There are differet kinds of chemical structures in ChEBI. Some of them are for molecular entities, some for groups, and some for classes. The latter ones have "R" symbols in them, e.g. alcohols (CHEBI:30879). But the mass of alcohols in NOT 29.01804. Even the lightest possible alcohol conforming to the CHEBI:30879 connectivity is heavier: methanol (CHEBI:17790) has mass = 32.04186. To get the minimal mass of alcohol, one has to replace Rs with Hs. Therefore it is misleading to call this calculated parameter "Mass". Now the question is whether it is actually useful number to display. I'd say it is but we need to find a good name for it.
Kirill
