The molar mass of hydrogen gas H2 = 2 g/mol (this value you can have from the periodic table)
Now: Moles = Mass / Molar mass
We need the mass. So, Molar mass . Moles = Mass of H2
2 g/mol . 7.5 moles = 15 g = Mass of H2
Answer: Mass of 7.5 moles of H2 = 15 g