Is a ratio of the parts of a compound exact?

Yes, the ratio of the parts of a compound is exact. In a chemical compound, the elements are combined in fixed proportions by mass, which is defined by their chemical formula. For example, water (H₂O) always consists of two hydrogen atoms for every one oxygen atom in a 2:1 ratio, regardless of the amount of water present. This precise ratio is fundamental to the compound's identity and properties.