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Atomic Weights. The mass of an individual atom, ion, or molecule is very small. Scientists use the atomic mass unit (amu) to express the mass of atoms or molecules. 1 amu = 1.66054 x 10 -24 g. Atomic number. Atomic weight (atomic mass). 5. 10.81. B. Element symbol. electronegativity.
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Atomic Weights • The mass of an individual atom, ion, or molecule is very small. • Scientists use the atomic mass unit (amu) to express the mass of atoms or molecules. • 1 amu = 1.66054 x 10-24 g
Atomic number Atomic weight (atomic mass) 5 10.81 B Elementsymbol electronegativity en = 2.04 boron Element name Atomic Weight • Atomic weight (atomic mass): • the average mass of one atom of an element expressed in amu • found on the periodic table • The atomic weight of an element is reported as the weighted average of the naturally occurring isotopes.
Atomic Weights • The atomic weight for silicon is found by taking a weighted average of the masses of the three naturally occurring isotopes of silicon: 28Si : 92.2297% of 27.9769 amu = 25.803 amu 29Si : 4.6832% of 28.9765 amu = 1.3570 amu 30Si: 3.0872% of 29.97377 amu = 0.92535 amu AW of Si = 28.085 amu
Formula Weight • The mass of a compound can be expressed using either its formula weight (all compounds) or its molecular weight (molecular compounds only). • Formula Weight: • the sum of the atomic weights of all of the atoms in any chemical formula • Molecular Weight: • the sum of the atomic weights of all of the atoms in a molecular formula
Formula Weight and Molecular Weight • Formula weight and molecular weight are found by the exact same process: • The molecular weight of CO2 is calculated as follows: FW = 1 (AW of C) + 2 (AW of O) = 1 (12.01 amu) + 2 (16.00 amu) = 44.01 amu
Formula Weight and Molecular Weight • The formula weight of CaCl2 is calculated as follows: • FW = 1(AW of Ca) + 2(AW of Cl) Ca: 1 (40.08 amu) = 40.08 amu Cl: 2 (35.45 amu) = 70.90 amu 110.98 amu
Formula Weight and Molecular Weight • How many significant figures should I use for the atomic weight??? • Use enough significant figures in the AW to avoid limiting the number of significant figures in any other calculation. • I want you to always use at least 2 decimal places (rounded appropriately) in the AW’s that you use in a calculation.
Molecular Weight Example: Calculate the MW of phosphoric acid Phosphoric acid = ????
Mole • Atoms and molecules are very small. • 1 cm length • 20 to 50 million atoms • Tiny speck of carbon • 60 million billion atoms (6 x 1016) • 1 cm3 box • billions of trillions of atoms
Mole • Virtually impossible to manually count the # of atoms in a reasonable size sample • Tiny speck of carbon • 60 million billion atoms (6 x 1016) • Counting 1 atom per second, 24 hours per day, 365 days per year it would take: • ~ 2 billion years
H H + O O H H Mole • Knowing the number of atoms or molecules in a sample is important. • Chemical reactions occur on an atomic or molecular level. 2 H2 + O2 2 H2O
Mole • Chemists usually measure out chemicals in grams not as individual atoms or molecules. • Need a way to • count the number of atoms or molecules in a sample and • relate the number of atoms or molecules to the mass of a sample
Mole • Mole: • the amount of a material that contains the same # of units as there are atoms in exactly 12 g of C-12 • the amount of material that contains 6.022 x 1023 atoms (or molecules or ions or any other chemical species)
Mole 1 mole 12C atoms = 6.022 x 1023 12C atoms 1 mole H2O molecules = 6.022 x 1023 H2O molecules 1 mole Na+ ions = 6.022 x 1023 Na+ ions
Mole • Simply a way to count, express, and compare the number of atoms or molecules present • pair of gloves (2) • dozen eggs (12) • case of pop (24) • gross (144) • Mole (contains 6.022 X 1023 units)
Avogadro’s Number • Avogadro’sNumber (N):the number of atoms, ions, or molecules per mole • a constant • N = 6.022 x 1023atoms, ions or molecules mole
Avogadro’s Number • How big is Avogadro’s Number? If you have a mole of pennies (i.e. Avogadro’s number) and divide them equally among the 6 billion people on Earth, how many dollars would each person get?
= $1 x 1012 person x $1 100 pennies Avogadro’s Number 6.022 X1023 pennies 6 x 109 people Each person would get ~ $ 1 trillion!!!
Comparing moles of different materials • 1 mole Cu = 6.022 x 1023 atoms = 63.55 g Cu 22 copper (pre-1983) pennies
Comparing moles of different materials • 1 mole He = 6.022 x 1023 atoms = 4.003 g He ~7 medium-sized He-filled ballons
Comparing moles of different materials • 1 mole H2O = 6.022 x 1023 molecules of H2O = 18.016 g H2O • ~18 mL of water
Mole • How do we relate the number of atoms (or molecules) in a mole to mass??? • 1 mole = 6.022 x 1023 atoms • The mass of 6.022 x 1023 atoms is a unique number for each element. • Atoms of each element have different masses • 1 mole of each element has a unique mass (molar mass)
Molar Mass • Molar mass • the mass in grams of 1 mole of substance • numerically equal to its formula weight (for ionic compounds), molecular weight (for molecules), or atomic weight (for atoms)
1 NO3- ion has a mass of 62.0 amu 1 mol NO3- ions has a mass of 62.0 g Molar Mass • 1 H2O molecule has a mass of 18.0 amu 1 mol H2O has a mass of 18.0 g
Molar Mass • Molar mass can be used as a conversion factor: • Relates the # grams and # moles • 1 mole of N2 has a mass of 28.0 g 28.0 g N2 1 mole N2 1 mole N2 28.0 g N2 OR
Calculating the number of moles in a sample Example: A medium-sized balloon contains 0.55 g of He gas. How many moles of He are in the balloon?
Calculating the # of moles Example:How many moles are present in 25.0 g of acetic acid ?
Calculating mass from moles Example:A 1-carat diamond, which is pure carbon, contains 0.0167 mol of carbon. What is the mass, in grams, of the diamond?
Calculating mass from moles Example:What is the mass in grams of 0.500 moles of carbon dioxide?
Avogadro’s Number • Avogadro’s number can also be used as a conversion factor: 1 mole = 6.022 x 1023 molecules (or atoms, ions, formula units) 6.022 x 1023 atoms 1 mole 1 mole 6.022 x 1023 atoms OR
Calculating # of molecules from # of moles Example:How many water molecules are in 2.5 moles of water?
Calculating # moles from # of molecules Example:How many moles are in 3.2 x 1022 molecules of water?
Use molar mass grams moles molecules Calculating Mass from # of Molecules or Vice Versa Use Avogadro’s number
Calculating mass from molecules Example:What is the mass in grams of 3.0 x 1024 molecules of nitric acid?
Calculating # of molecules from mass Example:How many molecules are in a 5.0 g sample of hydrogen?