Lecture Notes 5 Mole concept Molar mass Stoichiometry Mole defini8on A mole is the amount of any substance that contains the same number of elementary en88es as there are atoms in 12 grams of 12C. How many atoms are there in 12 g of 12C ? u=1.66054 x 10-­‐24 g u = atomic mass unit 1 atom of 12C =12u g 1atom 1
12g ×
= = 6.02217 × 10 23 atoms = N A
12ug u
How many atoms are there in 12 g of 12C ? u=1.66054 x 10-­‐24 g u = atomic mass unit 1 atom of 12C =12u g 1atom 1
12g ×
= = 6.02217 × 10 23 atoms = N A
12ug u
NA = 6.022 x 1023 Avogadro’s number Avogadro’s number The reciprocal of the atomic mass unit NA = 1/u = 6.022 x 1023 The number of en88es in 1 mole of anything How many molecules are in 10.0 moles of H2 gas? How many molecules are in 10.0 moles of SF6? How many molecules are in 10.0 moles of CH3CH3? How many molecules are in 10.0 moles of C60? Molar Mass-­‐ mass of 1 mole Atoms -­‐ atomic mass Molecules -­‐ molecular mass Formula mass -­‐ Ionic material like NaCl What is the mass of a H atom in amu? 1.008 amu Mass of 1 atom in grams? 1.008amu
ug
g
mass =
×
= 1.008u
atom
1amu
atom
What is the mass of a H atom in amu? 1.008 amu Mass of 1 atom in grams? 1.008amu
ug
g
mass =
×
= 1.008u
atom
1amu
atom
What is the mass of I mole of H atoms in grams? 1.008ug N A atom
g
mass 1mole =
×
= 1.008
atom
mole
mole
What is the mass of a H2 molecule in amu? 2 × 1.008 amu = 2.016 amu
What is the mass of 1 molecule of H2 in grams? ug
mass H 2 = 2.016 amu ×
= 2.016ug
1 amu
2.016amu × 1.66054x10 −24 g / amu = 3.210x10 −24 g
What is the molar mass of H2 in grams? g
molecule
g
mass = 2.016u
× NA
= 2.016
molecule
mole
mole
What is the mass of a C atom in amu? 12.01 amu What is the molar mass of C in grams? 12.01u g / atom × N A atom / mole
1
= 12.01u × = 12.01g / mole
u
The mass of an atom or molecule in amu is numerically the same as the mass of one mole of that atom or molecule in grams. What is the mass of a molecule of C60 in amu? mass = 60 atom × 12.01 amu / atom = 720.6 amu
What is the molar mass of C60 in grams? mass 1mole = 720.6u × N A = 720.6g
What is the mass of oxygen in amu? Not enough informa8on to say! What is the mass of an O atom in amu? mass = 16.00 amu
What is the mass of an O2 molecule in amu? mass = 2 atom × 16.00 amu / atom = 32.00amu
How many moles in 60.00 g of carbon? 1 mole
moles = 60.00g ×
= 4.996mole
12.01 g
How many C atoms ? 6.022 × 10 23 atom
4.996mole ×
= 30.08 × 10 23 atom
1 mole
What is the mass of a single N2 molecule in amu? mass 1 molecule = 2 × 14.01amu = 28.02amu
What is the mass of 7 moles of N2 in grams ? mass = 7mole × 28.02g / mole = 196.1g
Stoichiometry : measurement of the elements Composi8on of a compound is indicated by its chemical formula. NO2 : 1 molecule contains 1 atom of N and 2 atoms of O NO2 : 1 mole contains 1 mole of N and 2 moles of O The subscripts in the formula determine the mole ra8o’s of the cons8tuent elements and thus the stoichiometry What is the mass of 3 moles of water? Mass of 1 mole of H2O g
2 × 1.008 + 1× 16.00 = 18.016
mole
g
3mole × 18.016
= 54.05g
mole
How many moles of methane (CH4) in 64.0 grams? g
4 × 1.008 + 12.01 = 16.04
mole
1mole
64.0g ×
= 3.990mole ≡ 4.00mole
16.04g
Moles of H atoms ? Mass Percent Each atom in a compound contributes to the mass of the compound and the extent of this contribu8on is o\en quoted as a mass percent. If one knows the mass percent of the of the cons8tuent atoms one can deduce the empirical formula of the compound. Mass Percent Composi8on Example 3, lecture note 6 What is the % by mass of N in Ammonium Nitrate ? Given the formula the mass % can be determined. Ammonium Nitrate NH4NO3 = N2H4O3 Atomic masses N 14.01 g/mol H 1.008 g/mol O 16.00 g/mol Molecular mass 80.05 g/mol Mass N 28.02 g/mol mass N in molecule
%N =
× 100
mass molecule
28.02g / mol
%N =
x 100 = 35.00% N
80.05g / mol
Example 1 lecture notes 6 Given the mass % deduce the empirical formula An oxide of Copper contains 88.82 % Cu by mass.
What is the empirical formula? CuxOy Assume 100 grams of the unknown oxide 88.82 g of Cu 11.18 g of O 1mol
88.82g Cu ×
= 1.398mol Cu
63.55g
1mol
11.18g O ×
= 0.6988mol O
16.00g
Cu1.398O0.6988
Cu 1.398 O0.6988 = Cu2O
0.6988
0.6988
Name of Cu2O
Copper(I) oxide Example 2, lecture notes 6 A compound of B and H contains 78.14% B Empirical formula ? Assume 100 g of compound 78.14 g B and 21.86 g H 78.14g
moles B =
= 7.228 mole B
10.81g / mole
21.86g
moles H =
= 21.69 mole H
1.008g / mole
B7.228 H 21.69
B7.228 H 21.69 = BH 3
7.228
7.228
If the molar mass is 27.7 g/mol what is the molecular formula? Bx H 3x
Mass of molecular formula is an integral mul8ple of the empirical formula mass mass BH 3 = 3 × 1.008 + 10.81 = 13.83g / mole
27.7
= 2.00
13.83
B2 H 6
Example 5, lecture notes 6 1.256 g Sulfur , S reacts with fluorine, F2 to produce 5.722 g of a substance SFx What is x? grams of F in SFx = 5.722-­‐1.256 = 4.466 g moles of F in SFx = 4.466g/19.00 g/mol = 0.2351 moles moles of S in SFx = 1.256g/32.07 g/mol = 0.03916 moles
S0.03916 F0.2351 ≡ S 0.03916 F 0.2351 ≡ SF6
0.03916
0.03916