Analytical Chem istry - DePauw University

32 Analytical Chemistry 2.02E.2 Preparing Solutions by DilutionSolutions are often prepared by diluting a more concentrated stock solution.A known volume of the stock solution is transferred to a new containerPractice Exercise 2.6To prepare a standard solution of Zn 2+ you dissolve a 1.004 g sample ofZn wire in a minimal amount of HCl and dilute to volume in a 500-mLvolumetric flask. If you dilute 2.000 mL of this stock solution to 250.0mL, what is the concentration of Zn 2+ , in μg/mL, in your standard solution?Click here to review your answer to this exercise.Equation 2.2 applies only when the concentrationare written in terms of volume,as is the case with molarity. Using thisequation with a mass-based concentrationunit, such as % w/w, leads to an error. SeeRodríquez-López, M.; Carrasquillo, A. J.Chem. Educ. 2005, 82, 1327-1328 forfurther discussion.and brought to a new volume. Since the total amount of solute is the samebefore and after dilution, we know thatC × V = C × Vo o d d2.2where C o is the stock solution’s concentration, V o is the volume of stocksolution being diluted, C d is the dilute solution’s concentration, and V dis the volume of the dilute solution. Again, the type of glassware used tomeasure V o and V d depends on how exact the solution’s concentration mustbe known.Example 2.9A laboratory procedure calls for 250 mL of an approximately 0.10 M solutionof NH 3 . Describe how you would prepare this solution using a stocksolution of concentrated NH 3 (14.8 M).So l u t i o nSubstituting known volumes in equation 2.214. 8 M× V = 010 . M×025 . Loand solving for V o gives 1.69 × 10 -3 liters, or 1.7 mL. Since we are makinga solution that is approximately 0.10 M NH 3 we can use a graduatedcylinder to measure the 1.7 mL of concentrated NH 3 , transfer the NH 3 toa beaker, and add sufficient water to give a total volume of approximately250 mL.As shown in the following example, we can use equation 2.2 to calculatea solution’s original concentration using its known concentration afterdilution.