Question

What mass of water would you need to have 15.0 moles of `H_2O`?

Answer 1

This is a basic stoichiometry problem, which you will need to master early on in general chemistry. They are merely extensions of unit conversion, but chemistry-style!

Hence,

`15.0"mol" * (18g)/(H_2O) approx 270g`

of water are present given the data specified.