# General Approach to solving complex equilibria 1)Write all the pertinent reactions 2)Write the charge balance 3)Write the mass balance 4)Write the equilibrium.

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General Approach to solving complex equilibria 1)Write all the pertinent reactions 2)Write the charge balance 3)Write the mass balance 4)Write the equilibrium constants for each reaction 5)Count equations and unknowns 6)Solve

Charge Balance: sum of all the positive charges must equal all negative charges Write a charge balance for a solution containing H +, OH -, Ca 2+, HCO 3 -, CO 3 2-, Ca(HCO 3 ) +, CaOH +, K +, ClO 4 -

Mass balance: sum of the amount of all species in a solution containing a particular atom must equal the amount of that atom delivered to the solution The mass balance for a solution containing 0.0250 moles of H3PO4 in 1.0 liter is:

Complex equilibria: Example 1 Determine the concentration of [H + ] and [OH - ] in pure water

Complex equilibria example 2 What would the Ca 2+ in water be in a saturated solution of CaCO 3 at pH=4? Ignore the contribution of CO 2 from the atmosphere. –Relevant equilibria: CaCO 3 ==== Ca 2+ + CO 3 2- H 2 O ==== H + + OH - CO 3 2- + H + ==== HCO 3 - HCO 3 - + H + ==== H 2 CO 3

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