# Formula Stoichiometry What is stoichiometry n Deals with

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Formula Stoichiometry

What is stoichiometry? n Deals with the specifics of QUANTITY in chemical formula or chemical reaction.

Review: Counting Atoms n n Remember subscripts tell you how many atoms… … Coefficients tell you how many TOTAL molecules compounds

How many oxygen atoms in each? n n NH 4 NO 3 (3) C 8 H 8 O 4 (4) O 3 (3) C 3 H 5(NO 3)3 (9)

Counting Atoms Practice Example #1: (NH 4)2 CO 3 * First list the types of atoms and then count each. N: 2 H: 8 C: 1 O: 3

Counting Atoms Practice Example #2: Ca. Cr. O 4 * First list the types of atoms and then count each. Ca: 1 Cr: 1 O: 4

Counting Atoms Practice Example #3: Ca 3(PO 4)2 * First list the types of atoms and then count each. Ca: 3 P: 2 O: 8 **Do the rest on your own!**

Atomic Mass n n Mass of an atom of one element based on percent abundances and masses of all isotopes (on PT) Units: amu

Molecular Mass n n Mass of a molecular compound (ex: H 2 O, CO 2) Units: amu

Formula Mass n n Mass of an ionic compound (ex: Na. Cl, Mg. I 2) Units: amu

Gram Formula Mass n Mass expressed in grams

Gram Molecular Mass H 2 O O = Molecular Mass = Gram Molecular Mass = + + H + 15. 9994 amu + 1. 00794 amu +1. 00794 amu 18. 0 g

How do I calculate molecular or formula mass? n n n First: Identify and count atoms in the compound. Second: Locate atomic mass of each element. Third: Multiply mass x number of atoms and total all elements.

Molecular Mass Example (Covalent Compound) n n Example: H 2 O H- 2 x 1 O- 1 x 16 Molecular Mass = 18 g

Formula Mass Example (ionic compound) n n Example: Na. Cl Na- 1 x 23 Cl- 1 x 35. 5 Formula Mass = 58. 5 g

Percent Composition by Mass n n Experimentally. Use masses given in problem Mass of Element x Mass of Compound 100

Example: n A compound containing carbon and hydrogen has a mass of 16 grams. When decomposed, 12. 0 grams are found to be carbon. What is the percent by mass of carbon in the compound?

Percent Composition by Atomic Mass n n n Theoretically. Use Atomic Masses Formula (#atoms of element) x (atomic mass) x 100 Formula mass of compound

Hydrated Crystals Some ionic compounds are found to have surrounding water molecules. n These compounds are called Hydrated Crystals. n The percent water hydration can be found. Example Cu. SO 4. 5 H 2 O n The dot shows that 5 H 2 O molecules are attached to 1 Cu. SO 4 molecule. n

Percent Water Hydration % Hydration = Total mass of Water x 100 Formula Mass Cu. SO 4. 5 H 2 O Cu – 1 x 63. 5 = 63. 5 S – 1 x 32 = 32 O – 4 x 16 = 64 H 2 O – 5 x 18 = 90 Formula Mass = 249. 5 % Hydration = (90/249. 5) x 100 = 36% H 2 O

The Mole n n n Avagadro’s Number is the number of atoms in 1 mole of any substance. Avagadro’s Number is 6. 02 x 1023 That means that in 1 mole of any sample of matter there are 6. 02 x 1023 particles.

The Mole n There are 6. 02 x 1023 particles in one mole of any substance n One particle can be one atom or one molecule. n The mass of 1 mole will be different for all substances. n n Atomic / Formula / Molecular Mass = Molar Mass because it is the mass of 1 mole of any substance. Example: 1 mole of H 2 O = 18 g

Mole Relationships or Equalities n 1 mole = formula mass in grams n 1 mole = 6. 02 x 1023 particles n 1 mole = 22. 4 L (for gases) n Formula mass = 6. 02 x 1023 particles n 22. 4 L of a gas = 6. 02 x 1023 particles n Formula mass = 22. 4 L of a gas

Example Conversions Converting Grams to Moles given grams X 1 moles GFM grams Converting Moles to Grams given moles X GFM grams 1 moles

Table T n n n For problems that involve mass mole conversions you can use the “Mole Calculation” formula on Table T. You will usually have to calculate the gram formula mass of the substance unless it is given. See Table T

Examples Example #1: What is the mass of 1. 75 moles of oxygen gas (Hint: oxygen is diatomic)? 1 st find the molecular mass of oxygen remember oxygen is a diatomic element (O 2). O: 2 x 16 g= 32 g Convert from moles to grams using factor label method Factor Label Setup 1. 75 mol x __32 g _ = ? g 1 mol 56 g

Examples Example #2: How many moles are in 42 g of water? 1 st find the molecular mass of H 2 O. Convert from grams to moles… H: 2 x 1 = 2 42 g x __ 1 mol _ = 2. 3 ? mol O: 1 x 16 = 16 2 + 16 = _18 g_ 1 mol 18 g

Examples Example #3: How many moles of potassium chromate are in a 500 g sample? 1 st find the molecular mass of potassium dichromate (K 2 Cr. O 4). Convert from grams to moles… K: 2 x 39 = 78 Cr: 1 x 52 = 52 500 g x __ 1 mol _ = ? 2. 6 mol O: 4 x 16 = 64 194 g 78+104+112 = _194 g_ 1 mol

Empirical Mass Step 1: CH 2 O Empirical Mass C – 1 x 12 = 12 H– 2 x 1= 2 O – 1 x 16 = 16 30 Step 2: Calculate Multiple (Formula mass/Empirical mass) 180/30 = 6 Step 3: Multiply Empirical Formula by Multiple 6(CH 2 O) = C 6 H 12 O 6

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