Avogadro’s Number NotesAvogadro’s Number Notes

ConversionsConversions

In order to convert one unit to another:In order to convert one unit to another:

Given infoGiven info Units to convert intoUnits to convert into Units to convert into Units to convert into etcetc

Units to cancelUnits to cancel Units to cancelUnits to cancel etcetc

The “Given info is what you started with, and The “Given info is what you started with, and the final answer is everything multiplied and the final answer is everything multiplied and divided together (all units cancel out except divided together (all units cancel out except for the final units that you want to finish with)for the final units that you want to finish with)

Conversions—an exampleConversions—an example

Avogadro’s Number NotesAvogadro’s Number Notes

The word “dozen” represents the number 12. This The word “dozen” represents the number 12. This is independent of the actual items (can be is independent of the actual items (can be doughnuts, marbles, atoms, etc)doughnuts, marbles, atoms, etc)

In the same way, chemists use another word In the same way, chemists use another word called a “mole” to represent a different number.called a “mole” to represent a different number.

One mole (it is often abbreviated to mol.) =One mole (it is often abbreviated to mol.) =6.02 x 106.02 x 102323

oror602,000,000,000,000,000,000,000602,000,000,000,000,000,000,000

Avogadro’s Number NotesAvogadro’s Number Notes This number is important enough to get a name This number is important enough to get a name

(like dozen) because it can be used to calculate (like dozen) because it can be used to calculate how many atoms there are in an amount of how many atoms there are in an amount of substance. substance.

For any pure element, one mole of atoms will weigh For any pure element, one mole of atoms will weigh exactly the number in grams corresponding to the exactly the number in grams corresponding to the atomic weight of the element (found underneath the atomic weight of the element (found underneath the element symbol on the periodic table). element symbol on the periodic table).

For instance, 1 mol. of N atoms will weigh 14.007 gFor instance, 1 mol. of N atoms will weigh 14.007 gIt can also be said that 6.94 g of Lithium will contain It can also be said that 6.94 g of Lithium will contain one mole of Li atoms (6.02 x 10one mole of Li atoms (6.02 x 102323). ).

Avogadro’s Number NotesAvogadro’s Number Notes How many moles are there in 5 grams of Carbon?How many moles are there in 5 grams of Carbon?

5 g C x (1 mol. C atoms/12.01 g. C) = 5 g C x (1 mol. C atoms/12.01 g. C) = 0.4 moles C atoms0.4 moles C atoms

How many atoms are there in 5 grams of C?How many atoms are there in 5 grams of C?

0.4 mol C atoms x 6.02 x 100.4 mol C atoms x 6.02 x 102323 = 2.4 x 10 = 2.4 x 102323 C C atomsatoms

If you have 2.2 moles of Al, how many grams?If you have 2.2 moles of Al, how many grams?

2.2 mol Al x (26.98 g Al/1 mol Al atoms) = 59 g Al2.2 mol Al x (26.98 g Al/1 mol Al atoms) = 59 g Al

See p. 324, 328, 329See p. 324, 328, 329

Avogadro’s Number NotesAvogadro’s Number Notes

Calculating with moles is mostly helpful because Calculating with moles is mostly helpful because you will be able to predict the exact ratios of you will be able to predict the exact ratios of elements in a substance even though the atomic elements in a substance even though the atomic weights of the different elements may be different.weights of the different elements may be different.

For instance, in a mole of water molecules, what is For instance, in a mole of water molecules, what is the ratio of actual weights of hydrogen and oxygen the ratio of actual weights of hydrogen and oxygen (not the ratio of # of atoms)?(not the ratio of # of atoms)?

1 mole of water molecules will have 1 mole of 1 mole of water molecules will have 1 mole of oxygen atoms, and two moles of hydrogen atoms.oxygen atoms, and two moles of hydrogen atoms.

Avogadro’s Number NotesAvogadro’s Number Notes

To calculate the mass of 1 mole of oxygen atoms:To calculate the mass of 1 mole of oxygen atoms: 1 mol O x (15.99 g O/1 mol O atoms)= 15.99g O1 mol O x (15.99 g O/1 mol O atoms)= 15.99g O To calculate the mass of 2 moles of hydrogen To calculate the mass of 2 moles of hydrogen

atoms:atoms: 2 mol H x (1.007 g H/1 mol H atoms) = 2.01 g H2 mol H x (1.007 g H/1 mol H atoms) = 2.01 g H Total mass of 1 mole of water molecules = Total mass of 1 mole of water molecules =

15.99 g + 2.01 g = 18.0 grams15.99 g + 2.01 g = 18.0 grams See p. 335-337See p. 335-337

Avogadro’s Number NotesAvogadro’s Number Notes

Another calculation you can do is determine Another calculation you can do is determine the % composition (by weight) of one the % composition (by weight) of one element in a compound if you know the element in a compound if you know the name or the formula.name or the formula.

See p. 343.See p. 343.

Avogadro’s Number NotesAvogadro’s Number Notes

You can also infer the actual ratios of atoms You can also infer the actual ratios of atoms within compounds by comparing the weights within compounds by comparing the weights of the substances used to create it (finding of the substances used to create it (finding the the empirical formulaempirical formula). A multiple of the ). A multiple of the empirical formula in a covalent compound is empirical formula in a covalent compound is called a called a molecular formulamolecular formula..

See p. 345-350.See p. 345-350.

HydratesHydrates

In ionic compounds—In ionic compounds—empirical formula tells empirical formula tells the ratio of atoms, and the ratio of atoms, and the ions form lattice the ions form lattice structures of structures of alternating ions. alternating ions.

HydratesHydrates

Sometimes there are gaps within the ions in the Sometimes there are gaps within the ions in the lattice structure, leaving room for water lattice structure, leaving room for water molecules to join. The ratio of water molecules molecules to join. The ratio of water molecules to the other ions depends on the size of the ions, to the other ions depends on the size of the ions, the charge of the ions, and the type of latticethe charge of the ions, and the type of lattice

Such ionic compounds that can bind to water Such ionic compounds that can bind to water molecules are called molecules are called hydrateshydrates. .

HydratesHydrates

We can find the % of water or empirical formula We can find the % of water or empirical formula of a hydrate in the same way as when we did of a hydrate in the same way as when we did earlier for other substancesearlier for other substances

See p. 353.See p. 353.