2 solubility

what is molar solubility

amount of substance (mol) that can be dissolved in a given amount of solvent dm³

what is Ksp

solubility product

equilibrium constant for dissolution of sparingly soluble salts

dissociation equilibrium for MX_n (s)

Ksp equation (in terms of concentration) for MX_n (s)

• what does it represent (ratio)

• assumptions?

ratio of soluble ions to insoluble salt (activity of solid = 1)

assume standard conditions and solvent = water with activity 1

equation for Ksp in terms of activity

equation to find activity?

van’t Hoff equation + derivation in terms of Ksp

van’t Hoff plot

• axes

• shape

• gradient

• intercept

what is electroneutrality

sum of all negative charges in solution = sum of all positive charges in solution

electroneutrality equation for C^z+ and A^z-

what is charge concentration (for ions)

concentration x charge number

how to write charge balance equation

write sum of cation concentrations on the left, equal to sum of anion concentrations on the right - multiply each concentration by its charge number (not sign)

include H₂O, H⁺ and OH^-

what is Formal concentration

the total concentration of a species, in all forms

Formal concentration calculation and units for M

Formal concentation of M is the sum of [MX₂] and [MX^-] at equilibrium in mol dm^-3

how to write mass balance equation

• write formal concentration expressions for the different species

• write mass balance equation in terms of formal concentrations using dissolution equation

• substitute in expressions for the formal concentrations

how to make an ICE table

if the solubility is x then the concentration of a species is its stoichiometry multiplied by ±x

what is the common ion effect

a salt will be less soluble if ones of its ions is already in solution

Q & K for common ion effect and why

Q_sp > K_sp

K_sp is constant at standard conditions so if concentration of ione ionic species is increased the concentration of the other must decrease to oppose the change

how to use ICE table for common ion effect

set initial concentration of the ion to its conc in solution. still add x (or x x stoichiometry)

how to solve common ion effect problems when ICE table gives a cubic equation for x

Le Chatelier’s principle means increasing the concentration of the ion will decrease the solubility of the solid, so assume that the initial concentration of the ion is very close to the equilibrium and that x is small - K_sp << 1

hence x << initial concentration of ion in solution - equation can now be simplified to find x