Preparation of Solutions, Part 1


			Society for Amateur Scientists Preparation of Solutions, Part 1

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Preparation of Solutions, Part 1

Source: General Chemistry Lab, St. Mary's College of Maryland

Introduction

Many types of solutions, such as stock reagents, primary and secondary standards, indicators, acids, bases, and buffers are prepared for laboratory use. These solutions require specific preparations, and to understand these preparations, you must be familiar with the many ways by which chemists express concentrations.

Some common expressions of concentration are molarity, normality, parts per thousand, parts per million, parts per billion, weight percentage (or mass percentage), and weight-volume percentage. These various expressions have different uses and functions and the one that you employ is usually a matter of convenience or convention. Table I summarizes this information.

Table I: Concentration Expressions

Concentration
Expression Symbol Definition Units Use

Molarity M moles solute
liter solution mol/L general

Normality N equivalents solute
liter solution eq/L volumetric
methods

Parts per
thousand ppt grams of solute
kilogram solution g/kg quantitative
analysis

Parts per
million ppm milligrams solute
kilogram solution mg/kg quantitative
analysis

Parts per
billion ppb micrograms solute
kilogram solution Fg/kg toxicity
analysis

Weight
percentage W/W grams solute x 100
gram solution -- commercial
solutions

Weight-Volume
percentage W/V grams solute x 100
milliliter solution -- commercial
solutions

Molarity

Molarity is commonly used in describing the concentrations of general reagent solutions, such as 6-Molar HCl or 3-Molar NaOH. Molarity is defined as the number of moles of solute per liter of solution. Notice it means per liter of solution, not per liter of solvent. Therefore, the solute is dissolved in enough solvent to make a specified volume of resulting solution. This distinction decides the proper procedure for preparing such solutions.

Normality

Normality is used less frequently now than in the past, but its usage is still quite prevalent in chemical literature. It is presently most often used to express the concentrations of solutions involved in titrations. Normality is defined as the number of equivalents of solute per liter of solution. In acid-base reactions, one equivalentof an acid is that amount of an acid that will furnish one mole of hydrogen ions or that will react with one mole of hydroxide ions. Similarly, one equivalent of a base is that amount of a base that will furnish one mole of hydroxide ions or react with one mole of hydrogen ions. The definition of an equivalent is different in other applications, such as in oxidation-reduction reactions. These applications are not discussed here.

For acids and bases, normality is generally greater than or equal to the molarity of a solution.

For example:

H₂SO₄ (sulfuric acid) contains two equivalents of hydrogen ion per mole; therefore, as shown in Table II (below), an eighteen-molar solution of sulfuric acid is a thirty-six normal solution.

Concentration terms for small amounts

Concentration terms such as parts per thousand, parts per million, and parts per billion are generally used to express the concentrations of constituents that are present very small amounts, such as pollutants in environmental samples. Because lethal concentrations of toxic substances found in environmental samples are usually very small, these expressions are convenient to use.

For example, a certain water sample may contain 20 parts per billion mercury (20 ppb Hg).

Concentrations expressed as a percentage

Weight percentage (w/w) and weight-volume percentage (w/v) are concentrations expressed as a percentage (parts per hundred). Weight percentage is defined as the weight of solute multiplied by one hundred divided by the weight of the solution. Weight-volume percentage is the weight of the solute in grams multiplied by one hundred divided by the volume of solution in milliliters. Often, these expressions are called mass percentage and mass-volume percentage, respectively. Commercial aqueous reagents such as acid and bases are often labeled in concentrations of weight percentage. Table II lists the weight percentages for common commercial acids and bases. Weight-volume percentage is often used for solutions made from solid reagents.

For example, a 20% sodium chloride solution contains 20 g of NaCl per 100 mL of solution.

Table II: Technical Data on Commercial Acids and Bases

Solution	Formula Weight	Molarity	Normality	Weight Percent	Specific Gravity
Acetic acid [CH₃COOH]	60.05	17.4	17.4	99.8	1.05
Ammonia [NH₄OH]	35.05	14.8	14.8	57	0.90
Hydrochloric acid [HCl]	36.46	12.1	12.1	37	1.19
Nitric acid [HNO₃]	63.01	15.8	15.8	70	1.42
Sulfuric acid [H₂SO₄]	98.08	18.0	36.0	96	1.84
Phosphoric acid [H₃PO₄]	97.1	14.8	44.6	85	1.70

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