Measurements and Units

The basic mass unit is moles (mol) or, where appropriate grams (g).

1 mole of a substance is equal to its molecular weight expressed in grams. i.e. the molecular weight of sodium chloride (NaCl) is 58.44 therefore 58.44 g of NaCl = 1mole

The basic volume unit is litres (l or L) or liters if you are American

Concentration = Mass / Volume therefore the basic concentration unit = moles per litre (mol/l) (or g/l). Molar solutions may also be (and usually are) denoted by M

length is in metres (m) and time in seconds (s) or minutes (min)

Very large and very small values should be expressed in standard form i.e. multiplied by the appropriate power of 10 or by using the appropriate prefix.

The prefixes are:

multiplier

Standard form

prefix

symbol

1 000 000 000 000

1012

tera

T

1 000 000 000

109

giga

G

1 000 000

106

mega

M

1 000

103

kilo

k

1

0

-

-

0.1

10-1

deci

d

0.01

10-2

centi

c

0.001

10-3

milli

m

0.000 001

10-6

micro

m

0.000 000 001

10-9

nano

n

0.000 000 000 001

10-12

pico

p

0.000 000 000 000 001

10-15

femto

f

0.000 000 000 000 000 001

10-18

atto

a

 

Therefore:

0.00053g/l should be written as 5.3 x 10-4 g/l or as 530 mg/l

76000000 l should be written as 7.6 x 107 l or as 76 Ml

There are still some funny values and measurement systems floating around in old textbooks and practical schedules that need updating or proof reading properly.

Percentage solutions (%) means weight (grams) divided by volume (ml, usually) expressed as a percentage. Therefore 10 g in 100 ml is a 10% solution; 8.8 g of NaCl in 1000 ml is a 0.9% solution, which is isotonic with plasma i.e. the same as 0.15 M/l. Medics (at least old medics) seem very fond of percentage solutions and also of expressing concentrations in g/100 ml (which is weird).

Furthermore:

l is sometimes written as dm3 (cubic decimetres; which is correct but pedantic)

ml is sometimes written as cm3 (cubic centimetres)

ml is sometimes written as cc (cubic centimetre; which is old fashioned)

ml is sometimes written as mm3 (cubic millimetres)

g l-1 (etc.) means the same as g/l

 

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© Pete Smith 1998