Non-Ideal Solutions add-in

Question 1

Define:

osmotic pressure of a solution

Answer 1

Osmotic pressure is the physical pressure necessary to prevent solvent from flowing across a permeable membrane from a hypertonic to a hypotonic solution.

The larger the concentration difference, the higher the osmotic pressure will be.

Question 2

Give the equation for:

osmotic pressure of a solution

Answer 2

Π = iMRT
where:

Π = the solution’s osmotic pressure in atm
i = van’t Hoff factor (number of expected ions)
M = the solution’s molarity in mol/l
R = the ideal gas constant in L*atm / K*mol
T = the absolute temperature of the solution in K

Question 3

At a constant temperature, how will the osmotic pressure of a 1M NaCl solution compare to that of a 1M CaCl₂ solution?

Answer 3

The osmotic pressure of the CaCl₂ solution will be higher.

This is due to CaCl₂ having a van’t Hoff factor of 3 (Ca⁺², Cl^-, Cl^-) compared to 2 for NaCl (Na⁺, Cl^-).

Since Π = iMRT, and M,R,T are all constant, the determining factor is i=3 > i=2.

Question 4

How will increasing a solution’s temperature change its osmotic pressure?

Answer 4

The osmotic pressure will be greater at a higher temperature.

Since Π = iMRT, and i,M,R are all constant, the determining factor is T_new > T_old

Question 5

Define:

Non-ideal solutions

Answer 5

A non-ideal solution is one which does not follow Raoult’s Law. Non-ideal solutions can have pressures either greater or less than those predicted by Raoult’s Law.

Ex: A solution with a large attraction between solute and solvent, such as salt water, will have a lower pressure than that predicted by Raoult’s Law.

Question 6

What is the total vapor pressure above a mixture of liquids A and B?

Answer 6

P_tot = P_A + P_B
= (X_A* P⁰_A )+ (X_B * P⁰_B)

where

X_A = mole fraction of liquid A
X_B = mole fraction of liquid B
P⁰_A = vapor pressure of pure A
P⁰_B= vapor pressure of pure B

Question 7

How soluble are the salts of alkali metal cations such as Li⁺, Na⁺, K⁺, etc.?

Answer 7

All salts of alkali metals are highly soluble.

Question 8

How soluble are the salts of the ammonium cation such as NH₄⁺?

Answer 8

All ammonium salts are highly soluble.

Question 9

What determines solubilities of the salts of alkaline earth and transition metal cations such as Ca⁺², Mg⁺², Fe⁺³, etc.?

Answer 9

Salts of alkaline earth metals and transition metals have solubilities that vary based on the anion that the cation is paired with in the salt.

Highly soluble anions like chloride (Cl^-) will make soluble salts with alkaline earth and transition metals. Insoluble anions like phosphate (PO₄^-3) will make insoluble salts with these cations.

Question 10

Which anions make salts which are always fully soluble?

Answer 10

The following anions make salts which are always soluble:

• Nitrate (NO₃^-)
• Chlorate (ClO₃^-)
• Perchlorate (ClO₄^-)
• Acetate (CH₃COO^-)

Question 11

What is the solubility of salts made with halogen anions Cl^-, Br^-, I^-?

Answer 11

Halogen anion salts are soluble UNLESS the cation is Ag⁺, Pb²⁺, or Hg₂²⁺.

Question 12

What is the solubility of salts made with the sulfate anion, SO₄^2-?

Answer 12

Sulfate anion salts are soluble UNLESS the cation is Ag⁺, Pb²⁺, Hg₂²⁺, Ca²⁺, Sr²⁺, or Ba²⁺.

Question 13

Which anions make salts which are generally insoluble?

Answer 13

The following anions make salts which are generally insoluble:

• Hydroxide (OH^-)
• Carbonate (CO₃^-2)
• Phosphate (PO₄^-3)
• Sulfite (SO₃^-2)
• Chromate (CrO₄^2-)
• Sulfide (S^2-)

Exception: salts with these anions paired with alkali metal (such as NaOH) or ammonium cations (such as (NH₄)₂CO₃) will be soluble.