Page 50 - Physical chemistry understanding our chemical world

THE PRACTICE OF THERMODYNAMIC MEASUREMENT 17

Table 1.2 Several of the more common units that are not members of the Syst`eme
Internationale
Quantity Non-SI unit Abbreviation Conversion from non-SI to SI
Energy calorie cal 1 cal = 4.184 J
Length ˚ angstr¨ om ˚ A 1 ˚ A = 10 −10 m
Pressure atmosphere atm 1 atm = 101 325 Pa
5
Pressure bar bar 1 bar = 10 Pa
3
Volume litre dm 3 1dm = 10 −3 m 3
increasingly common to see the Avogadro constant given a different symbol than
L. The most popular alternative symbol at present seems to be N A .

Non-SI units
It is important to be consistent with units when we start a calculation. An enormously
expensive spacecraft crashed on the surface of the planet Mars in 1999 because a
distance was calculated by a NASA scientist in terms of inches rather than centimetres.
Several non-SI units persist in modern usage, the most common being listed in
Table 1.2. A calculation performed wholly in terms of SI units will be self-consistent.
Provided we know a suitable way to interchange between the SI and non-SI units,
we can still employ our old non-SI favourites.

Aside

In addition to the thermodynamic temperature T there is also the Celsius temperature
t,definedas

t = T − T 0

where T 0 = 273.15 K.
Sometimes, to avoid confusion with the use of t as the symbol for time, the Greek
symbol θ (theta) is substituted for the Celsius temperature t instead.
Throughout this book we adopt T to mean temperature. The context will make clear
whether T is required to be in degrees Celsius or kelvin. Beware, though, that most
formulae require the use of temperature in kelvin.

Why is the SI unit of mass the kilogram?

Multiples and the SI unit of mass m

The definition of mass in the Syst`eme Internationale scheme departs from the stated
aim of formulating a rigorous, self-consistent set of standards. The SI unit of ‘mass’