Fa bathtub and a cup contain water at the same temperature, which will have more thermal energy? why?

Explanation:

Short Yes.

The coefficients may not be conserved, but mass always has to be. Take this equation as an example

2 Mg3P2 ===>   6Mg + P4

There is a 2 on the left side and 6 and 1 on the right. I hope you mean that the coefficient 2 is not equal to 7.

But let's look a little closer. You have to look at the molecular structure of the left and right side.

2Mg3P2 has 6 Mgs and 4 Ps on the left side.

6Mg is on the right. They are free standing.

P4    has 1 molecule consisting of 4 Ps.

Everything balances.

This is a terrific question to be asking. You need to understand the internal balance numbers vs the molecular ones on the out side.

That sounds like a bit of gobbledygook and it takes a bit of study.

2 Mg3P2 means that Mg3P2 is written twice.

Mg3P2 ==> "3 Mg2+   +  2P3+   and there is another one written the same way.

Mg3P2 ==> "3 Mg2+   +  2P3+