The mass of 3.45 L of hydrogen gas at SATP

The easy way is knowing that all gases basically are 22.4 L/mol at SATP.

There fore 3.45 L/22.4 L/mol = 0.154 mol

Or use PV=nRT

n = (1 atm) (3.45L) / (273 K) (0.82 L atm) / (mol K) = 0.154 mol

same answer