Transcript Regents Chemistry

Regents Chemistry
• Properties of Solutions
Properties of Solutions
• Review - What’s a solution
– a solution is a homogeneous mixture of
substance in the same physical state
• Most chemical reactions take place in
solutions
• We will learn the nature and properties of
solutions and ways to express the
concentration of solutions
What do Solutions Contain?
• Solutions contain atoms, ions or molecules
in which one substance spread uniformly
throughout a second substance
• Ex: Salt water
Types of Solutions
• Solutions exist in all three states!
• A solid may be dissolved in another solid
– ex: Brass is a mixture if zinc and copper
• A Metal solution is called an alloy
• Air is a gaseous solution and can vary
depending on the conditions
– ex: amount of water vapor varies daily
Liquid Solutions
• We will mostly focus on solutions containing a
liquid
• We identify parts of a liquid solution by how it
is made
• Solute - is the substance that is being
dissolved, and it is the substance present in
the smaller amount
• Solvent - substance that dissolves the solute
- most common is water
NaCl
NaCl(s)
Na+(aq) + Cl-(aq)
Once the salt and water is stirred and the mixture
becomes homogeneous, the dissolved particles
will not settle - cannot filter!
Liquid solutions are clear but may have color and
light will pass through without being dispersed
Liquid Solution Summary
• 1. Solutions are homogeneous mixtures
• 2. Solutions are clear and do not disperse
light
• 3. Solutions can have a color
• 4. Solutions will pass through a filter
Solubility Factors
• Some things dissolve in solvents and some
don’t, so..
• Solubility - is how much of a solute will
dissolve in a certain amount of solvent at a
certain temperature
• Materials with high solubility are said to be
soluble
• Materials with a low solubility are said to be
insoluble
Nature of Solute and Solvent
• NaCl dissolves in water because its positively
and negatively charged ions are attracted to
oppositely charged ends of the polar water
molecule
• The attractive forces between the water
molecules and sodium ions are greater than
the attractive forces between the sodium and
chloride ions
• Same goes for the chloride ions and positive
end of water molecule
Like dissolves Like
• Ionic substance dissolve in ionic solvents
• Nonpolar substances, such as fats, dissolve
in nonpolar solvents
• So fats do not dissolve in water! No strong
attractive forces between water molecules and
fat molecules - must be dissolved in a nonpolar
solvent
• Why..because the forces are weak and they
simply mix together
Table summary
Solute Type
Nonpolar
Nonpolar Solvent
Soluble
Polar Solvent
insoluble
Polar
Insoluble
soluble
Ionic
Insoluble
soluble
Effect of Temperature
• As temperature increases, most solids
become more soluble in water
• A few exceptions exist:
– Gases react in the opposite manner
– As temperature increases, the solubility of all
gases in liquids decreases
Effect of Pressure
• Pressure has little or no effect on the solubility
of solid or liquid solutes
• Pressure does affect the solubility of gases in
liquids
• As pressure increases, the solubility of gases
in liquids increases
• Ex: opening a can of soda - the pressure
decreases
– CO2 is no longer as soluble at the lowered
pressure and escapes as bubbles
Regents Chemistry
• Solubility Graphs and saturated and
unsaturated solutions
Solubility Information
• Solubility information may be presented in
different ways
• Table G in your Reference Tables shows the
relationship between grams of solute that can
be dissolved at various temperatures
• Table F in Reference Tables provides some
general guidelines about the solubility of ionic
substances
Using Table G
• Shows the maximum number of grams
that can be dissolved in 100g H2O at
specific temperatures
• Most show increasing solubility as temp
increases, but a few don’t
– these are gaseous NH3, HCl and SO2
– gases decrease in solubility as temp
increases
Using Table G
• Any point that is below the curve of a
substance is considered unsaturated
• Any point that is on the curve of a
substance is considered saturated
• Any point that is above the curve of a
substance is considered supersaturated
Saturation
• Unsaturated solutions hold less solute than
maximum and no solid should be present
• Saturated solutions hold the max amount
and any additional will simply stay as a solid
• Supersaturated solutions occur when the
temperature is reduced but no crystals (solid)
form out of solution - any additional solute
added will cause crystals to form and solution
will return to saturated state
Recognizing Degree of Saturation
• Because solutions are clear, it is difficult to simply
look at a solution and determine whether it is
saturated, unsaturated or supersaturated
– So how can we tell?
• 1. We can compare the number of grams
dissolved in a given volume to table G
• 2. Add additional solute and see what
happens!
Using Table F
• Contains some guidelines for the solubility
of common ionic compounds
• YOU HAVE USED THIS TABLE BEFORE!
• Explains if a reaction will form
Table G Practice Problem
• Which substance on table G (solubility
curve) is saturated with 20g at 49 C?
• How many grams of HCl would have to be
added to a 70g in solution to make it
saturated at 10 C?
Regents Chemistry
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Concentrations of Solutions – Molarity
What’s Molarity
Let’s first review a mole…video clip
 We sometimes refer to solutions as
concentrated or dilute…but these are
not scientifically precise terms..
 We need to know specific strengths to
run reactions..

– This is the purpose of molarity!
Molarity

Molarity – is the number of moles of solute
in 1 Liter of solution
– This tells us the exact “strength” of the solution
– We add a specific amount of solute to a specific
amount of water..once this is made, the molarity doesn’t
change!
– The formula is below and on your reference tables
Molarity = moles of solute
liters of solution
=
g / mol
L
=
M
Solving Basic Molarity Problems

What is the molarity of a solution that
contains 4.0 mol of NaOH in 0.50 L of
solution?
M = mole of solute
liters of soln
Molarity = 8.0 M
= 4.0 moles NaOH
0.50 L
Molarity w / no moles given…

What is we are given a gram amount instead of
mole amount…can we still solve for molarity?
– Yes!
Practice Problem
What is the molarity of a solution containing 82.0 g
of Ca(NO3)3 in 2.0 L of solution?
1. Convert 82.0 grams to moles by using molar mass
2. Plug into Molarity equation and solve!
Additional Practice Problem

What is the molarity of a solution
containing 26.0 g KCl in 750 mL of
solution?
Rearranging the Equation
We can rearrange the equation to solve
for mole amount or liters of solution
Example
 How many moles of BaSO4 are in a 2.0
M solution originally made with 1.5 L of
solution?
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Regents Chemistry

% by mass, % by volume and ppm
Percent by Mass

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Common to find labels that list the
concentration of ingredients by mass
Percent Mass – is simply the mss of an
ingredient divided by the total mass
expressed as a percent
Percent mass is essentially the same as
percent composition – you have done
this in lab!
Percent by Mass
Percent mass =
mass of part
X 100%
mass of whole
What is the percent mass of sodium hydroxide
If 2.50 g of NaOH are added to 50.00 g of H2O?
Percent by Volume
When two liquids are mixed to form a
solution, it is common to express the
concentration of the solute as a percent
by volume
 For example, a solution of isopropyl
alcohol contains 70% alcohol by volume

Percent by volume =
Volume of solute
Volume of solution
X 100%
Practice Problem

What is the percent by volume of alcohol
if 50.0 mL of ethanol is dilluted with
water to form a total volume of 300 mL?
Parts per Million (ppm)

Parts per million is similar to % comp
because it compares masses
– It’s a ratio between mass of the solute to
total mass of the solution

This method of reporting concentrations
is useful for extremely dilute solutions
when molarity and % mass would be to
difficult to interpret
ppm

For example
– Chlorine is used as a disinfectant in
swimming pools. Only about 2g of chlorine
per 1,000,000 g of swimming pool water is
necessary to keep the pool sanitized
ppm =
Grams of solute
Grams of solution
x 1,000,000 ppm
Practice Problem
Approximately 0.0043 g of oxygen can
be dissolved in 100 mL of water at 20
degrees Celsius. Express this in terms of
ppm
 (assume 1mL water = 1.0 g water

Regents Chemistry
• Colligative Properties
What are Colligative Properties?
• Colligative properties are properties of a
substance that are affected by the nature
of a solute added to it
• In terms of water:
– Freezing and boiling points are colligative
properties that are affected by the nature of
the solute..as we shall see…
Molecular vs. Ionic
• Molecular substances affect the freezing
and boiling points of water different than
ionic substances..
• Why??
• Because ionic substance break apart into
ions and molecules do not!
– Ex: Salt vs. sugar
Salt vs. Sugar
C12H22O11 (s)  C12H22O11 (aq)
Vs.
NaCl (s)  Na+ (aq) + Cl- (aq)
1 mole of salt will raise the boiling point and
depress the freezing point twice as well as
1 mole of sugar!
Vapor Pressure and Boiling
Point
• When a substance that is normally a liquid
enters a vapor phase, it is called a vapor
• A liquid normally has molecules that
escape its surface
• The pressure that these molecules exert in
the surrounding atmosphere is called
vapor pressure
Vapor Pressure
• Why do these molecules escape?
• Liquids are held together by rather weak
intermolecular forces
• These forces are called dipole-dipole
forces
• As temperature increases, these forces
become less effective and more molecules
escape…thus VP increases!
Water is different…
• Water is different than most liquids..
• It participates in hydrogen bonding in addition to
dipole-dipole interactions..
• Thus it has a high boiling point and requires
more energy to break the intermolecular forces..
• This is seen by observing the relationship
between molecular weights and vapor pressure
Table H
• Table H on your reference tables shows us
the vapor pressure at various
temperatures..
• Notice the boiling point for each liquid
• Boiling Point – is when the vapor pressure
of a liquid is equal to the atmospheric
pressure…
• This occurs when we see bubbles!
Using Table H
• Find the Vapor Pressure of water at 75
degrees Celsius.
• Which of the substances has the weakest
intermolecular forces? Why?
• Which has the strongest intermolecular
forces? Why?