Okay so you convert mL to liters
50.0 mL/1000L = 0.05 liters
Then divide by 22.4 for moles
0.05/22.4 = 0.0022 moles of H2O we want hydrogen so for every two moles of hydrogen there’s one water
0.0022 x 2 H/ H2O = 0.0044 moles of hydrogen
Convert to mass by multiplying by its molar mass
0.0044 x 1.01 = 0.0045 grams of Hydrogen is the mass in 50 mL of water