Some implications of SI in chemistry

Volume
The SI unit of volume is the cubic metre, m³, which is rather large for practical purposes. The litre (L) and the millilitre (mL) are technically obsolete, but are widely used and glassware is still calibrated using them.

Mass
The SI unit for mass is the kilogram (kg) rather than the gram (g): this is unusual because the base unit has a prefix applied.

Amount of substance
You should use the mole (mol, i.e. Avogadro's constant, see Table 9.4) to express very large numbers. The mole gives the number of atoms in the atomic mass, a convenient constant.

Concentration
The SI unit of concentration, mol m^-3, is not convenient for general laboratory work. It is equivalent to the non-SI term 'millimolar' (mM) while 'molar' (M) becomes kmol m^-3. If the solvent is not specified, then it is assumed to be water.

Time
In general, use the second (s) when reporting physical quantities having a time element. Hours (h), days (d) and years should be used if seconds are clearly absurd (e.g. samples were taken over a 5-year period). Note, however, that you may have to convert these units to seconds when doing calculations.

Temperature
The SI unit is the kelvin, K. The degree Celsius scale has units of the same magnitude, °C, but starts at 273.15 K, the melting point of ice at STP. Temperature is similar to time in that the Celsius scale is in widespread use, but note that conversions to K may be required for calculations. Note also that you must not use the degree sign (°) with K and that this symbol must be in upper case to avoid confusion with k for kilo; however, you should retain the degree sign with °C to avoid confusion with the coulomb, C.

Interconversion of SI units
You will find that the use of SI units simplifies mathematical manipulations and ensures that you obtain the correct units for the parameter being calculated. Remember that you must convert all units into the appropriate SI units, e.g. masses must be expressed as kg, volumes as m³ and concentrations as kg m^-3 or mol m^-3, etc., and that you may need to use alternatives in derived units (Table 9.2). The application of these principles is shown in Box 9.2.