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Stoichiometry: Measurement of Elements

Stoichiometry: Measurement of Elements. Formula Weights. Sum of the atomic weights (amu) of each atom in a chemical formula. Called “ molecular weight ” or “ molecular mass ” for a molecule. Use “ formula weight ” for ionic cmpds. The Mole.

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Stoichiometry: Measurement of Elements

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  1. Stoichiometry: Measurement of Elements

  2. Formula Weights • Sum of the atomic weights (amu) of each atom in a chemical formula. • Called “molecular weight” or “molecular mass” for a molecule. • Use “formula weight” for ionic cmpds.

  3. The Mole • Chemists have derived a special counting unit for describing very large numbers of atoms or molecules. • Mole (mol) = the amount of matter that contains as many objects as the number of atoms in exactly 12g of isotopically pure carbon-12.

  4. Mole, cont. • Avogadro’s number: 6.022 x 10^23 • A mole of ANYTHING contains 6.022 x 10^23 objects! • I mole of marbles would cover the entire surface of the earth in a layer ________ thick. • Pennies placed side to side in a straight line would encircle the earth ________ times.

  5. Representative Particles • Species present in a substance.

  6. Moles, cont., cont. • Remember: • 1 mol of carbon = 6.022 X 10^23 atoms. • 1 molwater = ____ molecules. • 1 mol CN-ions = _____ ions. • 1 molNaCl = ____ formula units (f.u.’s). • 1 molelephants = _____ elephants.

  7. Mole-Particle/Particle-Mole Problems • Use dimensional analysis! • How many moles of magnesium are 1.25 x 10^24 atoms of magnesium? • How many moles are 2.80 x 10^24 atoms Si? • How many molecules are in 3.60 mol water? • How many hydrogen atoms are in 3.60 mol water????

  8. Molar Mass • A mole is always the same number but moles of different substances have different masses. • 1 mol water = 18.02 g; 1 mol lead = 207.2 g • Molar mass: the mass in grams of 1 mole of a substance. Unit is g/mol.

  9. Calculating Molar Mass • Calculate molar mass of CH4. • C: 1 atom x 12.01 g/mol = 12.01 g/mol • H: 4 atoms X 1.01 g/mol = 4.04 g/mol • Add together: • 12.01g/mol + 4.04 g/mol = 16.05 g/mol Watch your sig figs!!!

  10. Practice Calculating Molar Mass • Calculate the molar mass of NaCl. • Calculate the molar mass of calcium hydroxide. • Calculate the molar mass of Sr3(PO4)2. • Do you like this more than nomenclature?

  11. Mass-Mole/ Mole-Mass Problems • Use dimensional analysis! • Find the mass of 4.52 x 10^-3 mol water. • Find the mass of 0.342 mol nitrogen gas. • How many moles are in 72.96 g LiCl? • How many moles are in 12 28 g KOH? • Are we having fun yet?

  12. Mole-Volume Relationship • Avogadro’s Hypothesis: equal volumes of gases at the same temp and pressure contain an equal number of particles. • Note: the size or mass of gas particles does NOT matter because particles are so far apart from each other.

  13. Molar Volume of a Gas • STP = standard temp and pressure • Standard temp for gases = 0°C • Standard pressure = 1 atm • “Molar volume of a gas” means that at STP… I mole of ANY gas = 22.4L of gas

  14. Mole-Volume Problems • We use molar volume to do mole-volume problems! Yeah! • Calculate the volume of 3.20 x 10^-3 mol carbon dioxide at STP. • 0.0717 L carbon dioxide • Calculate the volume of 3.70 mol nitrogen gas at STP. • 82.9 L nitrogen gas

  15. More Molar Volume Calculations! • You have 0.200 L hydrogen gas at STP. How many moles? • 8.93 x 10^-3 mol • Dan has 0.375 mol of oxygen gas at STP. What is its volume? • 8.40 L oxygen

  16. Density & Molar Volume at STP • The density of gases is in g/L, NOT g/mL. WHY? • You can use density along with molar volume to get molar mass! Hurrah!

  17. Density & Molar Volume Problems • A gas has a density of 3.58 g/L at STP. What is the molar mass of the gas? • 80.2 g/mol • A gas has a density of 1.964 g/L at STP. What is its molar mass? • 43.99 g/mol • What is the density of krypton gas at STP? • 3.74 g/L

  18. Percentage Composition from Chemical Formula • Percentage by mass of each element in a substance. • % element = (# of atoms x molar mass of element/ molar mass of compound) x 100 • Total % of each element = 100%

  19. Calculating Percentage Composition • Calculate the % composition of C6H12O6. • % C = (6 x 12.01 g/mol/180.2 g/mol) x 100 = 40.00% % H = (12 X 1.01 g/mol/180.2 g/mol) x 100 = 6.73% % O = 100.00% - 40.00% - 6.73% = 53.27%

  20. Percent Composition Practice • Calculate the percent composition of C3H8. • C: 81.8 %; H: 18.2 % • Calculate the percent on nitrogen in NH3. • 82.4% nitrogen

  21. Percent Composition from Mass Data • Percent Composition = (mass of element in cmpd/mass of cmpd) x 100 • Total %’s of all elements in cmpd = 100%

  22. Percent Composition by Mass Practice • When a 13.60-g sample of a cmpd containing only Mg and O is decomposed, 5.40 g oxygen is obtained. What is the percent comp of the cmpd? • O: (5.40 g O/ 13.60 g) x 100 = 39.7% • Mg: [(13.60 g- 5.40 g)/ 13.60 g] x 100 = 60.3%

  23. More Percent Composition Practice • Calculate the number of grams of sulfur in 3.54 g H2S. • First, find the % of sulfur in H2S. • 94.1% S • Then, multiply .941 X 3.54 g H2S. • 3.33 g

  24. Empirical & Molecular Formulas • Empirical formula: gives the lowest whole number ratio of atoms in a compound. PbO, not Pb2O2 • Molecular formula: whole number multiples of the empirical formula.

  25. Determining Empirical Formulas • Sing the Empirical Formula Song (sung to tune of “Twinkle, Twinkle Little Star”): • “Percent to mass, mass to mole, divide by small, multiply ‘til whole.”

  26. Empirical Formula Practice • A compound is analyzed and found to contain 25.9% nitrogen and 74.1% oxygen. What is the empirical formula of the compound? • N2O5

  27. More Empirical Formula Calculations! • Find the empirical formula for the following cmpd: Hg: 67.6%; S: 10.8%; O: 21.6% • HgSO4 • Find the empirical formula for the following cmpd: H: 5.9%; O: 94.1% • HO

  28. Molecular Formulas • Once you have determined the empirical formula (e.f.), you can determine the molecular formula (m.f.) if you know the compound’s molar mass. • Divide the molar mass of the cmpd by the molar mass of the e.f. and get a whole number. Multiply this number by each subscript in the e.f. to get the m.f..

  29. Finding a Molecular Formula • You have the empirical formula HO. Find the molecular formula if its molar mass is 34.0 g/mol. • 34.0 g/mol/ 17.0 g/mol = 2 • The empirical formula HO becomes the molecular formula H2O2 !

  30. Finding Molecular Formulas :-) • Find the molecular formula of a cmpd with a molar mass of 60.0 g/mol and an empirical formula of CH4N. • C2H8N2 • Find the m.f. of a cmpd with a m.m of 62.0 g/mol and an e.f. of CH3O. • C2H6O2

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