what is the structure of C25H20?

Answer 1

This organic molecule is an alkane (a hydrocarbon). The structure for full saturation of alkane is CnH2n+2. In the case above, n=25, therefore a fully saturated alkane would have 52 Hs.
Since only 20 H present, then, 16 {(52-20)/2 }, is the unsaturation number, or degree of unsaturation of this molecule.

Any combination of rings, double bonds or triple bonds to add up to 16 is a candidate for the structure.

eg: 1,4 di(1,3,5-heptatriyne)-
2,ethyl-3-propylbenzene (this structure has one ring, 6 triple bonds, 3 double bond giving 16 (note a triple bond counts as 2 degrees of unsaturation)).

An IR spectrum would be extremely helpful to at least indicate a functional group present. This would narrow down the field of candidates considerably.

Answer 2

I suspect there are many chemical structures with that empirical structure. If you get a CRC handbook of Chemistry and Physics, in any library, you can look up that empirical structure and it will yield a reference (elsewhere in the book) of the corresponding structures matching it.

Answer 3

all you want to know here http://en.wikipedia.org/wiki/Tetraphenylmethane