AP Chemistry Unit 4

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AP ChemistryUnit 4

• Properties of Solutions
• Chapters 11-13

2
Concentrations
3
Molarity

• We covered this earlier
• M
• Moles solute/liter of solvent

4
Molality

• Moles of solute/kg solvent
• Symbolized by m

5
Normality

• Symbolized by N
• Typically used for acids or bases
• Equivalents of solute/liters of solvent
• Acids acidic protons x M
• Bases basic hydroxides x M

6
Mass Percent

• Percent of solute in a solution
• (mass solute/mass solution) x 100
• Units must be the same so they cancel

7
Mole Fraction

• Moles of one compound to total moles
• Symbolized by X
• X n1/(n1n2n3)

8
Practice

• 25 g of hydrochloric acid are dissolved in 100 mL
  of water. Calculate all possible concentrations.
• 3 moles of calcium hydroxide is in solution with
  45 mL of water. Calculate all possible
  concentrations.

9
Solubility
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Solubility

• Solubility is the amount of solute that dissolves
  in a given solvent at a certain temperature
• High solubility means a lot will dissolve
• Low solubility means not much will dissolve
• Insoluble means none will dissolve

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Solubility

• Affected by polarity of solutes and solvent
• Typically higher temperatures increase solubility
  of solids
• Higher temperatures always increases the rate of
  dissolution
• Typically higher temperatures decreases
  solubility of gases
• When no more solute can be dissolved the solution
  is called saturated

12
Solubility

• Polar solvents dissolve polar solutes
• Nonpolar solvents dissolve nonpolar solutes
• Polar solutes are hydrophilic, easily dissolved
  in water
• Nonpolar solutes are hydrophobic, not easily
  dissolved in water

13
Practice

• Would sodium chloride dissolve better in water or
  liquid methane (CH4)?
• Which would dissolve silicon tetrafluoride
  better water or liquid nitrogen?
• Is carbon dioxide highly or not very soluble in
  water? (Think pop)
• Is FOG (Fats, oils, and greases) polar or
  nonpolar?

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Pressure and Solubility

• Only affects the solubility of gases
• Thats why carbonated beverages are pressurized
• Makes aquatic life possible
• Pressure or solubility can be calculated using
  Henrys law

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Henrys Law

• S kHP
• S is solubility
• kH is Henrys Constants and depends on the gas,
  solvent, and temperature
• mol/Latm
• P is the partial pressure of the gas

16
Practice

• The kH of oxygen is 1.3 x 10-2 mol/Latm at 293
  K. At that temperature what is the solubility of
  oxygen in pond water at 1 atm?
• What is the Henry Constant for carbon dioxide
  dissolved in orange soda at 2 atm if 2 x 10-3 mol
  are dissolved in 500 mL?

17
Thermal Pollution

• When natural water is used to cool equipment
• Return water is hot, which means the amount of
  dissolved oxygen is lower

18
Vapor Pressure of Solutions
19
Vapor Pressure

• Solutions have different freezing and boiling
  points
• A nonvolatile (doesnt vaporize spontaneously)
  solute lowers the vapor pressure of solution
• The solution will vaporize less easily and
  therefore there will be less solvent molecules in
  the gas phase

20
Vapor Pressure

• Pressure of solution is related to the mole
  fraction of solvent and vapor pressure of pure
  solvent
• Called Raoults Law

21
Raoults Law

• Psltn Xslv Ppure
• Psltn is the vapor pressure of the solution
• Xslv is the mole fraction of the solvent
• Ppure is the vapor pressure of the pure solvent

22
Practice

• What is the vapor pressure of a 60/40 water and
  antifreeze mixture if the vapor pressure of pure
  water is 1 atm?
• 25 g of table salt is dissolved in 100 mL of
  water. What is the vapor pressure of the solution?

23
Vapor Pressure

• A solute in solution always lowers the vapor
  pressure of solution
• X can never be greater than 1
• When X 1 it is a pure solvent

24
Ideal Solution

• An ideal solution has a nonvolatile solute
• Nonvolatile solutes cannot exist as a gas, so the
  only vapor pressure to consider is the solvent
• An ideal solution obeys Raoults Law

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Nonideal Solutions

• A volatile solute makes the solution nonideal
• It doesnt follow Raoults Law
• Solute contributes to the vapor
• Can still behave as ideal solutions at low
  concentrations
• lt 0.1 M for nonelectrolytic solutes
• lt0.01 M for electrolytic solutes

26
Practice

• Are these ideal or nonideal? Why?
• Water and oil
• Water and ethanol
• Salt and ethanol
• Sugar and water
• Chlorine and water

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Vapor Pressure for Nonideal Solutions

• Ptotal X1Ppure1 X2Ppure2
• Ptotal is the total vapor pressure
• X1 and X2 are the mole fractions of compound 1
  and 2
• Ppure1 and Ppure2 are the vapor pressure of pure
  compound 1 and 2

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Practice

• What is the vapor pressure of a benzene and water
  mixture if 25 g of benzene are mixed with 40 mL
  of water? The vapor pressure of water is 1 atm
  and the vapor pressure of benzene is 1.3 atm.

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Colligative Properties
30
Colligative Properties

• Colligative properties are properties that depend
  on how much of something, not what it is
• Freezing Point Depression
• Boiling Point Elevation
• Osmotic Pressure

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Freezing Point Depression

• Most practical of the colligative properties
• The addition of a solute lowers the freezing
  point of the solvent
• Salt
• Antifreeze
• Test for impurities impure if freezing point is
  lower

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Freezing Point Depression

• ?T mkf
• ?T is the change in freezing point
• m is the molality of solute
• kf is the freezing point constant for the solvent
• Units of Ckg/mol
• Must be looked up or found experimentally

34
Practice

• Carbon tetrachloride has a freezing point
  constant of 30. If an unknown solute is dissolved
  in carbon tetrachloride, what is the
  concentration if the freezing point is depressed
  by 4 C?
• Bill Nye dissolves 5 g of salt in 200 mL of water
  and the freezing point is depressed by 1.85 K.
  What is the freezing point constant?

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Freezing Point Depression

• If the solute is an electrolyte it dissociates
• The concentration of the solute must be
  multiplied by a conversion factor, the van t
  Hoff Factor
• Really only reliable at very low concentrations
• van t Hoff Factor equal to the number of
  dissociated ions from a single compound

36
Practice

• What is the freezing point of 100 g of 0.000001 M
  hydrochloric acid if water has a freezing point
  constant of 1.86? (Assume the same density as
  water)

37
Boiling Point Elevation
38
Boiling Point Elevation

• A lowered vapor pressure increases the boiling
  point
• Solute must be nonvolatile

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Boiling Point Elevation

• ?T mkb
• ?T is the change in boiling point
• m is the molality of solute
• kb is the boiling point constant for the solvent
• Units of Ckg/mol
• Must be looked up or found experimentally

40
Practice

• Carbon tetrachloride has a boiling point constant
  of 4.95. If an unknown solute is dissolved in
  carbon tetrachloride, what is the concentration
  if the boiling point is elevated by 12 K?
• Mr. Science dissolves 12 g of salt in 98 mL of
  water and the boiling point is elevated by 6 C.
  What is the boiling point constant?
• How much is the boiling point elevated if 99 g of
  ammonium chloride is dissolved in 1 L of water,
  boiling point constant of 0.51.

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Osmotic Pressure
42
Osmotic Pressure

• If a solution and a pure solvent are divided by a
  semipermeable membrane the volume of solution
  will increase and solvent will decrease
• The membrane allows solvent to pass, but solute
  is too large
• The transfer is called osmosis

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Osmotic Pressure

• When the transfer stops the solution and solvent
  are in equilibrium
• The difference is pressures is the osmotic
  pressure
• The minimum pressure needed to stop osmosis is
  equal to the osmotic pressure

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Osmotic Pressure

• The van t Hoff Equation
• ? MRT
• ? is the osmotic pressure
• M is the molarity of solute
• R is the universal gas constant
• T is the temperature

45
Practice

• What is the osmotic pressure of a 0.05 M solution
  at 300 K?
• What is the concentration for a water and salt
  water solution with an osmotic pressure of 2.1
  atm at 17 C?
• What is the temperature of 10.0 mL of a 1.0 M
  solution if the atmospheric pressure is 1.1 atm?

46
Practice

• What is the molar mass of a compound if 1 g is
  dissolved in 10 mL at 23 C? The osmotic pressure
  was 0.05 atm.
• (Hint Find the concentration, then use the
  volume and mass. If you get stuck check pg 537)

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Dialysis

• A membrane allows solvent and small solutes to
  pass through
• Many living organisms use dialysis to filter
  blood or other nutrient systems

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Reverse Osmosis

• If the external pressure is higher than the
  osmotic pressure the fluid will flow to the
  solvent side
• Used in desalination plants to filter salt out of
  water

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Kinetics
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Chemical Kinetics

• Deals with reaction rates
• How fast a reaction takes place
• Reaction mechanisms how a reaction happens,
  what steps

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Rate of Reaction

• How quickly a reaction occurs
• Rate is equal to the change in concentration over
  change in time, just like rates in physics

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Rate of Reaction

• Rate ?X/?t
• X is the concentration of X
• t is time
• If rate is then X is being used
• If rate is then X is being formed

53
Practice

• In 400s the concentration of nitrogen monoxide
  goes from 0 to 0.0069 M while nitrogen dioxide
  goes from 0.0100 M to 0.0031 M. What is the rate?
• The rate of formation for water is 3.0 M/s from
  hydrogen and oxygen gas. Starting from 0
  molecules of water how long does it take to have
  1.0087 moles in 10 mL?

54
Practice

• The formation of ammonia from hydrogen and
  nitrogen has a reaction rate of 0.0023 M/s. After
  2 minutes there is a concentration of 2.0 M. What
  were the original concentrations and what is the
  final concentration of reactants?
• 295 g of aluminum oxide react with 250 g of
  hydrogen. If the rate of formation is 0.095 M/s
  how long does it take to fully react and how much
  water is produced in 0.025 L?

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Rate of Reaction

• Can be graphed, time on x, concentration on y
• Slope is rate
• Real rate is never a straight line, its a curve
• Rate slows with time for formation or
  disappearance

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Instantaneous Rate

• Solving the rate equation only gives the rate at
  a single point
• The rate is the slope of the tangent at that
  point
• Need calculus to find the curve of the slope,
  well only look at instantaneous rates

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Rate Law
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Rate Law

• Instantaneous rate only looks at formation or
  disappearance of a product or reactant, not both
  simultaneously
• This is a reversible reaction

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Rate Law

• Reactions are actually equilibria
• Reactants are forming products and some products
  are reforming reactants
• A good reaction forms product much faster than
  product reforms reactant

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Rate Law

• Rates then must account for the forward and
  reverse reactions
• We will make this assumption
• Rate forward gtgt Rate reverse

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Rate Law

• A rate law is a mathematical expression that only
  shows the rate as dependent upon the
  concentration of reactant
• Rate kAn
• k is the rate constant
• n is the rate order

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Rate Constant

• Gives what fraction of the molecules are reacting
  per unit time
• Units are usually s-1

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Rate Order

• Can be any integer including 0
• n1 means there is a direct relationship, if A
  doubles the rate doubles
• n2 means there is a square relationship, if A
  doubles the rate quadruples
• n0 means the rate does not depend on A
• n1/2 means if A quadruples the rate only
  doubles

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Reaction Order

• The overall reaction order is the sum of the rate
  orders
• Keep reaction order and overall reaction order
  seperate

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Rate Law

• Since we assume the reverse reaction is much
  slower than the forward we never show the
  product in a rate law
• The rate order cannot be determined from the
  equation, it can only be determined by
  experimentation

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REMEMBER YOUNG PADAWANS

• The reaction rate has the form Rate ?X/?t
• The rate law has the form Rate kXn
• The reaction rate can also be called the
  instantaneous rate

67
Practice

• Write the instantaneous rate and all rate laws
  for the formation of water.
• Write the instantaneous rate and all rate laws
  for the formation of ammonia from nitrogen and
  hydrogen.
• Show both of the above if hydrogen is first
  order, oxygen is 2nd order, and nitrogen is 0
  order. What do these mean in words?

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Solving Rate Laws
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Solving Rate Laws

• You must be given at least 3 sets of experimental
  data
• Data needs concentrations of all reactants and
  the rate in each experiment
• Write both in the form Ratex kAnBm
• Need 2 equations, one where A is the same and
  one where B are the same
• Lets you solve for each order

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Example
Write the rate laws for 1 and 2. 0.121
k(0.12)n(0.20)m 0.241 k(0.24)n(0.20)m
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Practice
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Practice
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Rate Law Summary
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Solving Rate Orders
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Solving Rate Orders

• More complex problems require a more complex
  solution

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Solving Rate Orders

• Use logarithms to solve for the order
• A Bx
• Log A x Log B
• Ln A x Ln B
• Logi A x Logi B

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Integrated Rate Law
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Integrated Rate Law

• Rate laws show dependence of rate on
  concentration
• Integrated rate laws show dependence of rate on
  time

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Integrated Rate Law

• Can be found by integrating the rate law
• You will do this in college, but not here

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Integrated Rate Laws

• Zero Order A -kt Ainit
• First Order lnA -kt lnAinit
• Second Order 1/A kt 1/Ainit

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Reaction Mechanisms
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Reaction Mechanisms

• Answers the why of rate laws
• Why are things different orders?
• Why does oxygen convert to ozone?
• Why do reactions happen the way they do?

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Reaction Mechanisms

• The simplest reactions are called elementary
  reactions
• A reaction mechanism is a set of elementary
  reactions which go from the initial reactants to
  the final desired product

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Reaction Mechanisms

• Compounds involved in an elementary reaction but
  arent reactants or products are called reaction
  intermediates
• If there is only 1 reactant it is unimolecular
• If there are 2 reactants it is bimolecular
• If there are 3 reactants it is termolecular

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Example

• Write the balanced equation of the conversion of
  ozone to oxygen

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Example

• Elementary reactions
• O3 ? O2 O
• O O3 ? O2 O2
• Overall Reaction
• O O3 O3 ? O2 O2 O2 O
• O3 O3 ? O2 O2 O2

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Modeling Kinetics
88
Modeling Kinetics

• Temperature increases reaction rate
• Why?

89
Collision Model

• Molecules react by colliding in to each other
• If they collide with enough force they stick
• Each collision has a certain percentage chance of
  creating a product molecule
• The higher the energy and percentage chance of
  creating a product molecule, the faster the rate

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Collision Model

• Svante Arrhenius figured out there is a minimum
  amount of energy needed to cause particles to
  react
• Reactant molecules without enough energy do not
  react when colliding
• Explains why adding energy (heat) increases the
  reaction rate

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Activation Energy

• This amount of energy is called the activation
  energy (Ea)
• Each reaction has a specific activation energy,
  in units of J/mol
• Must be looked up or found experimentally

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Activation Energy

• The mix of reactant and product at the peak of
  activation energy is called the transition state
• Bonds are partially formed and partially broken

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Transition State

• The energy needed to form the transition state,
  the activation energy, helps determine the rate
• If energy is lost from the reactants to products
  it is an exothermic reaction
• If energy is higher for products than reactants
  it is an endothermic reaction

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Reversibility

• Endothermic reactions are more likely to be
  reversible
• Why?

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Reversibility

• Activation energy is less for the reverse of an
  endothermic reaction
• REMEMBER Activation energy is the difference
  between the energy of reactants and the
  transition state

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Reactions

• Activation energy is not all that affects ability
  to react
• Molecular orientation affects likelihood to react

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Molecular Orientation

• How the molecules collide affects whether or not
  they can stick
• Some orientations dont react, or even repel

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Molecular Orientations

• Visual

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Arrhenius Equation

• Calculates the rate constant without
  experimentation
• Takes into account both activation energy and
  frequency of collisions with the correct
  orientation to react

102
Arrhenius Equation

• k Ae-Ea/RT
• k is the rate constant
• A is the frequency factor
• Ea is the activation energy
• R is the Universal Gas constant
• T is the temperature

103
Arrhenius Equation

• Take the natural log of each side to graph
• Ln(k) -Ea/RT Ln(A)
• y Ln(k)
• m -Ea/R
• x 1/T
• b Ln(A)

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Arrhenius Equation

• A graph gives the value of k and A
• Only data needed is temperature dependence of k
• This is usually how activation energy is
  calculated

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Solving

• Ln (k2/k1) Ea/R(1/T1-1/T2)
• This comes from the change in the 2 states, we
  subtract the second equation from the first
• Ln(A) is removed, leaving the values of k and T
  and our only unknown Ea

106
Practice

• At 300. K the rate constant for a reaction is
  3.91 L/mols and at 124 K it is 0.562 L/mols. What
  is the activation energy?
• Water at the freezing point has a rate constant
  of 2.2 L/molhr, and at the boiling point has a
  rate constant of 12.5 L/molhr. What is the
  activation energy?

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Catalysts
108
Catalysts

• A catalyst is not used up in a reaction, it is
  the same both before and after
• They increase the rate of reaction but not the
  amount of product made
• They do this by lowering the activation energy

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Catalysts

• Give an alternate reaction mechanism that
  requires less energy
• The reactant and product energies are the same,
  the transition state energy is significantly lower

110
Catalysts

• Enzymes are the most common catalysts
• Very large molecules (macromolecules)
• Increase the rate of biological processes in most
  living organisms

111
Farnesyl Diphosphate Synthase
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Homogeneous Catalysts

• Catalysts in the same state of matter (phase) as
  the reactants
• Usually liquids or gases, or at least in an
  aqueous solution

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Homogenous Catalysts

• Most common examples are atmospheric
• Ozone is very reactive, a good catalyst

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Heterogeneous Catalysts

• Exist in a different state of matter than the
  reactants
• Usually a metal catalyst and a liquid or gas
  reactant
• Ex Platinum and gold are excellent for adding
  hydrogen to carbon double bonds

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Adsorption

• Adsorption is the collection of a substance at
  the surface of another
• This is how heterogeneous catalysts work

116
Heterogeneous Catalyst Steps

• Adsorption of reactants to solid surface
• Attraction of reactants
• Reaction
• Desorption, product is removed from surface

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End of Unit

• Questions?