Objectives

Your performance will be satisfactory when you are able to:

• Correctly prepare a solution of a given molarity leaving a CLEAN lab area

• Do parallel and serial dilutions and distinguish between the two

• Determine whether to use a parallel or serial dilution in a given situation

• Use a microcentrifuge to pellet a precipitate

Introduction

A common task for any biotechnician is solution preparation. What is a solution? It is defined as a solute (smaller amount) dissolved in a solvent (larger amount). The concentration of a solution frequently must be known to a high degree of accuracy. An incorrectly prepared solution can destroy months of hard work or cost companies thousands of dollars. Therefore, companies usually have an SOP (Standard Operating Procedure) for the preparation of each solution to minimize mistakes. All calculations are recorded in the lab notebook, even if a calculator is used. Important calculations are double-checked by another person (and sometimes triple-checked). The exact mass and volume of reagents used is recorded in the notebook. This information, along with the date and the preparer’s name or initials, is recorded on a preparation form and on a label on the bottle itself; these forms are provided in Appendix C.

Units of Concentration

Percent concentrations may be expressed as:

1. weight per volume (wt/vol or w/v), which indicates the weight (in grams) of solute per 100 mL of solution (used to indicate the concentration of a solid in a liquid)

2. volume per volume (v/v), which indicates the volume (in mL) of solute per 100 mL of solution (used to indicate the concentration of a liquid dissolved in liquid)

3. weight per weight (w/w), which indicates the weight (in grams) of solute per 100 g solution (used to indicate the concentration of a solid mixed in another solid)

Note that in all cases a 100 mL (or 100 g) solution is used since percent means “out of 100”.

Weight per volume is a common unit of concentration in the biotechnology lab. This is often used for small amounts of chemicals and specialized biological reagents. For example, enzyme and nucleic acid concentrations are often given as weight per volume (for example, 1 µg/mL DNA).

Molarity is the most common unit of concentration in the biotechnology lab. The molarity of a solution is defined as the number of moles of solute per liter of solution. The symbol for molarity is M, but it can also be written as moles/Liter, or mol/L. A mole of any element always contains 6.02 X 10²³ (Avogadro’s number) atoms. Because some atoms are heavier than others, a mole of one element weighs a different amount than a mole of another element. The weight of a mole of a given element is equal to its atomic weight in grams. Consult a periodic table of elements to find the atomic weight of an element. For example, one mole of the element carbon weighs 12.0 g.

Example:

Using a periodic table, calculate the molar mass of chromium oxide (CrO₂).

The atomic weight of chromium is 52.00, and that of oxygen is 16.00. You must count the oxygen twice because there are two per formula unit of chromium oxide.

52.00 + 2(16.00) = 84.00 g/mol

Practice:

Using a periodic table, calculate the molar mass of potassium sulfate (K₂SO₄).

PART A: MAKING MOLAR SOLUTIONS

We can’t directly measure moles, but we can measure mass. To calculate the mass of a chemical needed to prepare a given volume of a solution of desired molarity, you must convert number of moles to mass, using the chemical’s molar mass as a conversion factor.

Mass = molarity x volume x molar mass

? g = moles/liter x L x g/mole

Don’t forget to convert mL to L, if necessary.

Example: To prepare 100 mL of 1 M NaOH (FW 40.0),

g = 40 g/mol x 1.0 mol/L x 0.1 L

g = 4 (dissolve 4 g of NaOH in 100 mL water)