Moles and Mass Reference Page
What Is a Mole?

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  • Video links go directly to the YouTube video.
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  • TFS (The Full Story) videos include interviews, additional explanation, and equations written on whiteboards. Same math, more context, more personality. Each has a matching JTF version.
  • Info Page links lead to definitions, expanded explanations, and related material—because sometimes you really do need to explain it.
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Purpose of Moles, Mass, Mole and Mass Fractions

The mole is one of the fundamental concepts in chemical engineering. It provides a convenient way to count extremely large numbers of molecules and allows engineers to relate chemical quantities to measurable masses.

In chemistry courses, the mole is usually expressed using grams. In chemical engineering practice, however, engineers often work with many different mass units, including pounds-mass \((\mathrm{lb_m})\), tons, and kilograms. For each mass unit there is a corresponding mole unit.

For a given chemical species, the numerical value of the molar mass does not change with the mass unit, only the unit itself changes. For example, sodium chloride (\(\mathrm{NaCl}\)) has a molar mass of \(58.443\). Therefore

  • a mole (gram-mole) has a mass of \(58.443\ \mathrm{g}\)
  • a pound-mole has a mass of \(58.443\ \mathrm{lb_m}\)
  • a ton-mole has a mass of \(58.443\ \mathrm{ton}\)

(One may need to specify whether the ton is a short ton or long ton.)

The videos on this page explain what a mole is and how it is used in basic chemical engineering calculations.

Unlike in chemistry, where a mole is a mole is a mole, chemical engineers must routinely translate among different mass units and the corresponding mole units.

What is a Mole? Just the Facts

The Just the Facts video gets straight to the definition of the mole and demonstrates how it is used in engineering calculations.

Visuals

What is a Mole? The Full Story

The Full Story video examines common conceptions and misconceptions about moles before explaining what a mole represents and how it is used in chemical engineering calculations.

Visuals

What the Frac?

This video introduces mass fractions and mole fractions, two common ways of expressing the composition of mixtures. It shows how to convert between mass-based and mole-based descriptions of composition and explains when each form is useful in engineering calculations.

Visuals

Definitions

Avogadro’s Number
Avogadro’s number, or the Avogadro constant \(N_\mathrm{A}\), has the exact value
\(6.022\ 140\ 76 \times 10^{23}\ \mathrm{mol^{-1}}\).
It represents the number of specified particles (atoms, molecules, ions, etc.) in one mole.
Gram Atom
The term gram atom historically referred to one mole of atoms of an element. The term is now largely obsolete but is occasionally encountered in older chemical engineering literature.
Molar Mass
The molar mass of a substance is the mass per mole of that substance. It is usually expressed in units of \(\mathrm{g/mol}\) or \(\mathrm{lb_m/lb\text{-}mol}\). See Wikipedia Molar mass for additional details.
Mole or Gram Mole
A mole (abbreviation \(\mathrm{mol}\)) is the amount of substance containing exactly \(6.022\ 140\ 76 \times 10^{23}\) specified entities. In chemical engineering the term gram mole (\(\mathrm{g\text{-}mol}\)) is sometimes used to emphasize that the associated mass unit is grams.
Pound Mass
A pound mass is the mass that exerts a force of one pound force when accelerated at the standard gravitational acceleration of \(32.174\ \mathrm{ft/s^2}\). NIST lists the pound mass as equal to \(0.453\ 592\ 37\ \mathrm{kg}\). The symbol is \(\mathrm{lb_m}\).
Pound Mole
A pound mole (abbreviation \(\mathrm{lb\text{-}mol}\)) is the amount of substance whose mass in pounds is numerically equal to the molar mass. For example, a pound mole of \(\mathrm{NaCl}\) has a mass of \(58.443\ \mathrm{lb_m}\).