9. Solutions

Question 1

What is hydration?

Answer 1

When a solvent is added to water.

Question 2

Are most dissolutions endothermic or exothermic?

FYI: Dissolution is the process where a solute in gaseous, liquid, or solid phase dissolves in a solvent to form a solution

Answer 2

Most are endothermic.

That is why they increase if you add heat.

Question 3

How do you define solubility?

Answer 3

It is the maximum amount of a solute that can be dissolved in a given solvent at a given temperature

It is often expressed as molar solubility- the molarity of the solute at saturation

Question 4

What is the definition of saturated?

Answer 4

The dissolved solute is in equilibrium with its undissolved state, and can dissolve no further.

If you add more solute, it will not dissolve any further.

Question 5

What are the most important solubility rules for aqueous solutions?

Answer 5

1. All salts containing ammonium (NH4+) and alkali metal (group 1) cations are water-soluble
2. All salts containing nitrate (NO3-) and acetate (CH3COO-) anions are water soluble

Question 6

What is a complex ion or coordination compound?

Answer 6

refers to a molecule in which a cation is bonded to at least one electron pair donor (which could include the water molecule)

Question 7

How does formation of complexion ion affect the solubility of ions?

Answer 7

Formation of complex ions increases the solubility of otherwise insoluble ions (the opposite of the common ion effect)

Question 8

What is chelation?

Answer 8

The central cation can be bonded to the same ligand in multiple places. This is often used to sequester toxic metals (lead, arsenic, mercury, and so on)

Question 9

Describe the process of solvation.

Answer 9

FYI: Solvation describes the interaction of a solvent with dissolved molecules.

Solvation refers to the breaking of intermolecular forces between solute particles and between solvent particles, with formation of intermolecular forces between solute and solvent particles. In aqueous solution, the water is the solvent.

Question 10

Describe the difference between solubility and saturation.

Answer 10

Solubility is the amount of solute contained in a solvent.

Saturation refers to the maximum solubility of a compound at a given temperature; one cannot dissolve any more of the solute just by adding more at this temperature

FYI: Google Definition: The solubility of a substance is the amount of that substance that is required to form a saturated solution in a given amount of solvent at a specified temperature.

Question 11

What is one way in which solubility of a compound can be increased? Gasses and solids?

Answer 11

Solubility of solids can be increased by increasing temperature.

Solubility of gases can be increased by decreasing temperature or increasing the partial pressure of the gas above the solvent (Henry’s Law)

Why increased solubility at lower temperatures for gases? A higher solubility is also observed at lower temperatures since the gas molecules dissolved in the liquid have less energy to escape into the gas phase.

Question 12

Name two ions that form salts that are always soluble.

There are actually 5 answers.

Answer 12

Group I metals, ammonium, nitrate, and acetate salts are always soluble.

Answer 13

Answer 14

Molarity (M)= moles of solute/ liters of solution

Answer 15

Molality (m)= moles of solute/kilograms of solvent

Question 16

What is the difference between molarity and molality?

Answer 16

Molarity= moles over total liters

Molality= moles over kilograms of SOLVENT only.

Question 17

If you wanted to find the concentration of a solution after diluting one, what equation would you use?

Answer 17

M_iV_i= M_fV_f

Question 18

What is the saturation point?

Answer 18

Where the solute concentration is at its maximum value for the given temperature and pressure.

Question 19

How would you define the solubility product constant?

Answer 19

The solubility product constant (K_sp ) is simply the equilibrium constant for a dissociation reaction

Example
AgCl(s)↔ Ag⁺ (aq) + Cl^- (aq)

The Law of mass action can be applied to great the equation for the solubility constant K_sp

K_sp= [Aⁿ⁺]^m[B^m-]ⁿ

K_sp= [Ag⁺][Cl^-]

Unlike the other equation, there is no denominator. This is because you have a solid (which =1)

Question 20

Why do complex ions increase the solubility of a solute?

Answer 20

This is because they contain multiple polar bonds between ligands and the central metal ion, so it can engage in a very large amount of dipole-dipole interactions

Question 21

Why does the common ion effect decrease a solubility of a compound?

Answer 21

The common ion effect decreases the solubility of a compound in a solution that already contains one of the ions in the compound. The presence of that ion in solution shifts the dissolution reaction to the left, decreasing its dissociation. (Le Chatlier’s Principle)

This makes sense. If you add NaCl to a solution that already contains a lot of Na, it’s unlikely that a lot of NaCl would dissolve.

Question 22

What will the comparison of the ion product and the K_sp tell you?

IP < K_sp

IP = K_sp

IP > K_sp

Answer 22

If IP < K_sp: The solution is unsaturated, and if more solute is added, it will dissolve

If IP = K_sp: The solution is saturated (at equilibrium), and there will be no change in concentrations

If IP > K_sp: The solution is supersaturated, and a precipitate will form

Question 23

What is a colligative property?

Answer 23

Colligative properties are physical properties of solutions that depend on the concentration of dissolved particles but not on their chemical identity.

Question 24

What is the equation for vapor pressure depression? or Raoult’s Law?

Answer 24

The presence of other solutes decreases the evaporation rate of a solvent without affecting its condensation rate, thus decreasing its vapor pressure

P_a=X_aP_a°

P_a= the vapor pressure of solvent A when the solutes are present
X_a = is the mole fraction of the solvent A in the solution (moles solvent/total moles (solvent + solute)
P_a°= the vapor pressure of solvent A in its pure state.

the presence of the solute molecules can block the evaporation of solvent molecules but not their condensation.

Question 25

What’s the equation for boiling point elevation when there’s a solute added to the solvent?

Answer 25

∆T_b= iK_bm

∆T_b= increase in boiling point

i= van’t Hoff Factor =corresponds to the number of particles into which a compound dissociates in a solution, for example, NaCl= 2, because NaCl dissolves into Na and Cl. Molecules like sugar do not dissociate so have a value of 1

K_b= proportionality constant of a particular solvent

m=molarity of solution

Question 26

What is the equation for freezing point depression?

Answer 26

∆T_f= iK_fm

∆T_f= increase in freezing point

i=van’t Hoff Factor =corresponds to the number of particles into which a compound dissociates in a solution, for example, NaCl= 2, because NaCl dissolves into Na and Cl. Molecules like sugar do not dissociate so have a value of 1

K_f= proportionality constant of a particular solvent

m=molarity of solution

Question 27

How are molality and molarity related for water? How are they related for other solvents?

Answer 27

Water: Molarity and molality are nearly equal at room temperature. This is only because 1 L solution is approximately equal to 1kg solvent for dilute solutions (the denominators of the molarity and molality equations, respectively).

Other Solvents: For other solvents, molarity and molality differ significantly because their densities are not 1 g/ml like water.