Solution Stoichiometry Glenn V. Lo Department of Physical Sciences Nicholls State University

Solution Stoichiometry Coefficients in balanced equation relate moles of reactants and products involved in a reaction Molarity and Volume can be used to calculate moles

Example Consider the neutralization of sulfuric acid by sodium hydroxide. H2SO4(aq) + 2 NaOH(aq)  2 H2O(l) + Na2SO4 How many moles of H2SO4 will be neutralized by 25.0 mL of 0.200M NaOH?

Titration Gradual addition of one reactant (from BURET) to another reactant (in FLASK) until the reactant in the flask is used up (EQUIVALENCE POINT) Moles or Molarity of reactant in flask is usually unknown. Unknown reactant is called the ANALYTE. Reactant in buret is called the TITRANT. Titrant is usually of well known concentration and is called the STANDARD. At the equivalence point: mole-to-mole ratio of reactants is equal to the ratio of their coefficients in balanced equation. Typical titration is designed so that a dramatic change occurs as close to equivalence point as possible, at which point titration is said to have reached the ENDPOINT.

Example NaOH(aq) of known concentration is often used as titrant to determine unknown amount of acid in flask. Phenolphthalein is added to flask; it makes the solution suddenly turn from colorless to pink at the endpoint. NaOH is easily contaminated by water vapor and CO2 upon exposure to air, so the concentration of NaOH(aq) cannot be reliably determined from the mass used to prepare the solution. “Standardization” of NaOH solution Determine its concentration by comparing with an acid that can be more reliably known: “KHP” (KHC8H4O4). “KHP” is an example of a primary standard. When used to titrate other acids, NaOH solution is called a secondary standard.

Example A solution of NaOH(aq) is standardized against 2.000 g KHP. Calculate the molarity of a NaOH(aq) solution if 15.00 mL of the solution is required to reach the endpoint.

Example A 0.4981 standard solution of NaOH(aq) is used to determine the amount of HCl in a sample. Calculate the moles of HCl in the sample if 15.00 mL of the solution is required to reach the endpoint.