4.6: Atomic Weight and Molecular Weight: Introduction and Indicator Words

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Learning Objectives

Identify the indicator words associated with atomic weight and molecular weight.

As stated previously, the mole is a unit that relates a variety of measurements to one another and to chemically-significant quantities. The previous sections of this chapter have defined and discussed Avogadro's number, 6.02 × 10²³, which quantifies the number of individual atoms, ions, or molecules that are present within a substance, and "component within" molar quantities, which indicate the relative ratios of the elements that are present within a compound or molecule.

A related set of fundamental chemical quantities will be discussed in below, and their associated equality patterns will be presented and applied in the next four sections of this chapter.

Atomic Weight & Molecular Weight

Section 1.1 defined and discussed mass, volume, length, temperature, and time. These five quantities are collectively known as "principle measurable quantities," because they are fundamental scientific measurements that can be combined to create additional units. When considered specifically in terms of chemical measurements, neither length nor time have practical applications. However, the masses, volumes, and temperatures of chemicals can be utilized in a multitude of contexts and, therefore, are all significant values. Unfortunately, recording volume and temperature data for certain classifications of chemicals can be challenging, which diminishes their scientific value. In contrast, measurements related to mass are not restricted by the properties of the chemical that is being considered. As a result, mass, which is defined as the amount of substance contained in an object, is the principle measurable property that is most often applied to chemical concepts.

Furthermore, due to the wide variety of chemicals that can be quantified by their masses, as well as the multitude of applications in which the corresponding data can be applied, mass is a chemically-significant quantity. Because pure chemical substances can be classified as either elements or compounds, chemists established two corresponding mass-based chemical quantities, atomic weight and molecular weight, respectively, as molar standards.

Atomic Weight & Molecular Weight Indicator Words

Atomic weight and molecular weight are molar quantities that relate to the mass of an element or a compound, respectively. Therefore, the word "mass" is the first indicator word that is associated with applying one of these values in a problem-solving context. Alternatively, mass units, such as "grams," "kilograms," or "milligrams," also serve as indicators for utilizing these molar relationships.

For example, consider a problem that would require a calculation of how many grams of Xe are present in 8.0 moles of Xe. The unit "grams" indicates that a mass-based conversion will be required to solve this problem. More specifically, because Xe, xenon, is an element, an atomic weight equality should be developed and applied to solve this problem.

Alternatively, consider a problem that would require a calculation of how many moles of copper (I) sulfite are present in 6,200 milligrams of copper (I) sulfite. The unit "milligrams" indicates that a mass-based conversion will be required to solve this problem. More specifically, because copper (I) sulfite, Cu₂SO₃, is a compound, a molecular weight should be calculated and applied to solve this problem.