Chemists calculate the normality of a solution by dividing the gram equivalent weight of the solute by the volume of the solution in liters, explains About.com. They calculate the equivalent weight by dividing the molecular weight by the number of positive or negative charges the substance provides through dissolution.
About.com indicates that normality is the only concentration dependent on the reaction. For acids that donate one mole of hydrogen atoms per mole of the compound, such as hydrochloric acid, the molarity and the normality of the acid remain the same. With other compounds, such as sulfuric acid, the acid provides two moles of hydrogen atoms per mole of the compound dissolved, so the hydrogen normality is twice the molarity. However, if the chemist is studying sulfate precipitation, sulfuric acid is only 1 normal for sulfate ions. It only provides one mole of SO4- ions per mole of H2SO4 dissolved in solution.
The medical field uses the term normal saline to describe a solution of sodium chloride in water. However, this solution is 9 percent by weight of sodium chloride, which is approximately 0.154 M NaCl. Since sodium chloride is an ionic solution, it dissociates completely into Na+ ions and Cl- ions, providing one mole of sodium per mole of sodium chloride. From the chemist's viewpoint, this solution is 0.154 N sodium chloride for either the sodium or chloride ion.