Calcium reacts with water to produce Calcium Hydroxide and Hydrogen gas;
                            Ca  +  2 H₂O    →    Ca(OH)₂  +  H₂
According to balance equation,
                 40 g (1 mole) Ca reacts to form  =  22.4 L (1 mole) H₂
So,
                           20.1 g of Ca will produce  =  X L of H₂
Solving for X,,
                                        X  =  (22.4 L × 20.1 g) ÷ 40 g
                                        X  =  11.25 L
Result:
          Theoretical Yield of H₂ at STP  =  11.25 Liter