Unit 6 Stoichiometry

6 units PresentationBy: Tyree Bradley

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Index

Moles

Empirical Formulas

Mole Conversion

Molecular Formulas

Practice Problems

Percent Composition

Stoichiometry

Moles (Mol)

Moles are the SI base unit of the amount in a substance. In a short definition it is a unit measurement.One mole equals : 6.022x1023 particles. This is called Avogadro's Number! Using moles help us to convert moles into atoms, molecules, and grams. A way to do moles is through molar mass!

Example

Molar Mass

KeysGram Moles or (g/mol) = Molar MassMolar mass in elements = Atomic Numbers Atomic number = amu or u

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Example: NO2N - 14.007O2- 16.00 ( Because there is a 2 with oxygen we multiple its atomic mass by 2)14.007 x 16.00(2) = 448.224 g/mol

Formula Mass

Average mass of one unit of a formula Measured = unified atomic mass units (U)Formula Unit = Compound or Molecule

Video:

Mole conversion or mole to mole conversion is a concept used in chemistry to convert the number of moles of one substance to the number of moles of another substance, based on the balanced chemical equation. It allows you to relate the amounts of reactants and products in a chemical reaction.

How to use the cheat sheet!

Mole Conversion

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Mole Conversion Example

2 H2 + O2 --- 2 H2O For this example, There are two moles of hydrogen gas ( H2 ) reacting with one moles of oxygen gas (O2) to produce two moles of water (H2O).Lets say you want to determine how many moles of water can produced from 4 moles of hydrogen gas. ( Then you do mole conversion! ) 1. Identify the given quantity: 4 Moles of H2 2. use the balance equation to set up conversion factor: 2 moles of H2 = 2 moles of H2O(This ratio is obstained from the cofficients of the balanced equation)3. Set up the conversion : ( 4 moles H2 ) x ( 2 moles H2O / 2 moles H2) = 4 moles H2O 4. The moles of H2 cancel out which leaves you with mols of H2O 5. Which leaves us with 4 moles of hydrogen gas which can produce 4 moles of water

Percent Compositon

Percent Composition is the percentage, by mass, of each element in the compound. Calculating Percent Composition % composition = Molar mass of element # of atoms/ Formula mass of compound x 100% How to calculate percent composition 1. Determine the number of atoms from each element 2. Put the formula mass for the compound and element 3. Use the formula to calculate Mass percent = ( mass of element in 1 mole of compound/ Molar mass in compound) x 100%

Percent Composition Example

First your problem is to determine the percent composition of hydrogen in H2O1. H(2 ) = 1.008u( multiply by 2) Oxygen = 16.00u 2. H2O = 1.008(2) + 16.00 = 32.256 u 3. ( 1.008(2)u) / 32.256u = 0.0625 4. 0.0625 x 100% = 6.25 Which means hydrogen make up 6.25% of H2O!

Empirical Formula

Empirical formula is the whole-number ratio of atoms in a compound

Step 1 % to Grams

Na : 22.45% - 22.45g H : 16.55% - 16.55gC: 45.06% - 45.06

Grams to moles

Na : 22.45g/ 22.99g/mol = 0.976 moles of NaH : 16.55g/1.008g/mol = 16.418 moles of HC : 45.06g/ 12.01g/mol = 3.751 moles of C

Divde all calculated moles by smallest number moles

Na : 0.976/ 0.976 = 1H : 16.418/ 0.976 = 16.821C : 3.751/ 0.976 = 3.843NaH 16 C 3

36.008/ 75.148 = 0.479

Molecular Formula

Molecular Formula is the actual whole-number ratio of atoms in a compund

Find the formula mass of empirical formula

Empirical formula: NaH 16 C 3 22.99 + 1.008 + 12.01 = Molecular mass - 36.008 22.99 + 1.008 x 16 +12.01 x 3 =Formula mass - 75.148

Divide molecular mass by empirical mass

Multiply the subscripts of the empirical formula by this number

Na : 0.479 x 1 = 0.479H : 0.479 x 16 = 7.664C : 0.479 x 3 = 1.437 NaH7C

Practice Problems

1. How many moles are in 25 grams of water ( H2O )?

2. What is the mass of 0.5 moles of carbon dioxide (CO2)?

3. How many molecules are in 2 moles of methane (CH4)?

4. What is the volume occupied by 0.2 moles of nitrogen gas (N2) at standard temperature and pressure (STP)?

5. How many moles of sodium chloride (NaCl) are produced from 3 moles of sodium (Na) reacting with excess chlorine (Cl2)?

6. What is the mass of 2 moles of sulfuric acid ( H2SO4) ?

Stoichiometry

Stoichiometry is the study of the relationships between reactants used and products formed in a chemical reaction Mass of Reactants = Mass of product CoefficientsThe coefficients tell us the number of moles for a element or compound Mole Ratios A ratio between the number of moles of any substances in a balanced chemical equation

Stoichiometry Example

Equation

Al + HCl -> AlCl3 + H2(Aluminum + Hydrochloric acid -> Aluminum chloride + Hydrogen gas)

Balance

1. 2 Al -> 2Al2. 6H -> 3H23. Cl -> Cl3 2Al + 2HCl -> 2AlCl3 +3H2

Multiply

Reactants -2x 26.98 = 53.96 +( 2 x 1.008) + ( 3 x 35.45) = 162.32Products - (2 x 26.98 ) + 35.45= 89.41 + ( 3 x 1.008) = 92.434 g

Practice Problems

7. How many moles of oxygen (O2) are required to react with 5 moles of ethylene (C2H4) in the combustion reaction?

10. What is the mass of 4 moles of calcium carbonate (CaCO3)?

8. What is the volume of 0.1 moles of carbon monoxide (CO) at STP?

9. How many moles of hydrogen gas (H2) are produced from the reaction of 1 moles of magnesium (Mg) with excess hydrochloric acid (HCl)?