Week 10. Osmosis

Question 1

Tonicity

Answer 1

Is a measure of the osmotic pressure of two solutions separated by a semi-permeable membrane

Question 2

Osmosis

Answer 2

Is a spontaneous passage or diffusion of water (or other solvent) through a semipermeable membrane (one that blocks the passage of dissolved substances).

Question 3

Osmotic pressure

Answer 3

Is the pressure that must be applied to a solution to stop movement when a semipermeable
membrane separates a solution from pure solvent.

Question 4

What kind of property is osmotic pressure and why?

Answer 4

Osmotic pressure is a colligative property.
Properties that depend on number of particles, but NOT the type of
solute particles; might depend on the type of solvent.

Colligative properties are:
◼Osmotic Pressure
◼Boiling Point
◼Freezing point
◼Vapor Pressure

Question 5

A dilute solution of glucose in water is placed in the right arm of a U-tube,
and the left arm is filled to the same height with pure water; a semipermeable
membrane separates the two arms. What will happen?

Answer 5

Osmosis occurs. The flow of pure solvent through the membrane from left to right
(from pure water => more solvent, higher concentration of water molecules
to the solution => less solvent, more solute, lower concentration of water molecules)
is greater than the flow of solvent in the reverse direction, the level of liquid in the right tube rises until an equilibrium is reached.
At equilibrium, the pressure differential is equal to the osmotic pressure of the
solution, equalizes the flow rate of solvent in both directions.

Question 6

With which formula do we calculate osmotic pressure?

Answer 6

π = i·cM·R·T [kPa]

π – osmotic pressure, kPa
i – isotonic coefficient
cM – molar concentration
R – universal gas constant; 𝑅 = 8.3145 𝐽/𝐾 ∙ 𝑚𝑜𝑙
T – absolute temperature, K; T(K) = t( ̊C) + 273.15

Question 7

What parameters do you need to calculate isotonic coefficient? Which formula do you use?

Answer 7

α – dissociation degree
N – number of particles from one molecule dissociating

i=1+α(N-1)

Question 8

State N for Al2(SO4)3 → 2Al3+ + 3SO22-.

Answer 8

N = 5

Question 9

What is the number of particles dissociating from one molecule that is a non-electrolyte? State an example.

Answer 9

Non-electrolytes do not split (do not dissociate)
=> So they do not produce any additional particles
apart from the non-electrolyte molecule itself
=> N=1

Example: Glucose C6H12O6 => X
=> 1 unsplit glucose molecule

Note that although non-electrolytes do not split, the
single, undissociated molecule is still present in the
solution causing some osmotic pressure

Question 10

What is the dissociation degree for strong, weak and non-electrolytes. Give calculation examples.

Answer 10

Strong electrolytes (NaCl)
=> α = 1; (even if not given in the task) or an exact (close to 1) value is given or calculated
=> α = 1, N = 2; i = 1 + 1(2-1) = 1+1 = 2

Weak electrolytes (AgCl)
=> α smaller than 1; An exact, (less than 1) value is always given or can
be calculated
=> α = 0.05 (given in the task, could be other values), N = 2, I = 1 + 0.05(2-1) = 1+0.05 = 1.05

Non-electrolytes (C6H12O6)
=> α = 0; (even if not given in the task) as 0% of non-electrolytes split into ions
=> α = 0 (does not split), N = 1, I = 1 + 0(1-1) = 1

Question 11

Under same conditions (same molar concentration cM, and temperature), what is the only value that matters?

Answer 11

Under same conditions (same molar concentration cM, and temperature), isotonic coefficient i is the
only value that depends on the particle and its electrolyte properties.

Question 12

Why do we need the 3 values cM, α and N for osmotic pressure calculations?

Answer 12

=> The 3 values give the total number of particles that affect osmotic pressure in a solution.

cM – shows how many substance molecules are there in a
solution (per 1 L volume)
α – shows the % of all molecules splitting (dissociating) into ions
N – shows how many ions (particles) are produced from every
molecule that splits

Question 13

What is tonicity?

Answer 13

Tonicity” is always compared to something. Two solutions can be called isotonic (having same osmotic pressure π even if their osmotic pressure is not the same that in blood or other body fluids/ cells).
=> Isotonic solution has SAME OSMOTIC PRESSURE as the solution/system to which it is added
=> Hypertonic solution has HIGHER OSMOTIC PRESSURE as the solution/system to which it is added
=> Hypotonic solution has LOWER OSMOTIC PRESSURE as the solution/system to which it is added

Question 14

What kind of tonic solution exist?

Answer 14

Isotonic solution => no net flow of water into or out of the cell; the cell’s volume remains stable
Hypertonic solution => net flow of water out of the cell; the cell will lose volume
Hypotonic solution => net flow of water into the cell; the cell will gain volume

Question 15

Why is tonicity important in a medical sense?

Answer 15

=> Red blood cells can burst or shrink, if they are placed in hypotonic or hypertonic solution.
=> maintain isotonic solution in body solutions such as blood
=> intravenous fluids to restore balance of extracellular and intracellular tonicity

Question 16

Water (solvent) movement is always…

Answer 16

=> To dilute the more concentrated solution (one that has more particles in it).
=> Two solutions do not necessarily need to have same concentration to achieve same osmotic
pressure!

Question 17

By which factor is osmotic pressure affected?

Answer 17

=> Osmotic pressure is affected by the total number of particles present in a solution.