BSCI 1510L Literature and Stats Guide: Appendix A: Calculation Final Concentrations

When you report concentrations of reagents used in a scientific paper, you should do so in terms of final concentration.

The stock concentration, or the initial concentration, is the original concentration that you may dilute or mix during the course of your procedure. The final concentration can be calculated using a simple equation:

(Concentration_initial)(Volume_initial) = (Concentration_final)(Volume_final)

The units for a final concentration should be reported in units of molarity (M), weight/volume (such as mg/mL or g/L), or percent (%).

Example 1:  You added 2 mL of a 1 M stock solution of Tris-HCl and brought it to a final volume of 100 mL. What is the final concentration of Tris-HCl?

(Concentration_initial)(Volume_initial) = (Concentration_final)(Volume_final)

(1 M)(2 mL) = (C_final)(100 mL)

C_final = 0.02 M Tris-HCl

Example 2: You mix 0.375 g of KCl into 500 mL of water. What is the final concentration of KCl in terms of molarity?

First calculate concentration in g/L:

(0.375 g KCl)/ 0.5 L = 0.75 g/L

Then, search for the molecular weight of KCl: MW of KCl = 74.6 g/mol

(74.6 g/mol)/ (0.75 g/L) = 0.01005 mol/L = 0.01005 M KCl