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C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 1
Chapter 13: Solutions Check MasteringChemistry and other deadlines *diagrams from the class textbook 2018 Pearson
Solutions:
Solutions are homogeneous mixtures of two or more substances.
Solutions come in all phases: gas (air), liquid (seawater, vodka,
carbonated water), and solid (brass, an alloy of Zn/Cu)
Solute: the part in the solution that is less
Solvent: the part in the solution that is more
Aqueous solutions have water as the solvent
Polarity:
Substances may be considered polar, nonpolar or ionic
Polar: Polar molecules have polar bonds that do not cancel each other
out. Determining polarity requires knowledge of chemical bonding
and molecular shapes that will be studied later.
For now, be aware of common examples: water and alcohols
(CxHyOH)
Nonpolar: Nonpolar molecules have nonpolar bonding or possibly polar
bonds that cancel each other out.
For now, be aware of examples: hydrocarbons (CxHy), like alkanes
or oils, diatomic elements
Ionic: examples include salts, acids and bases that are made of ion.
When dissolved ionic compounds break up into charged cations and
anions.
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 2
Like dissolves like:
Polar substances will dissolve in polar solvents
Nonpolar substances dissolve in nonpolar solvents
Soluble ionic compounds will dissolve in a strong polar substance like
water
Miscible: two or more substances blend to form a solution (water and
alcohol). Like polarities will blend to make a solution.
Immiscible: two or more substances create layers when added together
(oil and vinegar). Nonpolar and polar substances generally do not
blend into solutions.
Attractions:
In creating solutions the attractions within
the pure substance must be broken
(solute from solute / solvent from
solvent) and allow the formation of
new attractions between unlike
particles (solute and solvent).
Electrolyte solutions : Ionic solids
dissolve individual ions in water
because the polar water molecules
can form solvent cages around the
charged cation or anion in order to
disperse the ions in the solution.
Soluble ionic compounds form electrolyte solutions while soluble
polar or nonpolar molecules are nonelectrolyte solutions.
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 3
Solubility and Saturation:
• The solubility of a compound is the amount of the compound, usually in
grams, that dissolves in a certain amount of liquid, often 100 g of water.
The solubility of sodium chloride at 25 °C is 36 g NaCl per 100 g water.
• A saturated solution holds the maximum amount of solute under the
solution conditions. If additional solute is added, it will not dissolve.
• An unsaturated solution is holding less than the maximum amount of
solute. If additional solute is added, it will dissolve.
• A supersaturated solution is one holding more than the normal
maximum amount of solute. The solution is unstable, the solute will
normally precipitate from (or come out of) a supersaturated solution.
Any disturbance will cause the excess solute to come out.
Solubility
• Solubility rules give a qualitative description of the solubility of ionic
solids. For calcium carbonate, the attraction between Ca2+ ions and
CO32− ions is greater than solvent-solute attractions, and CaCO3 does not
dissolve in water (insoluble). The solubility of CaCO3is close to zero
grams per 100 g water. Ionic solids which dissolve nearly completely in
water (soluble) are strong electrolytes which break into ions: NaCl (aq)
Na+(aq) + Cl- (aq)
• Small polar molecular solids are
generally soluble in water. Table
sugar (C12H22O11) is polar and
soluble in water. Sugar molecules
stay as whole molecules and are
nonelectrolytes.
• Nonpolar molecular solids, such
as lard and vegetable shortening,
are insoluble in water.
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 4
Temperature:
• Generally the solubility of solid solutes in water
increase at higher temperatures. Look at KNO3
on the solubility graph.
• Generally the solubility of gas solutes in water
decrease at higher T. CO2 gas in soda is more
soluble at cold temps than room temp.
Temperature and Solubility to Purify Compounds
One way to purify a solid is a technique called
recrystallization.
Solid is added to a solvent at higher temperature to create a saturated
solution. As the solution cools, the solubility decreases, causing some of
the solid to precipitate, forming crystals as it comes out. The crystalline
structure reject impurities, resulting in a purer solid.
Recrystallization is a way to make rock candy (sugar).
Pressure:
The solubility of gas in water increases as the pressure of that gas above
the liquid increases.
Henry’s Law… Solubility = k Pgas
Liquids exposed to air contain some dissolved gases. Lakes and seawater
contain dissolved oxygen necessary for the survival of fish. Our blood
contains dissolved nitrogen, oxygen, and carbon dioxide. Even tap water
contains dissolved atmospheric gases.
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 5
Example 1:
a) Will C2H6 or CH3OH be more soluble in water? Explain.
b) Which is expected to be more soluble at higher temperatures
Choices: CO2 gas in water or KCl solid in water
c) Will CO2 gas be more soluble in water when the container is
pressurized with air or carbon dioxide?
Quantitative Concentrations:
molality (m) = moles of solute
kg of only the solvent
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 6
Dilution of Solutions:
M1V1 = M2V2
Laboratory Safety Note:
When diluting acids, always add the
concentrated acid to the water.
Never add water to concentrated acid
solutions.
Stoichiometry of Solutions:
Conversions:
Convert volume (or other units given) of A to moles A
Convert moles A to moles B
Convert moles B to desired units (grams, volume, molecules...) B
Example 2:
Solve for the number of grams of sodium hydroxide (NaOH) required
to make 250 ml of 3.00 M NaOH solution.
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 7
Example 3:
For 100 grams of aqueous solution that is 28.0% C2H5OH by mass and
has the density is 0.945 g/ml, solve for…
a) Grams of ethanol in 100 g of solution (mass %)
b) Moles of ethanol in 100 g of solution (MW)
c) Volume of 100 g of solution (density)
d) Molarity (mole/L)
Example 4:
How do you prepare 250 ml of 0.500 M H2SO4 from a stock solution
of 6.00 M H2SO4?
Example 5: (titration)
30.0 ml of 0.240 M NaOH is required to stoichiometrically react with
20.0 ml of an HCl solution. Write the balanced acid-base equation and
calculate and Molarity of the HCl solution?
C h e m i s t r y 1 2 C h 1 3 : S o l u t i o n s P a g e | 8
Colligative properties depend on amount of particles, not the type of solute
• Adding solute (impurities) to a liquid extends the temperature range in
which the liquid remains a liquid. The mixed solution has a lower
melting point and a higher boiling point than pure liquid; these effects are
called freezing point depression and boiling point elevation.
• For freezing point depression and boiling point elevation, the
concentration of the solution is expressed in molality (m), the number of
moles of solute per kilogram of solvent.
Freezing point depression:
The freezing point of a solution is lower than the pure solvent freezing
point.
Tf =iKf m Kf for water = 1.86 ˚C.kg/mol
i is the van’t hoff factor; i = 1 for nonelectrolytes, and a factor of the
number of particles that the compound breaks into for electrolytes:
NaCl (aq) i ≈ 2
ΔTf is the change in temperature of the freezing point in °C (compared
to the freezing point of the pure solvent).
m is the molality of the solution in (mol solute/kg solvent).
Kf is the freezing point depression constant for the solvent.
Different solvents have different values of Kf.
Boiling point elevation:
The boiling point of a solution is higher than the boiling point of the pure
solvent.
In automobiles, antifreeze not only prevents the freezing of coolant within
engine blocks in cold climates, but also prevents the boiling of engine
coolant in hot climates.
Tb =iKb m Kb for water = 0.512 ˚C.kg/mol
This equation is similar to the freezing point depression equation, ΔTb
is the change in temperature of the boiling point in °C, Kb is the
boiling point elevation constant for the solvent.
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Osmosis:
Osmosis is the flow of a solvent from a less concentrated solution to a
more concentrated solution in the attempt to equal the two concentrations.
Osmosis occurs when solutions containing a high concentration of solute
draw solvent from the lower concentration solution. As a result, fluid
rises on the higher concentration side until the weight of the excess fluid
creates enough pressure to stop the flow.
This pressure is the osmotic pressure of the solution.
Osmotic pressure is the pressure required to stop an osmotic flow.
Osmotic pressure is a colligative property depending only on the
concentration of the solute particles, not on the type of solute.
Why shouldn’t you drink seawater when lost at sea?
Membranes of living cells are semipermeable membranes. Seawater is
approximately 3.5% NaCl and cell tissues have the equivalent of 0.9%
NaCl solution. As seawater
flows through the stomach
and intestine, it draws water
out of cells, causing
dehydration.
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Red blood cells in solutions of different concentration
(a) In blood, the solute concentration of the surrounding fluid should be
equal to that within a blood cell, so there is no net osmotic flow, and the
red blood cell exhibits its typical shape.
(b) If a blood cell is placed in pure water and osmotic flow of water into
the cell causes it to swell up. Eventually, it may burst.
(c) If a blood cell is placed in a concentrated solution like seawater, then
osmosis draws water out of the cell, distorting its normal shape.
A) Intravenous solutions should have osmotic pressure equal to that of
bodily fluids. These solutions are isoosmotic.
B) Solutions having osmotic pressures less than bodily fluids are
hypoosmotic. These pump water into cells.
C) Solutions having osmotic pressures greater than bodily fluids are
hyperosmotic. These solutions take water out of cells and tissues.
When patients are given an IV in a hospital, the majority of the fluid is
usually an isoosmotic saline solution containing 0.9 g NaCl per 100 mL
of solution.
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