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SLOs
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10.1
Concentration
units
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10.1.1
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Units of Concentration
Once you have identified the solute and solvent in a
solution, you are ready to determine its concentration. Concentration may be
expressed several different ways, usingpercent composition by
mass, volume percent,mole fraction, molarity,molality,
or normality.
1.
Percent
Composition by Mass (%)
This is the mass of the solute divided by the mass of the
solution (mass of solute plus mass of solvent), multiplied by 100.
Example:
Determine the percent composition by mass of a 100 g salt solution which contains 20 g salt.
Solution:
20 g NaCl / 100 g solution x 100 = 20% NaCl solution
2.
Volume
Percent (% v/v)
Volume percent or volume/volume percent most often is used
when preparing solutions of liquids. Volume percent is defined as:
v/v % = [(volume of solute)/(volume of solution)] x 100%
Note that volume percent is relative to volume of
solution, not volume of solvent. For example, wine is about 12%
v/v ethanol. This means there are 12 ml ethanol for every 100 ml of wine. It
is important to realize liqud and gas volumes are not necessarily additive.
If you mix 12 ml of ethanol and 100 ml of wine, you will get less than 112 ml
of solution.
As another example. 70% v/v rubbing alcohol may be
prepared by taking 700 ml of isopropyl alcohol and adding sufficient water to
obtain 1000 ml of solution (which will not be 300 ml).
3.
Mole
Fraction (X)
This is the number of moles of a compound divided by the
total number of moles of all chemical species in the solution. Keep in mind,
the sum of all mole fractions in a solution always equals 1.
Example:
What are the mole fractions of the components of the solution formed when 92 g glycerol is mixed with 90 g water? (molecular weight water = 18; molecular weight of glycerol = 92)
Solution:
90 g water = 90 g x 1 mol / 18 g = 5 mol water 92 g glycerol = 92 g x 1 mol / 92 g = 1 mol glycerol total mol = 5 + 1 = 6 mol xwater = 5 mol / 6 mol = 0.833 x glycerol = 1 mol / 6 mol = 0.167 It's a good idea to check your math by making sure the mole fractions add up to 1: xwater + xglycerol = .833 + 0.167 = 1.000
4.
Molarity
(M)
Molarity is probably the most commonly used unit of
concentration. It is the number of moles of solute per liter of solution (not
necessarily the same as the volume of solvent!).
Example:
What is the molarity of a solution made when water is added to 11 g CaCl2 to make 100 mL of solution?
Solution:
11 g CaCl2 / (110 g CaCl2 / mol CaCl2) = 0.10 mol CaCl2 100 mL x 1 L / 1000 mL = 0.10 L molarity = 0.10 mol / 0.10 L molarity = 1.0 M
5.
Molality
(m)
Molality is the number of moles of solute per kilogram of
solvent. Because the density of water at 25°C is about 1 kilogram per liter,
molality is approximately equal to molarity for dilute aqueous solutions at
this temperature. This is a useful approximation, but remember that it is
only an approximation and doesn't apply when the solution is at a different
temperature, isn't dilute, or uses a solvent other than water.
Example:
What is the molality of a solution of 10 g NaOH in 500 g water?
Solution:
10 g NaOH / (40 g NaOH / 1 mol NaOH) = 0.25 mol NaOH 500 g water x 1 kg / 1000 g = 0.50 kg water molality = 0.25 mol / 0.50 kg molality = 0.05 M / kg molality = 0.50 m
Dilutions:You dilute a solution whenever you add solvent
to a solution. Adding solvent results in a solution of lower concentration.
You can calculate the concentration of a solution following a dilution by
applying this equation:MiVi = MfVf
where M is molarity, V is
volume, and the subscripts i and f refer to the initial and final values.
Example:
How many millilieters of 5.5 M NaOH are needed to prepare 300 mL of 1.2 M NaOH?
Solution:
5.5 M x V1 = 1.2 M x 0.3 L V1 = 1.2 M x 0.3 L / 5.5 M V1 = 0.065 L V1 = 65 mL Reference of the above material is
Parts per million (ppm)and Parts per billion (ppb)
The only difference in the calculations of parts per
million and parts per billion is factor used to multiply the ratio. The
formulas for these two are:
Parts per million
Parts per billion
Assignment: Percent,
ppm and ppb
1) Calculate the
concentration of salt in a solution of water in percent if 45 grams is
dissolved in 1,200 ml of water.
2) Calculate the mass
of solute in a 10% salt solution if the mass of the solvent is 350 grams.
3) Calculate the mass
of solvent in a 6 ppm solution of a drug if the mass of the solute is 0.050
milligrams.
4) What is the
concentration in ppm of selenium if 1.3 milligrams is found in 2,500 kg of
soil?
5) What is the
concentration in ppb of PCB's in a chemical spill, if their is 0.060 mg in
4,600 Kg of soil?
6) Calculate the mass
of solute of hydrogen peroxide in a 35 % solution if 450 ml of solution is
being used.
7) Calculate the mass
of solute PCB's in a 65 Kg person, if the concentration is 4 PPM?
8) What mass of nickel
is in a 2.4 Kg sample of propanol if the concentration is 20 ppb?
9) Calculate the mass
of solvent that would contain 3.0 mg of a drug if the concentration was 3.5%.
10) Calculate the
concentration in ppm if 8 grams of CaCl2 is dissolved in 250 ml of
water.
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10.2.1
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10.2.2
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10.2.3
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Solubility Curve Practice Problems
Worksheet 1
You'll notice that for most
substances, solubility increases as temperature increases. As discussed
earlier in solutions involving liquids and solids typically more solute can
be dissolved at higher temperatures. Can you find any exceptions on the
graph?____________________
Here's an example of how to read the
graph. Find the curve for KClO3.
At
30°C approximately 10g of KClO3 will dissolve in 100g of water. If
the temperature is increased to 80°C, approximately
______ of the substance will dissolve in 100g (or 100mL) of water.
Directions: Use the graph to answer the following
questions.
1) What
mass of solute will dissolve in 100mL of water at the following temperatures?
a.
KNO3 at 70°C =
____________
b.
NaCl at 100°C= ____________
c.
NH4Cl at 90°C=
____________
d.
Which of the above three
substances is most soluble in water at 15°C. = ____________
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10.3.1
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First Definition:
The
vapour pressure of a solution of a non-volatile solute is equal to the vapour
pressure of the pure solvent at that temperature multiplied by its mole
fraction.
Second definition:
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10.3.2
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10.3.3
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10.4.1
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Physical properties can be divided
into two categories. Extensive properties (such as mass and volume)
depend on the size of the sample. Intensive properties (such as
density and concentration) are characteristic properties of the substance;
they do not depend on the size of the sample being studied. This section
introduces a third category that is a subset of the intensive properties of a
system. This third category, known as colligative properties, can only
be applied to solutions. By definition, one of the properties of a solution
is a colligative property if it depends only on the ratio of the
number of particles of solute and solvent in the solution, not the identity
of the solute.
Very few of the physical
properties of a solution are colligative properties. As an example of this
limited set of physical properties, let's consider what happens to the vapor
pressure of the solvent when we add a solute to form a solution. We'll define
Po as the vapor pressure of the pure liquid
 the solvent and P as the
vapor pressure of the solvent after a solute has been added.
Po = vapor pressure of the pure liquid, or solvent
P = vapor pressure of the solvent in a solution
When the temperature of a liquid
is below its boiling point, we can assume that the only molecules that can
escape from the liquid to form a gas are those that lie near the surface of
the liquid.
When a solute is added to the
solvent, some of the solute molecules occupy the space near the surface of
the liquid, as shown in the figure below. When a solute is dissolved in a
solvent, the number of solvent molecules near the surface decreases, and the
vapor pressure of the solvent decreases.
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10.4.2
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10.4.3
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10.4.4
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10.5.1
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True Solution
True Solution is a homogeneous
mixture of two or more substances in which substance dissolved (solute) in
solvent has the particle size of less than 10-9 m or 1 nm. Simple solution of
sugar in water is an example of true solution. Particles of true solution
cannot be filtered through filter paper and are not visible to naked eye.
Suspensions
Suspension is a heterogeneous
mixture in which particle size of one or more components is greater than
1000nm.When mud is dissolved in water and stirred vigorously,
particles of mud are distributed evenly in water. After some time, the
particles of this solution settle under water due to influence of gravity.
This solution is an example of Suspension (see picture below). Contrary to
True Solution,particles of suspension are big enough to be seen with naked
eye.
Colloidal Solution
Colloidal Solution is a
heterogeneous mixture in which particle size of substance is intermediate of
true solution and suspension i.e. between 1-1000 nm. Smoke from a fire is
example of colloidal system in which tiny particles of solid float in air.
Just like true solutions, Colloidal particles are small enough and cannot be
seen through naked eye. They easily pass through filter paper. But colloidal
particles are big enough to be blocked by parchment paper or animal membrane.
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10.5.2
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Colloids
Colloids are mixtures
whose particles are larger than the size of a molecule but smaller than
particles that can be seen with the naked eye. Colloids are one of three
major types of mixtures, the other two being solutions and suspensions. The
three kinds of mixtures are distinguished by the size of the particles that
make them up. The particles in a solution are about the size of molecules,
approximately 1 nanometer (1 billionth of a meter) in diameter. Those that
make up suspensions are larger than 1,000 nanometers. Finally, colloidal
particles range in size between 1 and 1,000 nanometers. Colloids are also
called colloidal dispersions because the particles of which they are made are
dispersed, or spread out, through the mixture.
Types of Collids:
Colloids are common in
everyday life. Some examples include whipped cream, mayonnaise, milk, butter,
gelatin, jelly, muddy water, plaster, colored glass, and paper.
Every colloid consists
of two parts: colloidal particles and the dispersing medium. The dispersing
medium is the substance in which the colloidal particles are distributed. In
muddy water, for example, the colloidal particles are tiny grains of sand,
silt, and clay. The dispersing medium is the water in which these particles
are suspended.
Colloids can be made
from almost any combination of gas, liquid, and solid. The particles of which
the colloid is made are called the dispersed material. Any colloid consisting
of a solid dispersed in a gas is called a smoke. A liquid dispersed in a gas
is referred to as a fog.
Types of Colloids
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10.5.3
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10.5.4
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Friday, 3 August 2012
Notes of Solution
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Sir where r the remaining SLO notes After 10.3.1
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