def _calc_a(self, y_C, y_H, y_O, y_N, y_S):
return <NUM_LIT:1> / (y_C/mm("<STR_LIT:C>") + y_H/mm("<STR_LIT:H>") + y_O/mm("<STR_LIT:O>") + y_N/mm("<STR_LIT:N>") +<EOL>y_S/mm("<STR_LIT:S>"))<EOL>
Calculate the mean atomic weight for the specified element mass
fractions.

:param y_C: Carbon mass fraction
:param y_H: Hydrogen mass fraction
:param y_O: Oxygen mass fraction
:param y_N: Nitrogen mass fraction
:param y_S: Sulphur mass fraction

:returns: [kg/kmol] mean atomic weight

See equation at bottom of page 538 of Merrick1983a.