Notes 3-1 – Ch 3.1 – Ch. 3.4: Atomic Mass, Mass ...doccasagrande.net/Chem 2 Files/Chem 2 Chapter 03 - Mass Relation… · § Relative size of peak indicates percent abundance

Notes 3-1 – Ch 3.1 – Ch. 3.4: Atomic Mass, Mass Spectrometry, Moles, Molar/Molecular Mass • Atomic Mass

o Average Atomic Mass is mass on periodic table expressed in atomic mass units (amu) § 1 amu º 1/12 mass C-12 atom

o Weighted average of mass of all natural isotopes:

§

E.g. A sample of natural copper consists of 69.09% Cu-63 (mass = 62.928 amu) & 30.91 % Cu-65 (mass = 64.930 amu). What is the average atomic mass of a Cu atom?

o Second kind of problem (in homework): given atomic mass and two isotope masses, find abundances:

§ Set abundance1 = x and abundance2 = y, you have 1 equation and 2 unknowns:

• What info do you have that is not apparent?

• Mass Spectrometry o Mass Spectrometer: Device that separates particles according to mass

§ Particles in beam are ionized, accelerated through electric field, deflected by magnetic field § Deflection depends on charge/mass ratio (q/m): q is generally 1+, so more massive particles

deflected less o Mass Spectra

§ Each isotope produces peak on spectrum § Position of peak indicates mass of isotope

Mass X = Isotopic Abundance

100all isotopes∑ × Isotopic Mass

mass Cu = 69.09%100%

⎛⎝⎜

⎞⎠⎟

(62.928 amu)

Contribution of Cu-63! "################

+ 30.91%100%

⎛⎝⎜

⎞⎠⎟

(64.930 amu)

Contribution of Cu-65! "###############

= 63.55 amu

Mass X = Abundance1

100%⎛⎝⎜

⎞⎠⎟

(Mass1)+ Abundance2

100%⎛⎝⎜

⎞⎠⎟

(Mass2 )

Mass X = x100%

⎛⎝⎜

⎞⎠⎟

(Mass1)+ y100%

⎛⎝⎜

⎞⎠⎟

(Mass2 )

Carbon

Neon

Mercury

§ Relative size of peak indicates percent abundance § Elements with only 1 isotope show a single peak @ atomic mass

• Particles o 1 mol = 6.022´1023 particles (atoms, molecules, formula units, ions) o Dimensional analysis conversions:

o E.g. how many moles are in 8.73´1019 atoms C?

• Molar/Molecular Mass o Remember, for element MM is atomic mass expressed in grams o For compound, need to add up masses of all atoms:

§ e.g. H2O = 2(1.01 g) + 16.00 g = 18.02 g/mol § Ca(NO3)2 = 40.08 g + 2[14.01 g + 3(16.00 g)] = 164.10 g/mol

o Use molar mass to convert to/from grams § Use Dimensional Analysis! ID starting point, conversion factors in correct orientation:

§ e.g. what is the mass of 6.89 moles of nitrogen dioxide?

• Typical “challenging AP” problem: What is the mass, in g, of 1 atom of carbon? o Think: what information DO you know? o Molar mass: 1 mol C = 12.01 g C o Number of atoms in 1 mol C = 6.022´1023 atoms C o Want units of g/atom o Put them together:

# Particles = Moles× 6.022×1023 particles1 Mole

; Moles = # Particles× 1 Mole6.022×1023 Particles

? moles C = 8.73×1019 atoms C × 1 mole C6.022×1023 atoms C

= 1.45×10−4 moles C

mass = moles× molar mass (g)1 mole

; moles = mass× 1 molemolar mass (g)

Mass = 6.89 mol NO2 × 46.01 g NO2

1 mol NO2

= 317 g NO2

Mass 1 Atom C = 12.01 g C1 mol C

× 1 mol C6.022×1023 atom C

= 1.994×10–23 g C/atom C