 14 M stock solution must be used to make 250 mL of a 1.75 M solution?

2. You have 200 mL of 6.0 M HF. What concentration results if this is diluted to a total volume of 1 L?

3. 100 mL of 6.0 M CuSO4 must be diluted to what final volume so that the resulting solution is 1.5 M?

4. What concentration results from mixing 400 mL of 2.0 M HCl with 600 mL of 3.0 M HCl?

5. What is the concentration of NaCl when 3 L of 0.5 M NaCl are mixed with 2 L of 0.2 M NaCl?

6. What is the concentration of NaCl when 3 L of 0.5 M NaCl are mixed with 2 L of water?

7. Water is added to 4 L of 6 M antifreeze until it is 1.5 M. What is the total volume of the new solution?

8. There are 3 L of 0.2 M HF. 1.7 L of this is poured out, what is the concentration of the remaining HF?

DILUTION PROBLEMS (1-6, 6 TWO DILUTION PROBLEMS (1-6, 6 TWO WAYS)WAYS)

1.M1 = 14 M, V1 = ?, M2 = 1.75 M, V2 = 250 mLV1 = M2V2 / M1 = (1.75 M)(0.250 L) / (14 M)

V1 = 0.03125 L = 31.25 mL2.

M1 = 6 M, V1 = 0.2 L, M2 = ?, V2 = 1 LM2 = M1V1 / V2 = (6 M)(0.2 L) / (1 L)

M2 = 1.2 M3.

M1 = 6 M, V1 = 100 mL, M2 = 1.5 M, V2 = ?V2 = M1V1 / M2 = (6 M)(0.100 L) / (1.5 M)

V2 = 0.4 L or 400 mL

DILUTION PROBLEMS (4 - 6)DILUTION PROBLEMS (4 - 6)4. # mol = (2.0 mol/L)(0.4 L) + (3.0 mol/L)(0.6 L)

= 0.8 mol + 1.8 mol = 2.6 mol # L = 0.4 L + 0.6 L

# mol/L = 2.6 mol / 1 L = 2.6 mol/L

5. # mol = (0.5 mol/L)(3 L) + (0.2 mol/L)(2 L)

= 1.5 mol + 0.4 mol = 1.9 mol # mol/L = 1.9 mol / 5 L = 0.38 mol/L

6. # mol = (0.5 mol/L)(3 L) + (0 mol/L)(2 L) = 1.5 mol + 0 mol = 1.5 mol

# mol/L = 1.5 mol / 5 L = 0.3 mol/LOr, using M1V1 = M2V2,

M1 = 0.5 M, V1 = 3 L, M2 = ?, V2 = 5 L

DILUTION PROBLEMS (7, 8)DILUTION PROBLEMS (7, 8)7.

M1 = 6 M, V1 = 4 L, M2 = 1.5 M, V2 = ?

V2 = M1V1 / M2 = (6 M)(4 L) / (1.5 M)

V2 = 16 L

8. The concentration remains 0.2 M, both volume and moles are removed when the solution is poured out. Remember M is mol/L. Just like the density of a copper penny does not change if it is cut in half, the concentration of a solution does not change if it is cut in half.

% CONCENTRATION: % MASS EXAMPLE

3.5 g of CoCl2 is dissolved in 100mL solution.

Assuming the

density of the solution is 1.0 g/mL, what is concentration of the solution in % mass?

%m = 3.5 g CoCl2

100g H2O

= 3.5% (m/m)

CONCENTRATION: MOLARITY EXAMPLEIf 0.435 g of KMnO4 is dissolved in enough water to give 250. mL of solution, what is the molarity of KMnO4?

Now that the number of moles of substance is known, this can be combined with the volume of solution — which must be in liters — to give the molarity. Because 250. mL is equivalent to 0.250 L .

As is almost always the case, the first

step is to convert the mass of material to

moles.

0.435 g KMnO4 • 1 mol KMnO4 = 0.00275 mol KMnO4

158.0 g KMnO4

Molarity KMnO4 = 0.00275 mol KMnO4 = 0.0110 M

0.250 L solution

DILUTION

When a solution is diluted, solvent is added to lower its concentration.

The amount of solute remains constant before and after the dilution:

moles BEFORE = moles AFTER

C1V1 = C2V2

Suppose you have 0.500 M sucrose stock solution. How do you prepare 250 mL of 0.348 M sucrose solution ?

Concentration 0.500 M Sucrose

250 mL of 0.348 M sucrose

A bottle of 0.500 M standard sucrose stock solution is in the lab.

Give precise instructions to your assistant on how to use the stock solution to prepare 250.0 mL of a

0.348 M sucrose solution.

A bottle of 0.500 M standard sucrose stock solution is in the lab.

Give precise instructions to your assistant on how to use the stock solution to prepare 250.0 mL of a

0.348 M sucrose solution.

FACTORS AFFECTING SOLUBILITY

1. Nature of Solute / Solvent1. Nature of Solute / Solvent. - Like dissolves like (IMF)

2. Temperature -2. Temperature -i) Solids/Liquids- Solubility increases with Temperature

Increase K.E. increases motion and collision between solute / solvent.

ii) gas - Solubility decreases with Temperature

Increase K.E. result in gas escaping to atmosphere.

3. Pressure Factor -3. Pressure Factor -i) Solids/Liquids - Very little effect

Solids and Liquids are already close together, extra pressure will not increase solubility.

ii) gas - Solubility increases with Pressure.

Increase pressure squeezes gas solute into solvent.

SOLUBILITIES OF SOLIDS VS TEMPERATURESolubilities of several ionic solid as a function of temperature. MOST salts have greater solubility in hot water.

A few salts have negative heat of solution, (exothermic process) and they become less soluble with increasing temperature.

TEMPERATURE & THE SOLUBILITY OF GASESTHE SOLUBILITY OF GASES DECREASES AT HIGHER TEMPERATURES

SOFT DRINKS

Soft drinks contain “carbonated water” – water with dissolved carbon dioxide gas.

The drinks are bottled with a CO2 pressure greater than 1 atm.

When the bottle is opened, the pressure of CO2 decreases and the solubility of CO2 also decreases

Therefore, bubbles of CO2 escape from solution.

COLLIGATIVE PROPERTIES

Dissolving solute in pure liquid will change all physical properties of liquid, Density, Vapor Pressure, Boiling Point, Freezing Point, Osmotic Pressure

Colligative Properties are properties of a liquid that change when a solute is added.

The magnitude of the change depends on the numbernumber of solute particles in the solution, NOT on the identityidentity of the solute particles.