The number of moles of a substance can be used to calculate its mass, the number of atoms present in the compound as well as the number of moles of individual atoms/ions in the compound.
The number of moles of a compound or ion is defined as the ratio of its mass to its molar mass. The number of moles of a substance is very important when performing stoichiometric calculations.
If you have the number of moles of a substance you can find;
The mass of the substance
The number of atoms or ions in the substance
The number of moles of individual atoms/ ions in the substance.
The bracket just separate an atom or the ion and in the above it means there are two hydroxide molecules so bracket function is just to separate ions and molecules hope it helps
