Which type of C molecules? I think you want to say "X mol of Y. Weight measured in a vacuum, which necessitates an Y with very low evaporation."?
Since it isn't done like that, I assume it isn't easy to find an Y which is both easy to measure exactly and to keep pure. (I have a smattering of organic chemistry but no inorganic, so any more opinions from me would be without value.)
Edit: Other people here talk about Si. We can keep that clean, at least (see electronics).
They try it, the problem is that you can compare mass better than you can count molecules. So one of the approaches is, to manufacture a Si sphere as perfect as possible and calculate the mass from the radius and the properties of Si crystals. As far as I know, this had no practical results, except cool photos of strangely rendered looking spheres.
Since it isn't done like that, I assume it isn't easy to find an Y which is both easy to measure exactly and to keep pure. (I have a smattering of organic chemistry but no inorganic, so any more opinions from me would be without value.)
Edit: Other people here talk about Si. We can keep that clean, at least (see electronics).