Suppose two 200.0-L tanks are to be filled separately with the gases helium and hydrogen. What mass of each
gas is needed to produce a pressure of 135 atm in its respective
tank at 248C?

answer is: nuclear fusion produces elements that are heavier than helium.

nuclei combine to form a heavier nucleus, releasing energy.  

for example, fusion of hydrogen atoms at the sun.  

this fusion nuclear reaction is in the inner core of the sun and hydrogen (atomic number 1) is conveted into helium (atomic number 2).  

the inner core is the central region of the sun and all solar energy is produced in the core by nuclear fusion.