Box 21 How heavy is a mole

When you work in a laboratory, something you'll need to come to grips with sooner or later is the matter of quantifying the amounts and concentrations of substances used. Central to this is the mole, so before we go any further, let'sdefine this: A mole is the molecular mass of a compound expressed in grams. (The molecular mass is simply the sum of the atomic mass of all the atoms in a compound.)

So, to take sodium chloride as an example:

Molecular formula

Atomic mass of sodium Atomic mass of chlorine A Molecular mass

NaCl (one atom each of sodium and chlorine) 22.99 35.45 58.44

Thus one mole of sodium chloride equals 58.44 grams (58.44 g)

Concentrations are expressed in terms of mass per volume, so here we introduce the idea of the molar solution. This is a solution containing one mole dissolved in a final volume of 1 litre of an appropriate solvent (usually water).

Molar solution = one mole per litre

A one molar (1 m) solution of sodium chloride therefore contains 58.44 g dissolved in water and made up to 1 litre. A 2 m solution would contain 116.88 g in a litre, and so on.

In biological systems, a molar solution of anything is actually rather concentrated, so we tend to deal in solutions which are so many millimolar (mm, one thousandth of a mole per litre) or micromolar (^m, one millionth of a mole per litre).

Why bother with moles?

So far, so good, but why can'twe just deal in grams, or grams per litre? Consider the following example. You've been let loose in the laboratory, and been asked to compare the effects of supplementing the growth medium of a bacterial culture with several different amino acids. 'Easy',you think. 'Add X milligrams of each to the normal growth medium, and see which stimulates growth the most'.The problem is that although you may be adding the same weight of each amino acid, you're not adding the same number of molecules, because each has a different molecular mass. If you add the same number of moles (or millimoles or micromoles) of each instead, you would be comparing the effect of the same number of molecules of each, and thus obtain a much more meaningful comparison. This is because 1 mole of one compound contains the same number of molecules as a mole of any other compound. This number is called Avogadro'sNumber, and is 6.023 x 1023 molecules per mole.

Hydrogen atom

Carbon atom (b)

Hydrogen atom f) (H

Hydrogen molecule

Hydrogen atoms

Methane molecule

Hydrogen atoms

Methane molecule

Figure 2.2 The formation of molecules of (a) hydrogen and (b) methane by covalent bonding. Each atom achieves a full set of electrons in its outer shell by sharing with another atom. A shared pair of electrons constitutes a covalent bond

Figure 2.2 The formation of molecules of (a) hydrogen and (b) methane by covalent bonding. Each atom achieves a full set of electrons in its outer shell by sharing with another atom. A shared pair of electrons constitutes a covalent bond

Sodium atom Na

Chlorine atom Cl

Sodium ion Na+

Chloride ion Cl-

Figure 2.3 Ion formation. Sodium achieves stability by losing the lone electron from its outermost shell. The resulting sodium ion Na+ has 11 protons and 10 electrons, hence it carries a single positive charge. Chlorine becomes ionised to chloride (Cl-) when it gains an electron to complete its outer shell

When this happens, an ion is formed, which carries either a positive or negative charge. Positively charged ions are called cations and negatively charged ones anions. The sodium atom for example has 11 electrons, meaning that the inner two electron shells are filled and a lone electron occupies the third shell. If it were to lose this last electron, it would have more protons than electrons, and therefore have a net positive charge of one; if this happened, it would become a sodium ion, Na+ (Figure 2.3).

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