Explain the Normality in Physical Chemistry

Normality is a rarely used expression which indicates the concentration of a solution. It is defined as the gram equivalent weight per liter of solution.

The reason normality is rarely used lies in the definition of gram equivalent weight. Gram equivalent weight is determined by the amount of an ion that reacts, which could change depending on the reaction. The fact that the gram equivalent weight is not a consistent amount creates confusion and leads scientists to use other concentration values.

Normality, when used, is typically done so in acid base reactions. Here, a more favorable calculation for normality is considered, where the normality of a solution is equal to the molarity multiplied by the number of equivalents.

Normality(N) = Molarity(M) x number of equivalents

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NORMALITY DEFINITION

Normality is a measure of concentration equal to the gram equivalent weight per liter of solution. Gram equivalent weight is the measure of the reactive capacity of a molecule. The solute's role in the reaction determines the solution's normality. Normality is also known as the equivalent concentration of a solution.

NORMALITY EQUATION

Normality (N) is the molar concentration c_i divided by an equivalence factor f_eq:

N = c_i / f_eq

Another common equation is normality (N) equal to the gram equivalent weight divided by liters of solution:

N = gram equivalent weight / liters of solution (often expressed in g/L)

or it may be the molarity multiplied by the number of equivalents:

N = molarity x equivalents

UNITS OF NORMALITY

The capital letter N is used to indicate concentration in terms of normality. It may also be expressed as eq/L (equivalent per liter) or meq/L (milliequivalent per liter of 0.001 N, typically reserved for medical reporting).

EXAMPLES OF NORMALITY

For acid reactions, a 1 M H₂SO₄ solution will have a normality (N) of 2 N because 2 moles of H⁺ ions are present per liter of solution.

For sulfide precipitation reactions, where the SO₄^- ion is the important part, the same 1 M H₂SO₄ solution will have a normality of 1 N.

EXAMPLE PROBLEM

Find the normality of 0.1 M H₂SO₄ (sulfuric acid) for the reaction:

H₂SO₄ + 2 NaOH → Na₂SO₄ + 2 H₂O

According to the equation, 2 moles of H⁺ ions (2 equivalents) from sulfuric acid react with sodium hydroxide (NaOH) to form sodium sulfate (Na₂SO₄) and water. Using the equation:

N = molarity x equivalents
N = 0.1 x 2
N = 0.2 N

Don't be confused by the number of moles of sodium hydroxide and water in the equation.

Since you've been given the molarity of the acid, you don't need the additional information. All you need to figure out are how many moles of hydrogen ions are participating in the reaction. Since sulfuric acid is a strong acid, you know it completely dissociates into its ions.

POTENTIAL ISSUES USING N FOR CONCENTRATION

Although normality is a useful unit of concentration, it can't be used for all situations because its value depends on an equivalence factor that can change based on the type of chemical reaction of interest. As an example, a solution of magnesium chloride (MgCl₂) may be 1 N for the Mg²⁺ ion, yet 2 N for the Cl^- ion. While N is a good unit to know, it's not used as much as molarity or molality in actual labwork. It has value for acid-base titrations, precipitation reactions, and redox reactions. In acid-base reactions and precipitation reactions, 1/f_eq is an integer value. in redox reactions, 1/f_eqmay be a fraction.

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