# Complex Equilibria General Approach to solving complex equilibria Complex Equilibria General Approach to solving complex equilibria 1) 2) 3) 4) 5) 6)

Write all the pertinent reactions Write the charge balance Write the mass balance Write the equilibrium constants for each reaction Count equations and unknowns Solve Charge Balance: sum of all the positive charges must equal all negative charges

Write a charge balance for a solution containing H+, OH-, Ca2+, HCO3-, CO32-, Ca(HCO3)+, CaOH+, K+, ClO4- 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 Ca2+ in water be in a saturated solution of CaCO3 at pH=4? Ignore the contribution of CO2 from the atmosphere. Relevant equilibria:

CaCO3 ==== Ca2+ + CO32H2O ==== H+ + OHCO32- + H+ ==== HCO3HCO3- + H+ ==== H2CO3 Titrations Standard reagent is added to a solution containing analyte until the reaction between the analyte and the reagent is judged to be complete Types of Titrations

Volumetric: standard reagent is measured volumetrically Gravimetric: mass

of standard reagent is measured Coloumetric: Quantity of charge in coloumbs to complete a

Endpoint detection: example of NaOH + HCl Phenophalein Indicator error Methods of endpoint detection

1. 2. 3. 4. 5.

Color change Voltmeters including pH meters Refractometers turbidometers even thermometers Goal in a titration is to determine the amount of analyte Primary Standards

Is a highly purified compound that serves as a reference material in all volumetric and titrimetric methods. It must be: 1) Highly pure, established methods for

confirming purity should be available. 2) Stable toward air 3) Absence of hydrates of water so that the composition of the solid does not change with variations in relative humidity Primary standards continued

4) Readily available at reasonable cost. 5) Reasonably soluble in titration medium. 6) Reasonably large molar mass so that the relative error associated with

weighing the standard is minimized. Examples? KHP, AgNO3 Secondary standards

Hard to find primary standards therefore secondary standards are often used. Secondary standard is a compound whose purity has been established by chemical analysis and serves as a reference material for titrimetric methods of analysis. Examples? NaOH, EDTA

Titrations Advantages: Disadvantages: Redox titration example How many grams of oxalic acid should be dissolved in 500.0mL of water to make a 0.100N solution for titration with MnO4-?

Redox Review LEO the Lion says GER Redox titration example How many grams of oxalic acid should be dissolved in 500.0mL of water to make a 0.100N solution for titration with MnO4-? Coloumetric Titration

Cyclohexene is titrated with Br2 that has been electrolytically oxidized from Br-. If it requires 595.1 sec to complete the titration with a current of 4.825mA, how much cyclohexene was there in the sample?