Water has a molecuar mass of 18, this means it weights 18 g per mole. A mole is 6.0221415E23 molecules (Avogadro's number). If you have 1 g of water, you therefore have 1/18 of a mole, or 3.34E22 molecules.Now u have 250 g, hence,
half of it makes hydrogen and other half makes it oxygen
Hope this helps.