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Calculating Molecular Formula

Mole (mol) : The SI base unit that measures an amount of a substance; 1 mole of molecules has a mass equal to the molecular weight in grams. 1 mole = 6.02x10 23 atoms (molecules)

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Calculating Molecular Formula

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  1. Mole (mol): The SI base unit that measures an amount of a substance; 1 mole of molecules has a mass equal to the molecular weight in grams. • 1 mole = 6.02x1023 atoms (molecules) • When looking at a chemical reaction, the number of moles are indicated by the coefficients in front of the formula’s. • Avogadro’s Number: The number of molecules in one mole for any substance; 1mole of particles = 6.02 x 1023 2 moles of particles = 12.04 x 1023 3 moles of particles = 18.06 x 1023 etc . . .

  2. Calculating Molecular Formula • When supplied with the Empirical Formula, and the Molecular Mass, we are capable of calculating the Molecular Formula with the following equation: Mass MF Mass EF Example: The empirical formula is CH and the molecular mass is 26, what is the molecular formula?

  3. Gram Formula Mass (GFM) • The GFM is the total mass of any substance (element or compound) Example: K2CO3 (2 K atoms, 1 C atom, and 3 O atoms) Mass K = 39g Mass C = 12g Mass O = 16g (39x2) + (12x1) + (16x3) = 138 grams The gram formula mass of K2CO3 = 138g

  4. WHAT YOU KNOW!!!! GFM = 1 mole = 6.02 x 1023 molecules = 22.4 L • This is the most important information for you to memorize in order to master this unit. • The above means that the GFM of any substance (element or compound) is the same thing as saying 1 mole of that substance, which is the same as saying there are 6.02 x 1023 molecules of that substance, which is the same as saying there is 22.4 L of that same substance!!

  5. MoleMass / MassMole Converstions • Table T: mole = given mass GFM Example: If you have 54 grams of LiF, how many moles do you have? Example: If you have 2.3 moles of CuCl2, how many grams do you have?

  6. Stoichiometry Mole-Mole Problems: • Answers how many moles of one element or compounds react with a given number of moles of another element or compound. Example: How many moles of Ca are needed to react completely with 6 moles of H2O in the following reaction: Ca + 2H2O  Ca(OH)2 + H2 • Cross out anything in the equation that the problem is not considering. • Write the coefficient of each substance remaining under the element or compound. • On top of the element or compound write the number of moles given in the problem. • Set up a proportion:

  7. Percent Composition • The percentage by mass of each of the elements in compound • Table T: % composition = mass part x 100 mass whole Example: What is the percent by mass of magnesium in magnesium oxide (MgO)?

  8. Percent Composition • Hydrate: a compound that incorporates water molecules into its fundamental solid structure. The compound has a dot after it followed by the number of water molecules attached. • Ex: CaSO4•2H2O (SOLVE THE GFM) • The dot in the hydrate equates to an addition sign when calculating the GFM of a hydrate.

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