How do i work out what mass of oxygen reacts with 12g of magnesium?

Becquerel expected to find ( he developed the photographic plate that had sun-exposed minerals on top of it. becquerel expected to find ( he developed the photographic plate that had been in the closed drawer.

Asample of ammonia reacts with oxygen as shown. 4nh3(g) + 5o2(g) 4no(g) + 6h2o(g) what is the limiting reactant if 4.0 g of nh3 react with 8.0 g of oxygen? o2 because it produces only 0.20 mol of no. nh3 because it produces only 0.20 mol of no. o2 because it produces two times less no than nh3. nh3 because it produces three times more no than o2.

What is the value of keq for the reaction expressed in scientific notation?