The answer to that question depends on where Chimeric Mass is. When on the stack as a spell, the value of X in the converted mana cost of a spell is whatever was chosen when the spell was played. If you play Chimeric Mass and pay five mana, the converted mana cost of this spell will be five. However when in any other zone, the value of X in the converted mana cost of a card is zero. So once this Chimeric Mass card is in play, its converted mana cost is zero.