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Molarity, Molality and Normality

Molarity

The molarity (M ) is the number of moles of solute per liter of solution. For example, if 0.5 moles of solute dissolved in 100 mL of dissolution, that has a solute concentration of 0.5 M (0.5 molar). To prepare a solution of this concentration is usually first dissolved solute in a smaller volume, eg 30 mL, and transferred this solution to a volumetric flask, then fill it with more solvent until 100 mL.

is the most common method of expressing chemical concentration especially when working with chemical reactions and stoichiometry. However, it has the disadvantage that the volume changes with temperature.

Molality

The molality (m ) is the number of moles of solute per kilogram of solvent. To prepare solutions of a given molality in a solvent, not a volumetric flask is used as in the case of the molarity, but can be done in a beaker and weighing with an analytical balance, weight empty glass before so that we can subtract the corresponding value.

The main advantage of this method of action on the molality is that as the volume of a solution depends on the temperature and pressure, when they change, the volume change with them. Because the molality is not a function of volume, is independent of temperature and pressure, and can be measured more accurately.

used is less than the molarity. Normality

The normal ( N ) is the number equivalents (n ) of solute ( st) per liter of solution (sc ).

Normal acid-base

is the normality of a solution when used for a reaction as an acid or base. For this usually titrated using pH indicators.

In this case, the equivalent can be expressed as follows:

for an acid or base for.

Where:

• n: the amount of equivalents.
• moles: the amount moles. H
• ⁺ : The number of protons given by one mole of acid. HO
• ^- : The amount of hydroxyl ceded by one mole of the base.

For this, we can say the following:

for an acid or base for.

Where:

• N: is the normality of the solution.
• M: is the molarity of the solution. H
• ⁺ : The number of protons given by one mole of acid. HO
• ^- : Amount transferred hydroxyl by a mole of the base.

Examples:

• 1 M solution of HCl yields 1 M + H , therefore, is equal to 1 N.
• A solution of 1 M Ca (OH) 2 yields HO 2 M ^- , therefore, is equal to 2 N.

In this case, the equivalent can be expressed as follows:

.

Where:

• n: the amount of equivalents.
• moles: the number of moles.
• and ^- : The number of electrons exchanged in the oxidation hemireacción or reduction.

For this, we can say the following:

.

Where:

• N: is the normality of the solution.
• M: is the molarity of the solution.
• and ^- : The number of electrons exchanged in the oxidation or reduction hemireacción.

Examples:

• In the following case we see that the nitrate anion in acid (eg nitric acid, can act as an oxidant, wherein a solution 1 M, 3 N _{ox is} .

4 ^{+ H} _{+ NO 3 -1} ^{+ 3 e - ↔} _{NO + 2 H 2 O}

• In the following case we see that the iodide anion may act as a reductant, where a solution 1 M, is 1 N rd.

2 I ^- - 2 e ^- I _{2 ↔}

• In the following case we see that the cation of silver, can act as an oxidant, where a 1 M solution is 1 N ox.

^{1 Ag + 1 e + -} ↔ Ag ⁰