1. Solutions and Molarity
Describe the types of solutions.
Define the vocabulary words.
List and explain the factors that influence
solubility and the rate at which a solute
dissolves in a solvent.
Explain and calculate molarity.
2. Types of Solutions
Gas in a gas – Gases mix freely and will always form a
solution unless they react (Example: air)
Solid in a solid – Alloy: a homogeneous mixture of metals
(Example: brass is a mixture of copper and zinc)
Liquid in a liquid
Miscible – when liquids can be mixed together to form a
solution (Example: ethylene glycol and water form
antifreeze)
Immiscible – when liquids cannot be mixed (Example: oil
and water)
“Like dissolves like” – polar dissolves polar
Polar and nonpolar (vinegar and oil) are immiscible
3. Solid in a liquid
Energy is required to separate particles
of a solid (Endothermic)
Energy is released when solute particles
and solvent particles are attached
(Exothermic)
Goes back and forth: Dynamic
equilibrium
Differences are responsible for different
solubilities
4. Gases in liquids – when gas is attracted
to solvent particles---release energy
Free---move toward entropy
Examples: CO2 in soda and O2 in
seawater
Entropy (S) – disorder or randomness
Systems tend to go from a state of order
(low entropy) to a state of maximum
disorder (high entropy)
5. Solubility Curve
Solubility - quantity of solute that
will dissolve in a specific amount of
solvent at a certain temperature.
(pressure must also be specified for
gases).
-soluble vs. insoluble,
-saturated (on or above) vs.
unsaturated (below)
- solubility should NOT be confused
with the rate at which a substance
dissolves
Reading the curve:
At 30°C approximately 10g of
KClO3 will dissolve in 100g of
water
6. Refer to Solubility Curves
At 10 °C, 135 grams of KI will dissolve
At 50 °C, 85 grams of KNO3 will dissolve
At 30 °C, 42 grams of NH4Cl will dissolve
At 75 °C, 50 grams of KCl will dissolve
7. Factors Influencing the Rate at
which a Solute Dissolves in a
Solvent
1. Agitation – stirring; a surface
phenomenon
2. Particle size
3. Temperature – the only factor that
affects both the rate of solution and the
solubility
8. Solid in a liquid
Energy is required to separate particles
of a solid (Endothermic)
Energy is released when solute particles
and solvent particles are attached
(Exothermic)
Goes back and forth: Dynamic
equilibrium
Differences are responsible for different
solubilities
9. Gases in liquids – when gas is attracted
to solvent particles---release energy
Free---move toward entropy
Examples: CO2 in soda and O2 in
seawater
Entropy (S) – disorder or randomness
Systems tend to go from a state of order
(low entropy) to a state of maximum
disorder (high entropy)
10. Temperature Effects on
Solubility
In general, an increase in temperature
increases the solubility of solids in liquids
In general, an increase in temperature
decreases the solubility of gases in
liquids (gases escape)
11. Pressure Effects
Pressure increases the solubility of gases
in liquids (nail being hammered into wood)
Henry’s Law – At a given temperature, the
solubility of a gas in a liquid is directly
proportional to the pressure of the gas
above the liquid (Page 506)
S = solubility S1 S2
=
P = pressure P1 P2
12. Concentration of Solutions
Dilute – contains relatively little solute
Concentrated – contains relatively large
amount of solute
13. Vocabulary Words
Solubility: the amount of a substance that
dissolves in a given quantity of solvent at
specified conditions of temperature and
pressure to produce a saturated solution
Solute: dissolving particles
Solvent: the dissolving medium in a
solution (Example- water)
14. Saturated – a solution that holds as
much solid as it normally can at a given
temperature (If more solid is added, it
will not dissolve)
Unsaturated – a solution which has not
yet reached the limit of solubility at a
given temperature
Supersaturated – rare solution that
contains MORE dissolved solute than it
can normally hold at a given temperature
(Crystallization)
15. Suspension: if the particles are
so large that they settle out
unless the mixture is constantly
stirred or agitated.
suspension
Colloid: particles that are
intermediate in size between
those in solutions and
suspensions. AKA- colloidal
suspensions
colloid
16. Molarity
The number of moles of solute dissolved per
liter (1000mL) of solution
Water: 1 mL = 1 g; 1000 mL = 1 Kg
mol of solute
molarity (M) =
liters/Kg of solution
Moles of solute = liters of solution x molarity
Problem: What is the molarity of the solution
obtained by dissolving 90g of glucose (C6H12O6)
in 1000 grams of water?
17. Answer
1 mole of glucose = 180 g
90g ÷ 180 g = .5 mole
1000 g = 1000 mL
.5M per 1000 mL water
Problems: How many grams are needed
to make a molar solution of
a. 1M glucose
b. 2M glucose
c. .5M glucose
18. Answer
1 mole of glucose = 180 grams
a. 1M = 180 grams
b. 2M = 2 x 180 g = 360 grams
c. .5M = .5 x 180 g = 90 grams
19. Preparing Molar Solutions
The solute will take up some of the available
space in the volumetric flask.
Steps
1. The solute should be added to some of the
solvent and dissolved.
2. Then solvent is added to the 1L mark on the
volumetric flask.
If these steps are not followed, the total volume
of the mixture is likely to exceed the desired
volume.
A volumetric pipet measures volumes even
more accurately.
20. Making Dilutions
Dilution reduces the moles of solute per unit
volume, however, the total moles of solute in
solution does not change.
Moles of solute = molarity (M) x liters of solution
(V)
Moles of solute = M1 x V1 = M2 x V2
Problem: How many milliliters of a stock
solution of 2.00M MgSO4 would you need to
prepare 100.0 mL of 0.400 M MgSO4?
21. Answer
0.400M x 100.0 mL ÷ 2.00M = 20.0 mL
Thus, 20.0 mL of the initial solution must
be diluted by adding enough water to
raise the volume to 100.0 mL
OR
0.400M is 1/5th of 2.00M
1/5th of 100mL is 20 mL
22. Percent Solutions
If both the solute and solvent are liquids,
a convenient way to make a solution is
to measure volumes.
Example: 20 mL of pure alcohol is
diluted with water to a total volume of
100 mL – The concentration of alcohol is
20% (v/v)
A commonly used relationship for
solutions of solids dissolved in liquids is
percent (mass/volume).
23. Problems
1. What is the percent by volume of
ethanol (C2H6O), or ethyl alcohol, in the
final solution when 85 mL of ethanol is
diluted to a volume of 250 mL with
water?
2. How many grams of glucose
(C6H12O6) would you need to prepare
2.0 L of 2.8% glucose (m/v) solution?
24. Answers
1. 85/250 x 100% = 34% ethanol (v/v)
2. In a 2.8% solution, each 100 mL of
solution contains 2.8 grams of glucose
100mL is 1/10th of a liter, so you need
28 grams per liter
2 L x 28 g = 56 grams of glucose