Guide for the Use of the International System of Units (SI)

8.6.6 Volume fraction of B 
 
Quantity symbol:
φ
B
.
SI unit: one (1) (volume fraction is a quantity of dimension one). 
Definition: for a mixture of substances A, B, C, . . . ,

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Guide for the Use of the International System of Units (SI) 
*
A
m,
A
*
B
m,
B
B
V
x
V
x
∑
=
ϕ
where x
A
, x
B
, x
C
, . . . are the amount-of-substance fractions of A, B, C, . . ., 
 . . . are the 
molar volumes of the pure substances A, B, C, . . . at the same temperature and pressure, and where the 
summation is over all the substances A, B, C, . . . so that 
,
,
,
*
C
m,
*
B
m,
*
A
m,
V
V
V
∑
= .
1
A
x
 
8.6.7 Mass density; density 
 
Quantity symbol: ρ.
SI unit: kilogram per cubic meter (kg/m
3
). 
Definition: mass of a substance divided by its volume: ρ = m / V. 
Notes: 
1. This Guide prefers the name “mass density” for this quantity because there are several different 
“densities,” for example, number density of particles, n = N / V; and charge density, ρ = Q / V. 
2. Mass density is the reciprocal of specific volume (see Sec. 8.6.9): ρ = 1 / υ. 
8.6.8 Molality of solute B 
 
Quantity symbol: b
B
(also m
B
).
SI unit: mole per kilogram (mol/kg). 
Definition: amount of substance of solute B in a solution divided by the mass of the solvent: b
B
= n
B
/ m
A
. 
Note: The term molal and the symbol m should no longer be used because they are obsolete. One should 
use instead the term molality of solute B and the unit mol/kg or an appropriate decimal multiple or 
submultiple of this unit. (A solution having, for example, a molality of 1 mol/kg was often called a 
1 molal solution, written 1 m solution.)