Placenta facilitates the supply of `O_2` and nutrients to the embryo and removal of `CO_2` and excretory waste products produced by the embryo. Apart from this, placenta is considered as an endocrine tissue. Justify.

0.75 mole of solid A_(4) and 2 moles of gaseous O_(2) are heated in a sealed vessel, completely using up the reactants and producing only one compound. It is found that when the temperature is reduced to the initial temperature, the contents of the vessel exhibit a pressure equal to half the original pressure. What conclusions can be drawn from these data about the product of the reaction?