The Yellow River is the most important water resource for the dry north of China, playing an irreplaceable role in economic development, and agriculture. Since 1960 over 14 dams have been constructed on the river for hydroelectric power, which is vital to northern China's infrastructure.
It is D because nitrate reacts with the calcium and since na has a 2+ charge and no3 has a - charge we need 2 no3 to balance out the charge hence a 2 is placed outside the no3 bracket.
Since the acid and base react in a one to one ratio, you can use the equation M1V1 = M2V2 to solve for M2, or the molarity of NaOH.
M1 = 0.9 M HCl
V1 = 200.0 mL HCl
M2 = ?
V2 = 23.9 mL NaOH
(0.9 M)(200.0 mL) = M2(23.9 mL) —> M2 = 7.53 M NaOH
This is an oxidation-reduction (redox) reaction:
2 Mg0 - 4 e- → 2 MgII
(oxidation)
2 O0 + 4 e- → 2 O-II
(reduction)