consider the reaction: 2H2(g) + O2(g) ⇌ 2H2O(l). At equilibrium, the partial pressure of hydrogen was 0.705 atm and the partial pressure of oxygen was 2.21 atm. Calculate Kp

The reactants have four hydrogen atoms and four oxygen atoms. On the other hand, the products have just two hydrogen atoms and three oxygen atoms. So, to balance the chemical equation, there should be two more hydrogen atoms and one more oxygen atom. This imbalance can be corrected by placing the number 2 in front of H2O on the reactants' side.