Solution Concentration

•Molarity(M): Moles solute/1L solution

•Molality (m): Moles solute/1kg solvent

•Mole fraction (XA): Moles A* total moles solution

•Mass percent: Mass solute x 100

total mass of solution

*In some applications, one needs the mole fraction of solvent, not solute. Make sure you find the quantity you need!

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Changing Molarity to Molality

If we know the density of the solution, we can calculate the molality from the molarity and vice versa.

Converting molality to molarity

• Determine the molarity of a 0.273m aqueous solution of KCl?

• The density is 1.011g/mL

Converting molality to molarity

• What is the molarity of a 0.273m aqueous solution of KCl? The density if 1.011g/mL

• Ans: 0.271M

Converting molarity to molality

• Determine the molality of a 0.907M solution of Pb(NO3)2.

• The solution density is 1.252 g/mL.

Converting molarity to molality

• An aqueous solution is 0.907M Pb(NO3)2. What is the molaltiy of lead (II) nitrate in this solution? The density is 1.252 g/mL.

• Ans: 0.953m

Calculating Mass Percent

• How would you prepare 425g of an aqueous solution containing 2.40% by mass of sodium acetate?

Calculating Mass Percent

• How would you prepare 425g of an aqueous solution containing 2.40% by mass of sodium acetate (NaC2H3O2)?

• Ans: 10.2 g NaC2H3O2 in 414.8g of water

Mole Fraction

• Calculate the mole fraction of NaCl in an aqueous 5% solution of NaCl.

Mole Fraction

• Calculate the mole fraction of NaCl in an aqueous 5% solution of NaCl

• Mole fraction NaCl = 0.016

Solution Concentration

• Commercial concentrated aqueous ammonia is 27% NH3 by mass and has a density of 0.90 g/mL. What is the molarity of this solution?

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Solutions

• Solutions are homogeneous mixtures of two or more pure substances.

• In a solution, the solute is dispersed uniformly throughout the solvent.

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Solutions

The intermolecular forces between solute and solvent particles must be strong enough to compete with those between solute particles and those between solvent particles.

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How Does a Solution Form?

As a solution forms, the solvent pulls solute particles apart and surrounds, or solvates, them.

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How Does a Solution Form

If an ionic salt is soluble in water, it is because the ion-dipole interactions are strong enough to overcome the lattice energy of the salt crystal.

• Solutes dissolve in solvents by a process called solvationsolvation..

• Polar solvent dissolve polar solutes, non-polar solvents dissolve non-polar solutes. (aka: “llike dissolves likeike dissolves like”.

• If two liquids mix to an appreciable extent to form a solution, they are said to be misciblemiscible.

• In contrast, immiscibleimmiscible liquids do not mix to form a solution; they exist in contact with each other as separate layers.

The Solution ProcessThe Solution Process

When a cationcation exists in solution, it is surrounded by the negativenegative dipole ends of water molecules.

When as anionanion exists in solution, it is surrounded by the positivepositive dipole ends of water molecules.

+

Solvation of IonsSolvation of Ions

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Energy Changes in Solution

The enthalpy change of the overall process depends on H for each of these steps.

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Why Do Endothermic Processes Occur?

Things do not tend to occur spontaneously (i.e., without outside intervention) unless the energy of the system is lowered.

The reason is that increasing the disorder or randomness (known as entropy) of a system tends to lower the energy of the system.

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Types of Solutions

• Saturated– In a saturated solution,

the solvent holds as much solute as is possible at that temperature.

– Dissolved solute is in dynamic equilibrium with solid solute particles.

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Types of Solutions

• Unsaturated– If a solution is

unsaturated, less solute than can dissolve in the solvent at that temperature is dissolved in the solvent.

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Types of Solutions

• Supersaturated– In supersaturated solutions, the solvent holds

more solute than is normally possible at that temperature.

– These solutions are unstable; crystallization can usually be stimulated by adding a “seed crystal” or scratching the side of the flask.

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Factors Affecting Solubility

• Chemists use the axiom “like dissolves like."– Polar substances tend to dissolve in polar solvents.– Nonpolar substances tend to dissolve in nonpolar

solvents.

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Factors Affecting Solubility

The more similar the intermolecular attractions, the more likely one substance is to be soluble in another.

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Gases in Solution

• In general, the solubility of gases in water increases with increasing mass.

• Larger molecules have stronger dispersion forces.

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Gases in Solution

• The solubility of liquids and solids does not change appreciably with pressure.

• The solubility of a gas in a liquid is directly proportional to its pressure.

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Henry’s Law

Sg = kPg

where

• Sg is the solubility of the gas,

• k is the Henry’s Law constant for that gas in that solvent, and

• Pg is the partial pressure of the gas above the liquid.

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Temperature

Generally, the solubility of solid solutes in liquid solvents increases with increasing temperature.

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Temperature

• The opposite is true of gases.– Carbonated soft

drinks are more “bubbly” if stored in the refrigerator.

– Warm lakes have less O2 dissolved in them than cool lakes.