You can't even begin to convert volume to weight, or visa versa, without knowing the unit weight per volume (density) of what your measuring.
Let's just use a simple example - Morton Salt ...
1 teaspoon table salt = about 6 g
1 teaspoon kosher salt = about 4.8 g
so - in both cases you have a teaspoon of salt, but they don't weigh the same.
As for yeast example (which was probably a bad example to use - except to show that a tsp of dry yest didn't weigh the same as a tsp of water) ... dry yeast is only about 7% water, fresh yeast is about 70% water ... so even if they both weighed the same per tsp - there is about 2.4 times as much yeast in a tsp of dry yeast as in fresh yeast. That works out to a 0.25 oz pkg of dry yeast having the same leavening capacity of a 0.6 oz cake of fresh yeast.
"It ain't what you don't know that gets you in trouble. It's what you know for sure that just ain't so." - Mark Twain