Look at this equation: 2 H2 + O2 --> 2 H2O. Why isn't there any coefficient in front of oxygen? A. A one is assumed when no coefficient is present.

B. There is an unlimited amount of oxygen

C. The 2 after the oxygen is all that is needed

D. All of these

A is the correct choice. It's the reason one might write

instead of

They're both correct, but the 1s in the second form are redundant.

Multiplication by 1 is implied when it's left out. Similarly, we omit 0s when adding. We do this because 1 and 0 are multiplicative/additive identity elements, respectively. In other words, for all complex numbers a,

1 × a = a

0 + a = a