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Reactions and Reactivity: The Mole

Objective

In this lesson we will review the concept of the mole and how it is used to weigh and count atoms and molecules.

Previously we covered…

  • Equations for reactions occurring in aqueous solution can be written showing only the ions which react.
  • Molecular equations include all of the compounds and elements taking part in the reaction, including their physical state—solid, liquid, gas or aqueous.
  • Ionic equations are written when solutions of ionic compounds are involved in the reaction.
  • Net ionic equations are used to show only the ions which react to form products.
  • Spectator ions are not shown in net ionic equations.

The Mole

The concept of the mole is ubiquitous in chemistry and is involved in virtually all chemical calculations. The mole provides a way for chemists to count molecules by weighing (or massing) them. The mole is merely a very large number of items, or specifically 6.02 × 1023 items, just as one dozen is 12 items.

The concept of atomic mass has previously been discussed in terms of the mass of one atom having a mass in atomic mass units (amu) equal to the average atomic weight found on the Periodic Table. Or the mass of a molecule is equal to the sum of the masses of the atoms contained in the molecule. The sum of the masses in grams of all of the subatomic particles in a mole of any atom is remarkably close to the atomic mass shown on the periodic table for the element. (The small difference is due to the presence  of isotopes and the fact that a bound nucleus has a different mass than the unbound mass of the  articles—but that we’ll leave for yet another  lesson.)

Say, for example, we took a handful of paperclips and measured its mass. Then we measured the mass of exactly one dozen paper clips (12), we would have a way to determine how many dozen paperclips were in the handful. Simply divide the mass of the handful by the mass of one dozen. The mass of one mole of atoms of an element or the mass of one mole of molecules of a compound is called the molar mass. If a sample of sodium chloride is 454 g, and the molar mass of sodium chloride is 58.5 g, dividing the total mass by the mass of one mole will give the number of moles of sodium chloride in that sample

However, if given the same mass of sugar, 454 g, with a molar mass of 342 g, the number of moles in that sugar sample is 1.33. Because the sugar molecule is larger, it takes fewer moles to account for the same mass as the salt. If the number of moles of a substance is known, the number of grams present can be determined by multiplying the number of moles by the molar mass. If we are told that we need 0.0400 moles of sugar in a solution, we need to measure that amount using a balance. Balances, however, measure in grams.

To determine the number of grams of sugar, we multiply the number of moles by the molar mass of the sugar.

Question

Which of the following contains the largest number of molecules?

  1. 500 g of H2
  2. 500 g of N2
  3. 500 g of O2
  4. 500 g of NH3

Reveal Answer

The correct answer is A because H2 has the smallest molar mass of the four compounds. To obtain the number of moles, the mass is divided by the molar mass. The smaller the molar mass, the greater the number of moles.

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