$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$ In the given equation on using 4g of hydrogen, the amount of ${\mathrm{H}}_2\mathrm{O}$ Formed is

$2{\mathrm{H}}_2+{\mathrm{O}}_2\to 2{\mathrm{H}}_2\mathrm{O}$

2 moles of ${\mathrm{H}}_2$ react with one mole of Oxygen to produce 2 moles of ${\mathrm{H}}_2\mathrm{O}$ (36g of water) In the given equation 4g of Hydrogen means 2 moles, Hence 36g of water will be formed