When calculating the relative molecular mass (Mr) for hydrated compounds (compounds that include water molecules as part of their structure), it is essential to include the mass of the water molecules in the calculation. Hydrated compounds are often represented in their chemical formulas with a dot followed by the number of water molecules, for example, copper(II) sulfate pentahydrate, CuSO₄·5H₂O. To calculate the Mr of such a compound, one must sum the relative atomic masses of each atom in the compound, including those in the water molecules. Continuing with the example of CuSO₄·5H₂O, the calculation would involve the addition of the atomic masses of one copper atom, one sulfur atom, four oxygen atoms in the sulfate ion, ten hydrogen atoms, and five oxygen atoms from the five water molecules. This inclusion of the water molecules' mass is crucial as it significantly affects the total Mr, and consequently, any calculations involving the substance, such as determining its molar mass or using it in stoichiometric calculations. Ignoring the water molecules in such compounds would lead to incorrect results and a misunderstanding of the substance's actual mass and composition.

Relative molecular mass (Mr) itself does not directly give the actual mass of a substance but rather provides a relative measure compared to carbon-12. To determine the actual mass of a substance, Mr must be used in conjunction with the concept of moles. The molar mass of a substance, which is the mass of one mole of that substance, is numerically equal to its Mr but expressed in grams per mole (g/mol). By knowing the number of moles of a substance, one can calculate its actual mass using the formula: Actual mass = Number of moles × Molar mass. For example, if we have a substance with an Mr of 58.5 (like sodium chloride, NaCl) and we have 2 moles of it, the actual mass would be 2 moles × 58.5 g/mol = 117 grams. This calculation is fundamental in laboratory practices for measuring out substances and in stoichiometry for determining the mass of reactants and products in chemical reactions. Thus, while Mr is a dimensionless quantity providing a relative comparison, it forms a crucial part of the calculations needed to find the actual mass of substances in practical chemistry.

In a laboratory setting, the accuracy of the relative molecular mass (Mr) values used in calculations can vary depending on several factors, including isotopic abundance and the purity of substances. Isotopic abundance can affect Mr calculations, as the relative atomic mass of an element is an average value that takes into account the different isotopes of the element and their abundances in nature. These averages are generally consistent, but slight variations can occur in different geographical locations or in specific samples. Therefore, while the Mr values provided in the periodic table are precise for general use, they may not perfectly represent every sample.

Purity of substances is another critical factor. In practice, substances used in laboratories are not always 100% pure. Impurities can alter the actual Mr of a substance from its theoretical value. For instance, a sample of a compound might contain traces of other compounds or elements, which could skew the Mr calculation if not accounted for.

Despite these variables, the Mr values used in laboratory calculations are generally accurate enough for most educational and research purposes. High-precision work, such as in analytical chemistry or pharmaceuticals, might require more precise measurements of isotopic abundance and higher purity levels. In such cases, advanced techniques and equipment are used to ensure the utmost accuracy.

Despite the distinct structural differences between molecular and ionic compounds, the concept of relative molecular mass (Mr) is applicable to both due to its fundamental basis in atomic mass units. Molecular compounds are composed of molecules, which are groups of atoms covalently bonded together. On the other hand, ionic compounds consist of a lattice of positively and negatively charged ions. However, the core idea behind Mr is the summation of the relative atomic masses (Ar) of the constituent atoms or ions, regardless of the type of bonding or structure. This uniform approach simplifies the process of comparing and calculating the masses of different substances. In molecular compounds, Mr is the total of the atomic masses of the atoms in a molecule. In ionic compounds, despite the absence of discrete molecules, Mr still represents the sum of the masses of the constituent ions in their formula units. For example, in sodium chloride (NaCl), the Mr is calculated by adding the Ar of sodium (Na) and chlorine (Cl). This universal application of Mr across different compound types reflects the standardization of atomic mass units and facilitates a coherent and consistent method for mass calculations in chemistry.

The concept of isotopes significantly influences the calculation of relative molecular mass (Mr). Isotopes are different forms of an element, having the same number of protons but different numbers of neutrons. This variation in neutron number leads to different atomic masses for the isotopes. For instance, chlorine has two main isotopes, ^35Cl and ^37Cl, with relative atomic masses of approximately 35 and 37, respectively. When calculating the Mr of a compound containing chlorine, we must consider the average relative atomic mass of chlorine, which accounts for the natural abundance of its isotopes. This average is often not a whole number. For example, the average relative atomic mass of chlorine is about 35.5, considering the relative abundances of its isotopes. Therefore, in a compound like sodium chloride (NaCl), the Mr would be calculated using the average atomic mass of chlorine (35.5) instead of a whole number. This inclusion of isotopic masses ensures that the calculated Mr closely reflects the actual mass of the molecules as they exist in nature.