3.6 Introduction to Molarity

Chemical Concept Demonstrated: Molarity of aqueous solutions


  • Combine 100 mL of 0.1M Pb(NO3)2 and 100 mL of either 0.1M KI or K2Cr2O7.
  • Propose a balanced chemical equation for this reaction, and predict how much product is produced.


    After the two solutions are mixed, a precipitate forms within the beaker.

Explanations (including important chemical equations):

    Pb(NO3)2 + 2 KI --> PbI2 + 2 KNO3

    Pb(NO3)2 + K2Cr2O7 --> PbCr2O7  + 2 KNO3

    Weighing the two solutions is not a useful approach to finding the amount of product formed.  When working with solutions, it is necessary to know the molarity of the solution rather than its weight.  Molarity, by definition, is the measure of the number of moles of solute in each liter of solution. By knowing the volumes of the solutions used, it is then possible to calculate the moles of reactants in the solution. The moles of reactants can then be used to predict the amount of product formed.