What is the theoretical yield in liters of hydrogen at stp if 20.1g of calcium are completely reacted

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

Spontaneity tell us the direction of the reaction, but not how fast it goes. gibbs free energy brings together enthalpy, entropy and temperature in one easy formula. g = h - ts. and, finally the sign of delta g is always negative for a spontaneous reaction.

It shows that the experiment is designed properly.