How many grams of K2HPO4 are there in a teaspoon? Tablespoon? How do I figure this out?

I don't know that you can. K2HPO4 is a solid. The volume taken up by a solid depends upon particle size. Technically, if you follow the question really close, there is no K2HPO4 is a teaspoon or a tablespoon. Metals or plastic, yes. KH2PO4, no.