Find the % by mass of hydrogen if a compound is decomposed by an electric current giving 19.98 g O and 2.52 g H.

Ithink the answer is 56.

Yes, endothermic means it turns cold. and some chemical changes include change of tempature.

Ibelieve the answer is d

The problem here is that according to aufbau principle before 3d orbital 4s subshell comes.after filling 3p orbital with electrons you have 4 electrons left, you place two in 4s with opposite spin and the other two in 3d in any of its two orbitals of total five orbitals with the same spin.