MassEl
The MassEL program allows to calculate masses of individual elements for synthesis of alloys, compounds or mixtures of specific compositions. For instance, in order to get 1 g of the FeSe compound you will have to mix 0.414 g of Fe and 0.586 g of Se. Mass of a specific element (e.g. you have 0.5 g of Fe for your synthesis) can be used as a reference as well.