3.7: Equilibrium Continued and Introduction to Acid-Base Concepts (Worksheet) Knowing how to set up and solve equilibrium problems for gas-phase systems is essential preparation for applying equilibrium concepts to more complicated systems, such as acid-base chemistry. The mixture of reactants and products can often be altered by applying a stress to the system (changing species concentrations, changing pressures, changing temperature, etc.), and the shift in the position of the equilibrium can be understood and predicted on the basis of LeChatelier’s Principle. 3.8: Weak Acid and Base Equilibria (Worksheet) We have seen that the calculation of [H3O+] and pH for solutions of strong acids and base. To carry out a calculation of all species present in a solution of a pure weak acid in water requires use of the equilibrium constant for the acid’s hydrolysis, called Ka. The strengths of acids and their conjugate bases are related to their molecular structures. Knowing the trends allows us to predict whether an acid is strong or weak, and if weak how it compares in strength to other similar weak acids. 3.9: Common Ion Effect and Buffers (Worksheet) Last week we looked at how to calculate the concentrations of all species and pH or pOH in a solution of a pure acid or base in water, with no additional amounts of the conjugate added. Now, we need to look at the effect of adding extra amounts of the conjugate base or acid to the solution. The shift in the position of the equilibrium, called the common ion effect, changes the pH and imbues the solution with certain properties that are the basis for formulating a buffer. 3.10: Titration (Worksheet) Titration is the addition of a standard solution of precisely known concentration (the titrant) to a precisely measured volume of a solution with unknown concentration (the analyte) to react according to a known stoichiometry. It is an important technique in analytical chemistry. In an acid-base titration, the reaction is neutralization, and the acid or base can be either the titrant or the analyte. Titrations offer an opportunity to review all the kinds of calculations we have seen.