How many moles of H2 would occupy 45.6 liters at 10.0 C and 1.25 ATM

Use the ideal gas law pv=nrt

Since you're looking for moles, the formula will become n=pv/rt

P=1.25 atm

V=4.56 L

R=0.08206 (L•atm/moles•K)

T=10 C but you want it in kelvins so add 273.15 and your temp will be 283.15

Plug those all into the formula and all the units excite the moles should cancel out and that will be your final answer.