• Solubility?is defined as the?number of grams?or?moles?of compound needed to?saturate?100 g of?water, or it can also be defined in terms of 1 kg of water, at a given temperature
  • For example, sodium chloride (NaCl) is considered to be a?soluble?salt as a saturated solution contains 36 g of NaCl per 100 g of water
  • Lead chloride (PbCl₂) on the other hand is an?insoluble?salt as a saturated solution only contains 0.99 g of PbCl₂?per 100 g of water

Solubility product

• The?solubility product?(K_sp) is:
  • The product of the concentrations of each ion in a saturated solution of a relatively soluble salt
  • At 298 K
  • Raised to the power of their relative concentrations

C (s) ? aA^x+?(aq) + bB^y-?(aq)

K_sp?= [A^x+?(aq)]^a?[B^y-?(aq)]^b

• When an?undissolved ionic compound?is in contact with a?saturated solution of its ions, an equilibrium is established
• The ions move from the solid to the saturated solution at the same rate as they move from the solution to the solid
  • For example, the undissolved magnesium chloride (MgCl₂) is in equilibrium with a saturated solution of its ions

MgCl₂?(s) ? Mg²⁺?(aq) + 2Cl^- (aq)

When the undissolved MgCl₂?salt gets in contact with its ions in a saturated solution, an equilibrium between the salt and ions is established

• • The?solubility product?for this equilibrium is:

K_sp?=?[Mg²⁺?(aq)] [Cl^-?(aq)]²

• The?K_sp?is only useful for?sparingly soluble salts
• The smaller the value of?K_sp, the lower the solubility of the salt

• The general equilibrium expression for the?solubility product?(K_sp) is:

C (s) ? aA^x+?(aq) + bB^y-?(aq)

K_sp= [A^x+?(aq)]^a?[B^y-?(aq)]^b

Worked Example: Expressing?K_sp?of ionic compounds

Answer

Expressing?K_sp?of ionic compounds answers table