well u find the reaciton and balance it. and u know taht 200mL of HCL is used. so u find the amu of HCL and divide 200 by it. thats how many moles u got. then if u've balanced it properly, u divide the mole number by 2 cause theres half as much H2 gas produced. Lastly u multiply whatever answer u got by 24.71.