Avogadro’s Number Notes

1. Avogadro’s Number Notes

2. Conversions • In order to convert one unit to another: Given info Units to convert into Units to convert into etc Units to cancel Units to cancel etc The “Given info is what you started with, and the final answer is everything multiplied and divided together (all units cancel out except for the final units that you want to finish with)

3. Conversions—an example

4. Avogadro’s Number Notes • The word “dozen” represents the number 12. This is independent of the actual items (can be doughnuts, marbles, atoms, etc) • In the same way, chemists use another word called a “mole” to represent a different number. • One mole (it is often abbreviated to mol.) =6.02 x 1023or602,000,000,000,000,000,000,000

5. Avogadro’s Number Notes • This number is important enough to get a name (like dozen) because it can be used to calculate how many atoms there are in an amount of substance. • For any pure element, one mole of atoms will weigh exactly the number in grams corresponding to the atomic weight of the element (found underneath the element symbol on the periodic table). • For instance, 1 mol. of N atoms will weigh 14.007 gIt can also be said that 6.94 g of Lithium will contain one mole of Li atoms (6.02 x 1023).

6. Avogadro’s Number Notes • How many moles are there in 5 grams of Carbon? • 5 g C x (1 mol. C atoms/12.01 g. C) = 0.4 moles C atoms • How many atoms are there in 5 grams of C? • 0.4 mol C atoms x 6.02 x 1023 = 2.4 x 1023 C atoms • 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 Al • See p. 324, 328, 329

7. Avogadro’s Number Notes • Calculating with moles is mostly helpful because you will be able to predict the exact ratios of elements in a substance even though the atomic weights of the different elements may be different. • For instance, in a mole of water molecules, what is the ratio of actual weights of hydrogen and oxygen (not the ratio of # of atoms)? • 1 mole of water molecules will have 1 mole of oxygen atoms, and two moles of hydrogen atoms.

8. Avogadro’s Number Notes • To calculate the mass of 1 mole of oxygen atoms: • 1 mol O x (15.99 g O/1 mol O atoms)= 15.99g O • To calculate the mass of 2 moles of hydrogen atoms: • 2 mol H x (1.007 g H/1 mol H atoms) = 2.01 g H • Total mass of 1 mole of water molecules = 15.99 g + 2.01 g = 18.0 grams • See p. 335-337

15. Avogadro’s Number Notes • Another calculation you can do is determine the % composition (by weight) of one element in a compound if you know the name or the formula. • See p. 343.

16. Avogadro’s Number Notes • You can also infer the actual ratios of atoms within compounds by comparing the weights of the substances used to create it (finding the empirical formula). A multiple of the empirical formula in a covalent compound is called a molecular formula. • See p. 345-350.

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

21. Hydrates • Sometimes there are gaps within the ions in the lattice structure, leaving room for water molecules to join. The ratio of water molecules to the other ions depends on the size of the ions, the charge of the ions, and the type of lattice • Such ionic compounds that can bind to water molecules are called hydrates.

22. Hydrates • We can find the % of water or empirical formula of a hydrate in the same way as when we did earlier for other substances • See p. 353.