Link1)https://media.cheggcdn.com/media/d0a/d0a10eb2-c650-4a2a-9da9-42efc0a34767/phpqIF1yp.png

Link2)Molar mass of Hydrogen gas is 2.0g/mol

Calculation for number of moles of Hydrogen

Number of moles hydrogen gas =( mass of Hydrogen)/(molar mass of Hydrogen)

Number of moles of Hydrogen gas = 28.3g/(2.0g/mol)

Number of moles of Hydrogen gas = 14.65 mole

Calculation for concentration of Hydrogen gas

concentration = (number of moles of Hydrogen gas)/(volume of water)

concentration = 14.65mol/2.8L

concentration = 5.2 mol/L

Link3)https://media.cheggcdn.com/media/956/956df838-7c54-4742-b08d-41de68838c62/phpHCV7c9.png

Link4)https://media.cheggcdn.com/media/349/3497ca61-2ca0-4c2d-be4c-a894281a5641/phpOspuQ1.png

Link5)Given , volume of gas at STP = 2.1 L

And number of moles of oxygen gas can be calculated by

No. Of moles = volume of gas at STP/ 22.4 L

HERE 22.4 L is the molar volume at STP

so, no. Of moles = 2.1/22.4 = 0.093 mol