Relative molecular mass (Mr) plays a crucial role in stoichiometry, the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. In stoichiometry, Mr is used to convert between moles and grams, which is essential for calculating the quantities of substances involved in a chemical reaction. Knowing the Mr of each substance allows chemists to determine how much of each reactant is needed to completely react with other reactants and how much of each product will be formed. This is particularly important in industrial chemistry where precise quantities of reactants are required for cost-effective and efficient production. Mr is also crucial in balancing chemical equations, as it helps ensure that the mass and atoms are conserved in a reaction. Furthermore, in the laboratory, understanding Mr assists in predicting the outcomes of reactions, planning experiments, and interpreting results. Accurate stoichiometric calculations are essential for successful chemical synthesis, analysis, and research, making Mr an indispensable tool in the field.

Understanding relative atomic and molecular masses is fundamental in environmental science, particularly in the analysis and monitoring of pollutants. For instance, in air and water quality testing, the identification and quantification of pollutants such as heavy metals, particulates, and organic compounds rely on the knowledge of their atomic and molecular masses. This knowledge allows for the accurate determination of the concentration of these pollutants in different mediums. Additionally, in the study of atmospheric chemistry, the understanding of molecular masses of greenhouse gases like carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) is crucial for assessing their relative impacts on global warming and climate change. The molecular masses of these gases, in relation to their volumes, contribute to the calculation of their Global Warming Potential (GWP). Furthermore, in the field of ecotoxicology, understanding the relative atomic and molecular masses of various substances helps in evaluating the toxicity levels of different compounds and their potential impact on ecosystems and wildlife. Thus, in environmental science, the comprehension of these masses is essential for both the analysis of environmental problems and the development of strategies for pollution control and sustainable management of natural resources.

The concept of relative atomic mass is intrinsically linked to the periodic table, as each element's position in the table is associated with its atomic number and average relative atomic mass. The atomic number of an element, which determines its position in the periodic table, is the number of protons in the nucleus of an atom of that element. The relative atomic mass, listed under each element symbol in the periodic table, represents the weighted average mass of all the isotopes of that element, taking into account their natural abundance. This weighted average is crucial for understanding the properties of elements and predicting how they will react chemically. The periodic table thus provides a concise and comprehensive summary of the relative atomic masses of elements, facilitating quick reference and comparison. Additionally, trends in the periodic table, such as atomic radius, electronegativity, and ionization energy, are often explained and predicted based on relative atomic masses and electronic configurations of the elements.

The concept of relative atomic mass can be applied to determine the purity of a sample through a process known as isotopic analysis. This technique involves measuring the relative abundance of different isotopes in a sample and comparing it to the expected isotopic composition. For instance, if a sample of copper is suspected to be impure, its isotopic composition can be analyzed. Pure copper should have a specific ratio of its two stable isotopes, copper-63 and copper-65. Deviations from the expected isotopic ratio could indicate the presence of impurities. This method is particularly useful in fields such as geochemistry, environmental science, and archaeology, where the isotopic composition of a sample can provide valuable information about its origin, age, or the processes it has undergone. Isotopic analysis leverages the principles of relative atomic mass to provide insights that are not possible with conventional purity tests, making it a powerful tool in analytical chemistry.

Carbon-12 is chosen as the standard for defining relative atomic mass because of its stable nature and widespread presence in organic compounds. Prior to the adoption of carbon-12, oxygen had been used as a reference standard. However, the problem with oxygen was its multiple naturally occurring isotopes (oxygen-16, oxygen-17, and oxygen-18), which caused inconsistencies in the standard. Carbon-12, on the other hand, has a unique advantage. It is a stable isotope and constitutes a significant part of all known life, making it readily available and practical for use as a standard. The use of a single isotope eliminates the discrepancies caused by the presence of multiple isotopes in the standard. Furthermore, the decision to define the relative atomic mass based on 1/12th the mass of a carbon-12 atom provides a convenient and consistent scale for measuring atomic masses. This scale ensures uniformity and precision in the measurement of atomic masses across different laboratories and studies.