Concentration of reactants in solution: Molarity
For a chemical reaction to occur, the reacting molecules or ions must come into contact. This means that the reactants must have considerable mobility, which in turn means that most chemical reactions are carried out in the liquid state or in solution rather than in the solid state. It’s therefore necessary to have a standard means for describing exact quantities of reactants in solution.
As we’ve seen, stoichiometry calculations for chemical reactions always require working in moles. Thus, the most generally useful means of expressing a solution’s concentration is molarity (M), the number of moles of a substance (the solute) dissolved in each liter of solution. For example, a solution made by dissolving 1.00 mol (58.5 g) of NaCl in enough water to give 1.00 L of solution has a concentration of 1.00 mol/L, or 1.00 M. The molarity of any solution is found by dividing the number of moles of solute by the number of liters of solution.
Molarity (M) = (Moles of solute)/(Volume of Solution)
Note that it is the final volume of the solution that is important, not the starting volume of the solvent used. The final volume of the solution might be a bit larger than the volume of the solvent because of the additional volume of the solute. In practice, a solution of known molarity is prepared by weighing an appropriate amount of solute and placing it in a volumetric flask. Enough solvent is added to dissolve the solute, and further solvent is added until an accurately calibrated final volume is reached. The solution is then shaken until it is uniformly mixed.
Molarity can be used as a conversion factor to relate a solution’s volume to the number of moles of solute. If we know the molarity and volume of a solution, we can calculate the number of moles of solute. If we know the number of moles of solute and the molarity of the solution, we can find the solution’s volume.
Molarity = (Moles of solute)/(Volume of Solution)
Moles of solute = (Molarity)x(Volume of solution)
Volume of solution = (Moles of solute)/(Molarity)
volumetric flask: bình định mức