# Diffusion coefficient (Mass diffusivity) for gases

## Description

Diffusivity or diffusion coefficient is a proportionality constant between the molar flux due to molecular diffusion and the gradient in the concentration of the species (or the driving force for diffusion). The dependence of the diffusion coefficient on temperature for gases can be expressed using Chapman–Enskog theory.

Related formulas## Variables

D | diffusion coefficient (cm^2/s) (dimensionless) |

T | absolute temperature (K) (dimensionless) |

M_{1} | molar mass molecule 1 (g/mol) (dimensionless) |

M_{2} | molar mass molecule 2 (g/mol) (dimensionless) |

p | pressure (atm) (dimensionless) |

σ_{12} | average collision diameter (Å) (dimensionless) |

Ω | temperature-dependent collision integral (dimensionless) (dimensionless) |