How to set coefficients in chemistry?

The coefficients show the number of molecules in the composition, they are put down in large numbers in front of the molecular formula.

All coefficients in the formula are put down according to the law of conservation of energy. All substances do not arise from anywhere and do not disappear anywhere. The number of atoms in the original substance should be equal to the number of atoms in the resulting substance. Let’s consider this on a specific example of the interaction of phosphorus and oxygen:

P + O2 → P2O5

It turns out that before the arrow, oxygen has 2 atoms, and after – 5. Therefore, the next step is to find the smallest common multiple, which is divisible by both 2 and 5. That number is 10. Divide 10 by 2 and 5, we get the coefficients and arrange their:

P + 5O2 → 2P2O5

Then you need to level the number of phosphorus atoms. In the first part of the equation we have one atom, and in the second – 4, so in the first part we put 4 in front of P.

4P + 5O2 → 2P2O5

It is important to remember that 1, as a coefficient, is never written, it is always worth starting to equalize the number of atoms with oxygen, the coefficient refers to the entire formula, and not to a specific atom, and is placed in front of it.