Chapter 11 - The MoleChapter 11 - The Mole

Measuring MatterMeasuring Matter

Sections 11.1 Sections 11.1 11.3 11.3

Measuring MatterMeasuring MatterOne pair = 2One pair = 2 One dozen = 12One dozen = 12

One gross = 144One gross = 144 One ream = 500One ream = 500

Measuring a MoleMeasuring a Mole

Because atoms are so tiny, when chemists Because atoms are so tiny, when chemists count atoms, they use a unit of count atoms, they use a unit of measurement called a “mole” which measurement called a “mole” which equals 6.02 x 10 equals 6.02 x 10 23 23 atoms. atoms. This is a very This is a very large number because it measures large number because it measures extremely tiny items.extremely tiny items.

602,000,000,000,000,000,000,000602,000,000,000,000,000,000,000

Putting the Mole into PerspectivePutting the Mole into PerspectiveOne mole of pennies would allow every person on Earth to One mole of pennies would allow every person on Earth to spend a million dollars an hour, day and night, and still spend a million dollars an hour, day and night, and still have half of it unspent when they diedhave half of it unspent when they died

One mole of popcorn would cover every dry One mole of popcorn would cover every dry surface of the Earth to a depth of six milessurface of the Earth to a depth of six miles

One mole of stacked pennies would reach to the moon One mole of stacked pennies would reach to the moon and back 16 timesand back 16 timesOne mole of marshmallows One mole of marshmallows

would cover the United States would cover the United States to a depth of 650 milesto a depth of 650 miles

History of Avogadro’s NumberHistory of Avogadro’s NumberAvogadro: 1811 – equal volumes of gases at the Avogadro: 1811 – equal volumes of gases at the same temperature and pressure contain equal same temperature and pressure contain equal numbers of moleculesnumbers of molecules Cannizzaro: 1861 – used Avogadro’s hypothesis, Cannizzaro: 1861 – used Avogadro’s hypothesis, developed a set of atomic weights for the known developed a set of atomic weights for the known elements based on comparing the weights of equal elements based on comparing the weights of equal volumes of gasvolumes of gasLoschmidt: 1865 – determined the number of atoms Loschmidt: 1865 – determined the number of atoms present in a given volume of airpresent in a given volume of airOstwald: 1902 – developed the concept of the moleOstwald: 1902 – developed the concept of the mole1971 – the true value, 1971 – the true value, 6.02x106.02x102323, was actually , was actually determined and is called determined and is called Avogadro’s Avogadro’s number since number since the entire string of events started with his findings in the entire string of events started with his findings in 18111811

Molecular Mass of AtomsMolecular Mass of Atoms

Determined by number of protons and neutrons it Determined by number of protons and neutrons it containscontainsExample 1Example 1: Hydrogen atom – has one proton : Hydrogen atom – has one proton

and therefore has a molecular mass and therefore has a molecular mass

equal to 1 amuequal to 1 amu

Example 2Example 2: Oxygen atom – 8 protons and 8 neutrons : Oxygen atom – 8 protons and 8 neutrons

and therefore has a molecular mass of and therefore has a molecular mass of

16 amu, making it 16 times heavier than 16 amu, making it 16 times heavier than

a hydrogen atoma hydrogen atom

Molecular Masses and Ratios of Molecular Masses and Ratios of AtomsAtoms

The molecular mass of Hydrogen is 1 amu and the molecular The molecular mass of Hydrogen is 1 amu and the molecular mass of Oxygen is 16 amu, so the ratio by mass of Hydrogen mass of Oxygen is 16 amu, so the ratio by mass of Hydrogen to Oxygen is 1:16. to Oxygen is 1:16.

An atom of Oxygen (8 pAn atom of Oxygen (8 p++, 8 n, 8 n00) is 16 times heavier than an ) is 16 times heavier than an atom of Hydrogen (1 patom of Hydrogen (1 p++))

Hydrogen = 1 amu Oxygen = 16 amuHydrogen = 1 amu Oxygen = 16 amu

Lithium = 7 amu Chlorine = 35 amuLithium = 7 amu Chlorine = 35 amu

The Law of Fixed Proportions and The Law of Fixed Proportions and Ratios of AtomsRatios of Atoms

Law of Fixed Proportions – atoms combine in Law of Fixed Proportions – atoms combine in small whole number ratios to form compoundssmall whole number ratios to form compounds

Example: Water- two atoms of Hydrogen will Example: Water- two atoms of Hydrogen will always combine with one atom of Oxygen to always combine with one atom of Oxygen to produce a molecule of waterproduce a molecule of water

2 Hydrogen atoms + 1 Oxygen atom = 1 H2 Hydrogen atoms + 1 Oxygen atom = 1 H22O moleculeO molecule

4 Hydrogen atoms + 2 Oxygen atoms = 2 H4 Hydrogen atoms + 2 Oxygen atoms = 2 H22O moleculesO molecules

8 Hydrogen atoms + 4 Oxygen atoms = 4 H8 Hydrogen atoms + 4 Oxygen atoms = 4 H22O moleculesO molecules

Unit of Measurement for Unit of Measurement for Molar MassMolar Mass

To ensure chemists have the correct number of atoms to To ensure chemists have the correct number of atoms to form a compound they use the concept of the mole, or form a compound they use the concept of the mole, or molar massmolar massA convenient metric measurement of mass for humans is A convenient metric measurement of mass for humans is the gram; therefore chemists utilize a unit called the “gram the gram; therefore chemists utilize a unit called the “gram mole”mole”Since a Hydrogen atom has a mass of 1 amu, then the Since a Hydrogen atom has a mass of 1 amu, then the Hydrogen atom would have a molar mass of one gram moleHydrogen atom would have a molar mass of one gram moleSince Oxygen has a mass of 16 amu’s, then Oxygen would Since Oxygen has a mass of 16 amu’s, then Oxygen would have a molar mass of 16 gram moleshave a molar mass of 16 gram molesThis keeps the mass ratios between atoms the same This keeps the mass ratios between atoms the same because the mass ratio between hydrogen and oxygen is because the mass ratio between hydrogen and oxygen is still 1:16still 1:16

The atomic mass ratio of hydrogen to oxygen is 1 amu : 16 amu

The gram mole ratio between hydrogen and oxygen is 1 g : 16 g

So, chemists can determine how many atoms are present in a sample.

One gram of hydrogen would contain exactly 6.02x1023 atoms, or Avogadro’s Number of atoms. Two grams of hydrogen atoms would contain 12.04x1023 hydrogen atoms.

Also, 16 gram moles of oxygen would contain 6.02x1023 atoms. Remember that an atom of oxygen is 16 times heavier than an atom of hydrogen, and that is why the number of atoms is the same, but the mass is 16 grams.

Therefore: 1 gram of hydrogen = 6.02x1023 atoms = 1 mole

2 grams of hydrogen = 12.04x1023 atoms = 2 moles

16 grams of oxygen = 6.02x1023 atoms = 1 mole

32 grams of oxygen = 12.04x1023 atoms = 2 moles

The Law of Conservation of Mass The Law of Conservation of Mass and Ratios of Atomsand Ratios of Atoms

According to the Law of Conservation of Mass, atoms According to the Law of Conservation of Mass, atoms combine in small whole number ratios to form combine in small whole number ratios to form compounds. So the production of one water molecule compounds. So the production of one water molecule requires 2 hydrogen atoms and 1 oxygen atom.requires 2 hydrogen atoms and 1 oxygen atom.

Therefore:Therefore:

two atoms of hydrogen + one atom of oxygen two atoms of hydrogen + one atom of oxygen 1 water molecule 1 water molecule

AndAnd

12.04x1012.04x102323 hydrogen atoms + 6.02x10 hydrogen atoms + 6.02x102323 oxygen atoms oxygen atoms 6.02x106.02x102323 water molecules water molecules

In conclusion:

2 atoms of hydrogen + 1 atom of oxygen = 1 water molecule

2 amu of hydrogen + 16 amu’s of oxygen = 18 amu’s of water molecules

2 g of hydrogen + 16 grams of oxygen = 18 grams of water molecules

12.04x1023 atoms H + 6.02x1023 atoms O = 6.02x1023 atoms H2O molecules

2 moles of H atoms + 1 mole of O atoms = 1 mole of water molecules