If 25.0g of water are decomposed at RTP how would you find the volume of hydrogen gas produced?

Write the equation.

Convert 25.0 g H2O to moles.
Using the coefficients in the balanced equation, convert moles H2O to moles H2.
Convert mole H2 to L by volume = 22.4 x moles.
Post your work if you get stuck.
Convert moles