An alkaline earth metal oxide A and an oxide of nitrogen B react with water. A forms ONLY compound X , and B forms ONLY compound Y . X is the hydroxide of the Group 2 metal belonging to the 4^ th period. Y is an oxoacid of nitrogen in which nitrogen is in a +5 oxidation state. The total number of atoms in one molecule of A and one molecule of B is ______

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The Group 2 metal belonging to the 4^ th period is Calcium (Ca). Thus, the hydroxide X is Ca(OH) ₂ . The alkaline earth metal oxide A that reacts with water to form Ca(OH) ₂ is CaO . The atomicity (number of atoms per molecule) of CaO is 1 + 1 = 2 . The oxoacid of nitrogen Y with nitrogen in the +5 oxidation state is nitric acid, HNO ₃ . The oxide of nitrogen B that reacts with water to form ONLY HNO ₃ is dinitrogen pentoxide, N ₂ O ₅ . N ₂ O ₅ + H ₂ O 2 HNO ₃ The atomicity of N ₂ O ₅ is 2 + 5 = 7 . The total numbe