As a random guess, I'd have to say 20? I'm a bit rusty on my chemistry, but this is my thought process: If there's 1 proton and 0 neutrons in a single hydrogen atom, and each proton and neutron is 1g each, then i would assume that 20 atoms of H times (1 proton + 0 neutrons) would equal 20. Please correct me if I'm wrong :)