flinnsci.com SAFETY REFERENCE LABORATORY SOLUTION PREPARATION
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Laboratory Solution Preparation
Many of the reagents used in science are in the form of solutions that need to
be purchased or prepared. For many purposes, the exact value of concentration
is not critical; in other cases, the concentration of the solution and its method
of preparation must be as accurate as possible. The Flinn Laboratory Solution
Preparation reference section is designed for both the novice and experienced
solution maker. It provides valuable information on the basic concepts of preparing
solutions and instructions for preparing most solutions required in the high school
science laboratory. Professional-quality solutions are possible when high-quality
and fresh chemicals and solvents are used and meticulous procedures are
followed. Many of the solutions described in this section are available ready-made
from Flinn Scientific to save valuable laboratory preparation time.
Basic Concepts of Preparing Solutions
Molarity
The most common unit of solution concentration is molarity (M). The molarity
of a solution is defined as the number of moles of solute per one liter of solution.
Note that the unit of volume for molarity is liters, not milliliters or some other
unit. Also note that one liter of solution contains both the solute and the solvent.
Molarity, therefore, is a ratio between moles of solute and liters of solution. To
prepare laboratory solutions, usually a given volume and molarity are required.
To determine molarity, the formula weight or molar mass of the solute is needed.
The following examples illustrate the calculations for preparing solutions.
If starting with a solid, use the following procedure:
1. Determine the mass in grams of one mole of solute, the molar mass, MMs.
2. Decide volume of solution required, in liters, V.
3. Decide molarity of solution required, M.
4. Calculate grams of solute (gs) required using Equation 1.
Eq. 1. gs = MMs x M x V
Example: Prepare 800 mL of 2 M sodium chloride.
(MMNaCl = 58.45 g/mol)
gNaCl = 58.45 g/mol x 2 mol/L x 0.8 L
gNaCl = 93.52 g NaCl
5. Dissolve 93.52 g of NaCl in about 400 mL of distilled water, then add more
water until final volume is 800 mL.
If starting with a solution or liquid reagent:
1. When diluting more concentrated solutions, decide what volume (V2) and
molarity (M2) the final solution should be. Volume can be expressed in liters
or milliliters.
2. Determine molarity (M1) of starting, more concentrated solution.
The section is divided into several parts for your convenience.
Basic Concepts of Preparing Solutions
Preparation of Simple Inorganic Salt Solutions
Preparations of Acid and Base Solutions
Recipes for Biological, Histological and Chemical Solutions
3. Calculate volume of starting solution (V1) required using Equation 2.
Note: V1 must be in the same units as V2.
Eq. 2. M1V1 = M2V2
Example: Prepare 100 mL of 1.0 M hydrochloric acid from concentrated
(12.1 M) hydrochloric acid.
M1V1 = M2V2
(12.1 M)(V1) = (1.0 M)(100 mL)
V1 = 8.26 mL conc. HCl
4. Add 8.26 mL of concentrated HCl to about 50 mL of distilled water, stir, then
add water up to 100 mL.
Percent Solutions
Mass percent solutions are defined based on the grams of solute per 100
grams of solution.
Example: 20 g of sodium chloride in 100 g of solution is a 20% by mass
solution.
Volume percent solutions are defined as milliliters of solute per 100 mL of
solution.
Example: 10 mL of ethyl alcohol plus 90 mL of H2O (making approx. 100 mL of
solution) is a 10% by volume solution.
Mass-volume percent solutions are also very common. These solutions are
indicated by w/v% and are defined as the grams of solute per 100 milliliters
of solution.
Example: 1 g of phenolphthalein in 100 mL of 95% ethyl alcohol is a
1 w/v% solution.
BASIC CONCEPTS OF PREPARING SOLUTIONS continued on next page.
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