code: def standard(target, mol_weight='pore.molecular_weight',
               molar_density='pore.molar_density'):
      r
      MW = target[mol_weight]
      rho = target[molar_density]
      value = rho*MW
      return value
docs: r"""
      Calculates the mass density from the molecular weight and molar density
  
      Parameters
      ----------
      mol_weight : string
          The dictionary key containing the molecular weight values
  
      molar_density : string
          The dictionary key containing the molar density values