Solutions with small concentrations are often prepared by diluting a more concentrated stock solution. A known volume of the stock solution is transferred to a new container and brought to a new volume. Since the total amount of solute is the same before and after dilution, we know that
where Co is the concentration of the stock solution, Vo is the volume of the stock solution being diluted, Cd is the concentration of the dilute solution, and Vd is the volume of the dilute solution. Again, the type of glassware used to measure Vo and Vd depends on how exact the solution’s concentration must be known.
For example, a laboratory procedure calls for 250 mL of an approximately 0.10 mol/L solution of NH3, and we would prepare this solution using a stock solution of concentrated NH3 (14.8 mol/L). Substituting known volumes in equation
14.8 mol/L* Vo =0.10 mol/L*0.25 L
and solving for Vo gives 1.69*10-3 L, or 1.7 mL. Since we are trying to make a solution that is approximately 0.10 mol/L NH3, we can measure the appropriate amount of concentrated NH3 using a graduated cylinder, transfer the NH3 to a beaker, and add sufficient water to bring the total solution volume to approximately 250 mL.