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STOICHIOMETRY

STOICHIOMETRY. 436. 5.6 – To determine, using stoichiometric calculations, the quantity of a substance involved in a chemical reaction. 5.8 – To solve numerical problems related to stoichiometry. Recall:.

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STOICHIOMETRY

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  1. STOICHIOMETRY 436 • 5.6 – To determine, using stoichiometric calculations, the quantity of a substance involved in a chemical reaction. • 5.8 – To solve numerical problems related to stoichiometry.

  2. Recall: A mole is the amount of substance that contains 6.02 x 1023 particles of that substance n (moles) = mass of substance (g) molar mass of substance (g/mol)

  3. Stoichiometry: • Stoichiometry: • uses ratios of a chemical reaction to determine the amounts of elements/ compounds (reactants & products) involved in a chemical change

  4. Ex. 2Mg + O2 2MgO Magnesium metal reacts with oxygen to produce magnesium oxide. • Mole Ratio – • describes the ratio in which the substances combine • Ex. for every 2 Mg, 1 O2 needed • used to predict what will happen • Ex. for every 2 Mg & 1 O2, 2 MgO will be produced 2 : 1 : 2

  5. Ex #1 2Mg + O2 2MgO 4 moles of magnesium metal reacts with oxygen to produce magnesium oxide. How many moles of MgO are produced? First 2 : 1 : 2 Mole Ratio Second from mole ratio Set up ratio 2 moles Mg 2 moles MgO = 4 moles Mg ? moles MgO from question

  6. Ex #1 cont... Third Solve by cross multiplying 2 moles Mg 2 moles MgO = 4 moles Mg ? moles MgO 4 x 2 ÷ 2 4 moles MgO are produced

  7. Ex #2 4Fe + 3O2 2Fe2O3 Iron metal reacts with oxygen to produce 3 moles of iron (III) oxide. How many moles of oxygen were needed in the reaction? First 4 : 3 : 2 Mole Ratio Second from mole ratio Set up ratio 3 moles O2 2 moles Fe2O3 = ? moles O2 3 moles Fe2O3 from question

  8. Ex #2 cont... Third Solve by cross multiplying 3 moles O2 2 moles Fe2O3 = ? moles O2 3 moles Fe2O3 3 x 3 ÷ 2 4.5 moles O2are needed

  9. Ex #3 N2 + 3H2 2NH3 How many moles of nitrogen gas & hydrogen gas are required to make 7.5 mol of ammonia? 1 : 3 : 2 Solve for moles of N2: 11.25 moles H2are required 1 mole N2 2 moles NH3 = ? moles N2 7.5 moles NH3 Solve for moles of H2: 3.75 moles N2are required 3 mole H2 2 moles NH3 = ? moles H2 7.5 moles NH3

  10. n (moles) = mass of substance (g) molar mass of substance (g/mol) We can now add mass into problems involving stoichiometry. Recall, Ex. How many moles are in 12.5g of Ca? n (moles) = 12.5g 40g/mol 0.3125 mol of Ca Ex. How many moles are in 2.4g of NO2? n (moles) = 2.4g 46g/mol 0.052 mol of NO2

  11. Ex #4 2Li + F2 2LiF How many moles of Li are needed to react with fluorine gas to make 435g of lithium fluoride? 2 : 1 : 2 First Solve for moles of LiF: Second: n (moles) = 435g 26g/mol Solve for moles of Li: 16.73 mol of LiF 2 moles Li 2 moles LiF = ? moles Li 16.73 mol LiF 16.73 moles Liare needed

  12. How many grams of NaClO3 must be decomposed to produce sodium chloride & 1.65 moles of oxygen? Ex #5 2NaClO3 2NaCl + 3O2 2 : 2 : 3 First Solve for moles of NaClO3: 2 mol NaClO3 3 mol O2 = ? mol NaClO3 1.65 mol O2 1.1 mol of NaClO3 Second Solve for mass of NaClO3: 116.6g of NaClO3 1.1 mol = ? g 106 g/mol

  13. 3g of magnesium react with how many grams of hydrochloric acid to produce hydrogen gas & magnesium chloride? Ex #6 Mg + 2HCl  MgCl2 + H2 1 : 2 : 1 : 1 First Solve for moles of Mg: Third n = 3 g 24 g/mol Solve for mass of HCl: 0.25mol = ? g 36 g/mol 0.125 mol of Mg Second 9g of HCl Solve for moles of HCl: 1 mol Mg 2 mol HCl = 0.25 mol of HCl 0.125 mol Mg ?mol HCl

  14. Key Points: • Always find moles first!!! • Use mole ratio to find the moles of each reactant & product • Remember, the equation must be balanced to give you the mole ratio n (moles) = mass of substance (g) molar mass of substance (g/mol)

  15. Practice: • Worksheet #

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