what amout of zinc will be required to generate sufficient hydrogen to completely combined with oxygen obtained by complete decomposition of 30 g of potassium clorate?
You are talking about two separate reactions. Oxygen (O2) is being made with the reaction
2 KClO3 -> 2 KCl + 3 O2
while H2 is (we will assume) being made with the reaction
Zn + 2 HCl -> ZnCl2 + H2
First, convert 30 g of KClO3 to moles. One mole of potassium chlorate is 122.6 g, so you start with 0.2447 moles of KClO3. From that, you get 0.3670 moles of O2. To convert all that O2 to H2O, you need 0.734 moles of H2. That will require an equal number of moles of Zn. Convert that to grams.