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How to Calculate an Equivalent Point

Chemists use a technique called titration to determine an unknown quantity of an acid or base, called an analyte, dissolved in solution. Titration requires monitoring the pH level of the solution while adding controlled volumes of a known solution to the unknown. When the chemical reaction reaches equilibrium, the pH level changes rapidly, marking the point at which the amount of reactants in the known and unknown solutions are equal. Chemists can calculate the equivalence point using a method known as stoichiometry, which represents chemical reactions mathematically.

Instructions

- 1
  Determine the stoichiometric equation for the reaction. Look for the ratio at which the reactants combine to form the products. For example, strong bases and acids react at a one-to-one ratio such as "HCl + NaOH <-> NaCl + H2O."
- 2
  Calculate the amount of analyte. Multiply the concentration of the solution (usually in molars or moles per liter) by the volume of the solution. For example, 1 liter of 0.2 molar HCl contains 0.2 moles of HCl.
- 3
  Calculate the amount of reactant required. Multiply the amount of analyte by the stoichiometric ratio (1:1 in the example) to determine the amount required to use all of the analyte.
- 4
  Determine the volume of reactant solution required to reach the equivalence point. Divide the amount of reactant required by the concentration of reactant. For example, 0.2 moles of NaOH divided by 0.1 molar equals 2 liters of NaOH solution.
- 5
  Record the result as the volume of titrant necessary to reach the equivalence point. Record the amount of reactant necessary to react all of the analyte as the equivalence point.

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