Regents Chemistry

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Regents Chemistry

• Properties of Solutions

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Properties of Solutions

• Review - Whats 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

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What do Solutions Contain?

• Solutions contain atoms, ions or molecules in
  which one substance spread uniformly throughout a
  second substance
• Ex Salt water

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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

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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

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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
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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

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Solubility Factors

• Some things dissolve in solvents and some dont,
  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

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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

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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

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Table summary
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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

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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

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Regents Chemistry

• Solubility Graphs and saturated and unsaturated
  solutions

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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

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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 dont
• these are gaseous NH3, HCl and SO2
• gases decrease in solubility as temp increases

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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

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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

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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!

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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

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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?

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Regents Chemistry

• Concentrations of Solutions Molarity

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Whats Molarity

• Lets first review a molevideo clip
• We sometimes refer to solutions as concentrated
  or dilutebut these are not scientifically
  precise terms..
• We need to know specific strengths to run
  reactions..
• This is the purpose of molarity!

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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
  doesnt change!
• The formula is below and on your reference tables

Molarity moles of solute
g / mol
M


liters of solution
L
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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

4.0 moles NaOH
liters of soln
0.50 L
Molarity 8.0 M
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Molarity w / no moles given

• What is we are given a gram amount instead of
  mole amountcan 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!

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Additional Practice Problem

• What is the molarity of a solution containing
  26.0 g KCl in 750 mL of solution?

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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

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Percent by Mass

• 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!

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Percent by Mass
mass of part
X 100
Percent mass

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?
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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

Volume of solute
X 100
Percent by volume

Volume of solution
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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?

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Parts per Million (ppm)

• Parts per million is similar to comp because it
  compares masses
• Its 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

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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

Grams of solute
x 1,000,000 ppm
ppm

Grams of solution
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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

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Regents Chemistry

• Colligative Properties

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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

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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

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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!
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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

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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 escapethus VP
  increases!

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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

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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!

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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?