FORMULA MASS. 17.2 Conservation of mass  Lavoisier showed that a closed system must be used when studying chemical reactions.  When chemicals are reacted.

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Presentation transcript:

FORMULA MASS

17.2 Conservation of mass  Lavoisier showed that a closed system must be used when studying chemical reactions.  When chemicals are reacted in a closed container, you can show that the mass before and after the reaction is the same.

17.2 Formula mass  The sum of the atomic mass values of the atoms in a chemical formula is called the formula mass.

Ex 1 Find the formula mass of O 2 Ex 2 Find the formula mass of CO 2

Atomic & Molecular Weights Chemical formulas and chemical equations have quantitative significance. H 2 O means 2 atoms of hydrogen and 1 atom of oxygen. C 3 H 8 + 5O 2 → 3CO H 2 O means 5 moles of oxygen gas

Definitions:  Molecular Formula: the actual number of atoms of each element in the compound.  Molar mass: the mass of one mole of any substance  Atomic mass unit: amu

17.2 Molar Mass The mass (in grams) of one mole of a compound is called its molar mass.

What is the molar mass of one mole of CaCO 3 ? 1.Looking for: – … molar mass of CaCO 3 2.Given – … chemical formula 3.Relationships: – no. amu in formula = molar mass in grams Solving Problems

4.Solution Solving Problems Formula mass CaC0 3 = amu 1 mole CaC0 3 = g CaCO 3

17.2 Avogadro’s Number  The Avogadro number was named in honor of Amedeo Avogadro who discovered that a mole of any gas under the same conditions has the same number of molecules.  Johann Josef Loschmidt, a German physicist, named and discovered the Avogadro number.  Loschmidt realized that a mole of any substance—be it a gas, liquid, or solid— contains 6.02 x atoms or molecules.

Avogadro’s Number  Avogadro’s number states that for one mole of any substance, whether element or compound, there 6.02x10 23 Particles present in the sample.  It is also the number of molecules in the formula mass of a compound when these masses are expressed in grams.  One set of 6.02x10 23 atoms or molecules is also referred to as a mole of that substance.

Remember 1 gram = x10 23 amu The atomic mass unit(amu) is assigned by assigning exactly 12 amu to the 12 C isotope Elements occur in Nature as isotopes.

 1 oxygen atom = 16.0grams  1mole of oxygen atoms=6.02x10 23 atoms=16.0grams

Formula Weight & Molecular Weight Formula Weight – Ionic Compounds Molecular Weight – Molecular/Covalent Compounds ****We tend to use interchangeably**** To find the Formula/Molecular Weight Simply find the sum of the atomic weights of each atom in the compound.