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# Ch. 8 Formula Stoichiometry

Ch. 8 Formula Stoichiometry. Stoichiometry – studies the mathematical relationships involving chemical formulas and equations. Atomic Mass. The mass of an atom is very very small: oxygen atom 2.66 x 10 -23 grams hydrogen atom 1.67 x 10 -24 grams . Atomic Mass.

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## Ch. 8 Formula Stoichiometry

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1. Ch. 8 Formula Stoichiometry Stoichiometry – studies the mathematical relationships involving chemical formulas and equations.

2. Atomic Mass • The mass of an atom is very very small: • oxygen atom 2.66 x 10-23 grams • hydrogen atom 1.67 x 10-24 grams

3. Atomic Mass • Although it is possible to measure the actual mass of an atom, it is more convenient to use relative masses. • The relative mass of an atom is a comparison of the atom’s mass to the mass of a carbon-12 atom. • Carbon-12 was chosen as the standard for measuring relative atomic masses.

4. Atomic Mass Unit • The mass of a carbon-12 atom = exactly 12 u • One atomic mass unit (amu or u) =1/12 the mass of a carbon-12 atom.

5. Some atomic masses: • Hydrogen 1.008 u • Oxygen 16.00 u • Iron 55.85 u What is the atomic mass of: Nitrogen 14.01 u Chlorine 35.45 u Magnesium 24.31 u

6. Formula Mass • The sum of the atomic masses of all the atoms in a chemical formula. • Formula mass can refer to an element or an ionic or molecular compound. • EX: Formula mass of CO2 is: 12.01 + 2 x 16.00 = 44.01 u

7. What is the formula mass of Mg(NO2)2? • 1 Mg + 2 N + 4 O • 24.31 u + (2 x 14.01 u) + (4 x 16.00 u) = 116.3 u • Calculate the formula mass of: • 1) NaCl 2) NO2 3) CaCO3 • 4) Ba(NO3)2 5) (NH4)3PO4

8. Molecular Mass • molecular mass is the formula mass of a molecular substance. It is the mass of a molecule of that substance. • EX: the molecular mass of C6H12O6 = 6 X 12.01 + 12 X 1.008 + 6 X 16.00 • = 180.2 u

9. The Mole

10. The Mole: Basic Concepts Measuring Matter • Chemists need a convenient method for counting accurately the number of atoms, molecules, or formula units in a sample of a substance. • As you know, atoms and molecules are extremely small. There are so many of them in even the smallest sample that it’s impossible to actually count them. • That’s why chemists created their own counting unit called the mole.

11. The mole, commonly abbreviated mol, is the SI base unit used to measure the amount of a substance. • 1 mole (mol) = 6.02 x 1023 particles • “particles” may be atoms, molecules, or formula units The number 6.02 x 1023 is called Avogadro’s number (in honor of Italian Amedeo Avogadro who determined the volume of one mole of a gas.) • If you write out Avogadro’s number, it looks like this: 602 000 000 000 000 000 000 000

12. A representative particle is any kind of particle such as atoms, molecules, formula units, electrons, or ions. • We will now look at one-mole quantities of three substances, each with a different representative particle.

13. The representative particle in a mole of water is the water molecule. • 1 mol H2O = 6.02 x 1023 H2O molecules

14. The representative particle in a mole of copper is the copper atom. • 1 mol Cu = 6.02 x 1023 Cu atoms

15. The representative particle in a mole of sodium chloride is the formula unit. • 1 mol NaCl =6.02 x 1023 NaClformula units • The term “formula unit”is used to represent one“unit” of an ionic compound.

16. Mole  Particle Conversions Avagadro's number is used as a conversion factor to convert between # of moles and # of particles (atoms, molecules, ions, etc.)

17. Converting Moles to Particles • Suppose you want to determine how many particles of sucrose are in 3.50 moles of sucrose. You know that one mole contains 6.02 x 1023 representative particles. • Therefore, you can write a conversion factor, Avogadro’s number, that relates representative particles to moles of a substance.

18. There are 2.11 x 1024 molecules of sucrose in 3.50 moles of sucrose. EX: How many iron atoms are in 0.025 mol of Fe?

19. Converting Particles to Moles • Now, suppose you want to find out how many moles are represented by a certain number of representative particles. • You can use the inverse of Avogadro’s number as a conversion factor.

20. Zinc is used as a corrosion-resistant coating on iron and steel. It is also an essential traceelement in your diet. • Calculate the number of moles that contain 4.50 x 1024 atoms of zinc (Zn).

21. The Mass of a Mole • Technically, the mole is defined as the number of carbon-12 atoms in exactly 12 g of pure carbon-12. (This has experimentally been shown to be 6.02 x 1023).

22. Thus, the mass of one mole of carbon-12 atoms is 12 g. What about other elements? (Whether you are considering a single atom or Avogadro’s number of atoms (a mole), the masses of all atoms are established relative to the mass of carbon-12. ) (Play Video) Molar mass – the mass of one mole of asubstance. The molar mass of a substanceis equal to its formula mass expressed in grams. (units for molar mass are g/mol)

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