I1i: Definitions; fluids & electrolytes
Terms
Mole
Defintion
SI unit for amount of a substance
The amount of substance that contains the no. of molecules equal to Avogadro’s no.
Terms
Avogadros No.
Definition
The number of molecules in 1 mole of a substance (6 x 1023)
Terms
Molality
Definition
No. of moles of solute per kg of solvent
Terms
Molarity
Definition
No. of moles of solute per L of solution
Terms
Osmole
Definition
No. of moles that a solute contributes to the osmotic pressure of a solution
Terms
Osmotic P
Definition
= What drives osmosis
= The hydrostatic P required to oppose the movement of H2O through a semi-permeable membrane
= One of the colligative properties of a solution
= y x C/M x RT
- y = no. of particles
- c = [ ] (g/L)
- M = MW of molecules
- R = universal gas constant
- T = absolute temp in Kelvin
Example
- 1L water contains 1mol of NaCl
- 1 mol NaCl = 2 osmoles of NaCl because NaCl dissociates completely
- ∴1 mol Na + 1 mol Cl = 2 osmoles in 1L of solution = 2 osmoles/L
OSMOLALITY = no. of osmoles in weight (kg) of solvent
OSMOLARITY = no. of osmoles in volume (L) of solvent
- You measure osmolality (kg in solvent) using OSMOMETER
- You then use measured osmolality to calculate OSMOLARITY
- Plasma Osmolarity = 280 – 300 mOsm/L
→ We tend to use milli-osmoles (mOsm) because such small no. we are measuring
OSMOLAR GAP = measured osmolality – calculated osmolarity
- ≤ 10 difference because down [ ] particles are not measured e. (Ca, Mg & their anions)
TONICITY = Effective osmolality = the sum of the concentration of solutes which have the capacity to exert osmotic force across membrane