B. Mg + O2  MgO + O C. Mg2 + O2  2 MgO D. 2 Mg + O2  2 MgO QUESTION: Which of the following is the balanced chemical equation for the following reaction: Mg + O2  MgO A. Mg + O  MgO B. Mg + O2  MgO + O C. Mg2 + O2  2 MgO D. 2 Mg + O2  2 MgO In a chemical equation, number of atoms must be the same on both sides of the arrow, for each element. True for all four choices.. Once formulas of reactants are established, you cannot change them (do not change subscripts!), and you cannot add another formula. The only thing you’re allowed to do is change coefficients. equation for the following reaction: Mg + O2  MgO A. Mg + O  MgO B. Mg + O2  MgO + O C. Mg2 + O2  2 MgO D. 2 Mg + O2  2 MgO PAUSE CLICK In a chemical equation, number of atoms must be the same on both sides of the arrow, for each element. This is true for all four choices given above. However, only one of the four is properly balanced. Once formulas of reactants are established, you cannot change them. By changing subscripts, you change the formulas. This means that you cannot change the subscripts. Therefore, choice A is wrong. CROSS OUT O in choice A Here, the subscript of Oxygen in O2 was changed. Choice C is also wrong. CROSS OUT 2 in Mg2 Here, the subscript of magnesium was changed. You’re also not allowed to add the formula of another substance. Therefore, choice B is wrong. CROSS OUT + O in choice B The only thing you’re allowed to do is change coefficients, and that is what was done in choice D. Let’s verify that we, in fact, have a balanced equation in choice D. Remember that to count atoms, we multiply coefficient and subscript. So for magnesium, we have two on the left… Two… CALLOUT “coefficient of Mg” Times 1 CALLOUT “subscript 1 implied” Equals 2. And two on the right… CALLOUT “coefficient of MgO” For oxygen, we have two on the left One… CALLOUT “coefficient 1 implied” Times 2 CALLOUT “subscript of O” And two on the right as well. The correct answer is D. CLICK PAUSE END RECORDING

Video ID: 1-31-2 © 2008, Project VALUE (Video Assessment Library for Undergraduate Education), Department of Physical Sciences Nicholls State University Author: Glenn V. Lo Narrator: Funded by Louisiana Board of Regents Contract No. LA-DL-SELECT-13-07/08