What is Composition (mole and mass fractions)?

Which species a gas contains and how much of each — as a share of molecules (mole fraction X) or of mass (mass fraction Y).

At the same temperature and pressure, a different composition burns differently and has different properties. The fuel-to-air ratio is one of the most important inputs to any combustion calculation.

Air is about 79 % nitrogen and 21 % oxygen by moles. By mass, oxygen is about 23 %, because an oxygen molecule is heavier.

Enter species with relative amounts; they are normalized to sum to 1 (for example CH4 1, O2 2, N2 7.52). Choose a mole or mass basis. Species names must exist in the reaction mechanism.

X_i = \frac{n_i}{n}, \qquad Y_i = \frac{m_i}{m}
They are related through the species molecular weight W_i and the mean molecular weight \bar{W}:
Y_i = X_i\,\frac{W_i}{\bar{W}}

• Equivalence ratio
• Adiabatic mixing
• Reaction mechanism

• Cantera documentation