Chapter 16 - Solutions Many chemical reactions occur when the reactants are in the aqueous phase....

Chapter 16 - Solutions

Many chemical reactions occur when the reactants are in the aqueous phase.

Therefore, we need a way to quantify the amount of reactants in an aqueous solution. We quantify products and reactants in an aqueous solution by

measuring the concentration.

Solutions

• Factors that affect the solubility of a compound.

• The process of a solute dissolving in a solvent is called solvation.

Solutions


• Agitation – When a solute is agitated, or stirred, more water molecules can come in contact with the solute, thus speeding up solvation.

Solutions


• Heat – An increase in the temperature causes the water molecules to move faster. So there will be more contacts between

the water molecules and the solute, increasing the process of solvation.

Solutions


• Increasing the surface area of the solute – breaking up or crushing the solute increase the surface area of the solute. As a result, more water molecules can contact the solute making is solvate faster.

Solutions• Calculating Solution Concentration• Expressing Concentrations• Molarity (M)• Molality (m)• % by volume or mass• Mole fraction

Solutions• Calculating Solution Concentration

• Molarity (M): Molarity is defined as the number of moles of solute per liter of solution.

Molarity (M) = moles of solute 1.0 L of H2O

Solutions• What is the molarity of a solution that

was made by dissolving 5.0 grams of NaCl in 500. mL of water.

Solutions• How many moles of calcium chloride

would be contained in 30.0 mL of a 1.5 M calcium chloride solution?


• Molality (m): Molality (m) is defined as the number of moles of solute per kilogram of solvent.

Molality (m) = moles of solute kg of solvent


• Calculate the molality of solution that was made by dissolving 3.0 grams of ammonium chloride in 100.0 mL of water.


• How many grams of water must 60.0 grams of sodium sulfate be dissolved in to make a 0.80 m sodium sulfate solution?

Aqueous Solutions• Calculating Solution Concentration

• % by volume: It is what the name says it is, the percentage of the whole solution that is the solute.

% by volume = volume of solute volume of

solution


• % by mass: The percentage of the mass of the solution that is the solute.

% by mass = mass of solute mass of solution


• Vinegar is made by adding 3.0 grams of concentrated acetic acid to 97.0 grams of water. What is the % by mass of acetic acid in vinegar?


• Most red wines are 12.0 % ethyl alcohol by volume. How many milliters of ethyl alcohol are contained in a 1.0 L bottle of red wine?


• Mole Fractions – Is defined as the number of moles of solute per moles of solvent plus the number of moles of solute.

Mole fraction = moles solute moles solvent + moles solute


• Calculate the mole fraction of a solution that is made by dissolving 15.0 grams of sucrose, C12H22O11, in 600.0 mL of water.

Aqueous Solutions• Colligative Properties

o Why do we put salt on the roadways when they are icy?

o Why do we put antifreeze in the radiators of our cars?

o How can some species of frogs hibernate in subzero temperatures?


o A physical property of a substance that varies depending on the number of solute particles dissolved in the solution.

1. The vapor pressure of a solution

2. The boiling point of a solution

3. The freezing point of a solution

4. The rate of diffusion of water particles (osmosis)

Aqueous Solutions

• Colligative Properties1. Vapor Pressure – The pressure exerted by

the vapor of a liquid once dynamic equilibrium has been established.


1. Vapor pressure • Dissolved solute particles will interfere with

solvent molecules trying to evaporate.• Therefore a solution will have a lower vapor

pressure than the pure solvent.


2. Boiling Point • Boiling occurs when a the vapor pressure

above a liquid becomes equal to the atmospheric pressure.

• If the vapor pressure of above a liquid is lowered, it will need to be heated to higher temperature to reach the atmospheric temperature.


2. Boiling Point• Dissolved solute particles will increase the

boiling point of pure liquid.

Aqueous Solutions

• Colligative Properties2. Boiling Point Elevation• The change in the boiling temperature (ΔTb) is equal

to the boiling point constant for water (kb) times the molality of the

solution (m) times the Van Hoff Factor (i)*.

ΔTb = kb . m . i

* i = the # of particles the Solid produces when itdissolves.

Aqueous Solutions• Colligative Properties 2. Boiling Point Elevation

Calculate the boiling point of a solution that is made by dissolving 40.0 grams of NaCl in 500.0 mL of water.

ΔTb = kb . m . i(kb = 0.512 °C/m)

Aqueous Solutions

• Colligative Properties 3. Freezing Point Depression

• Solute particles can interfere with the arrangement of water molecules when they freeze. As a result, a

colder temperature must be reached before water molecules can arrange themselves to form the solid (ice).

Aqueous Solutions


Aqueous Solutions


• The change in the freezing point of water (ΔTf) is equal equal to the freezing point constant of water(kf) times the molality of the solution (m) times the Van Hoff Factor (i).

ΔTf = kf . m . i(kf = 1.86 °C/m)

Aqueous Solutions


• Calculate the freezing point of an aqueous solution consisting of 10.0 grams of CaCl2 dissolved in 1.0 L of water.

ΔTf = kf . m . i(kf = 1.86 °C/m)

Solutions• Colligative Properties• Osmotic Pressure• Osmosis is the movement of water molecules through a

semi-permeable membrane. Osmotic pressure is a measure of the pressure that the water molecules expert on the semi-permeable membrane.

• Solute particles can cause water molecules to migrate to area where the water molecules are less concentrated.

Solutions• Colligative Properties• Osmotic Pressure

• Now you know why salt and snails don’t mix.

Solutions• Solubility of Gases and Liquids

• Solubility of Gases in Water – Generally, as the temperature of water increases, the solubility of a gas decreases.

• Fast moving water molecules are no longer in contact with dissolved gases long enough to keep it dissolved.


• Solubility of Gases in water• Consequences -


• Solubility of Gases in Water• Henry’s Law – The solubility of a gas

increases as the pressure of the gas increases.

S1 = S2P1 P2


• Solubility of Gases in Water• Henry’s Law Problem – At standard pressure,

the solubiligy of oxygen gas is 0.041 g/L. If you wanted to increase the solubility of O2 to 0. 100 g/L, to which pressure would you have to pressurize the O2?

S1 = S2P1 P2


• Solubility of Solids in Water - • Generally, the solubility of a solid increases as the temperature of the water increases.• Some solids become less soluble as the temperature of the water increases.


• Solubility of Solids in Water - • The solubility graph represents the maximum amount of solute that can dissolve in 1 gram of water as a function of the temperature.• Saturation occurs when a solution can no longer hold any more solute.


• Solubility of Solids in Water - • An unsaturated solution does not have the maximum amount of solute dissolved.• A supersaturated solution has more than the maximum allowable dissolved solute.


• Solubility of Solids in Water - • What mass of KNO3 would precipitate from a saturated solution that was cooled from 23°C to 10°C?

Chapter 16 - Solutions Many chemical reactions occur when the reactants are in the aqueous phase....

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