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Stoichiometry and the Mole Study Guide

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Updated on Sep 24, 2011

Introduction

Atoms and molecules are too small to count and would require extremely large numbers. Chemists must have an understanding and a unit of measurement to answer "How much is there?" The mole (6.022 * 1023) is a unit of measurement that describes matter just as a gross or case describes a quantity of consumer products.

The Mole

In this lesson you'll learn how to define how many atoms, molecules, and/or ions are in a sample. Because these particles are extremely small, a very large number would be required to describe a sample size. Eggs are sold by the dozen (12), soda by the case (24), and paper by the ream (500). Amedeo Avogadro's work on gases helped define a new unit of measurement for chemistry and physics: the mole. A mole of a particular substance is equal to the number of atoms in exactly 12 g of the carbon-12 isotope. Experiments established that number to be 6.022142 * 1023 particles.

Avogadro's number = NA = 6.022 * 1023 items/mole

In other words, one mole equals 6.022 * 1023 items. The units of the mole are modified depending on the units needed in a calculation. Units could be molecules, atoms, ions, or even everyday items. Is anyone craving one mole of donuts?

Molar Masses

Because the number of grams per element defines the mole, the atomic masses on the periodic table can be given in the units g/mol. Carbon has a molar mass of 12.01 g/mol, and iron has a molar mass of 55.85 g/mol. The molar mass of a molecule is calculated by adding all the atomic molar masses.

Molar Calculations

Example 1:

What is the molar mass of carbon dioxide?

Solution 1:

Carbon dioxide is CO2, which contains 1 atom of carbon and 2 atoms of oxygen.

Molar mass of CO2 = 12.01 + 2(16.00) = 44.01 g/mol

Example 2:

How many moles are there in 62.5 grams of NaHCO3?

Solution 2:

The molar mass of sodium bicarbonate is needed.

22.99 + 1.008 + 12.01 + 3(16.00) = 84.008 g/mol

62.5 g = 0.744 moles NaHCO3

Example 3:

How many atoms are there in 46 grams of carbon?

Solution 3:

The first step is to use the molar mass to convert grams of carbon to moles of carbon. Once the moles are obtained, Avogadro's number can be used to calculate the number of atoms.

46 g carbon = 2.3 * 1024 atoms

Consider the following:

  • One mole of carbon is 12.01 g and contains 6.022 * 1023 atoms.
  • One mole of oxygen (O2) is 32.00 g and contains 6.022 * 1023 molecules.
  • One mole of carbon dioxide (CO2) is 44.01 g and contains 6.022 * 1023 molecules.
  • One mole of sodium chloride (NaCl) contains 58.44 g and contains 1 mole of sodium ions (Na+) and 1 mole of chloride ions (Cl).

A molar mass of a molecule can also be called a molecular mass.

Note

The molecular weight is the sum of the atomic weights of all the atoms in a molecular formula. It is the same number as the molar mass (in grams) without the unit.

Percent Composition

The number of atoms or the mass percent of its elements can describe a compound's composition. The percent composition by mass can be calculated by comparing the mass of one mole of each element to the molar mass.

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