Question

Which of the following is the formation reaction for CaO(s) ?


A. Ca2+(aq) + O2–(aq) CaO(s)
B. Ca(s) + .5O2(g) CaO(s).
C. CaO(s) Ca(s) + .5O2(g)
D. 2 Ca(s) + O2(g) 2CaO(s)
E. 2 CaO(s) 2 Ca(s) + O2(g)

Left with B and D. Please Help

Answer 1

B isn't balanced so throw it out.
That leaves D as the only possible answer.
By the way, you should show the arrows; otherwise, we don't know the difference between reactants and products.
Although I would pick D as the answer, I looked for
Ca + 1/2 O2 ==> CaO since these usually are quoted for 1 mol of substance and not 2 mols of substance.