Simplest

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Simplest formula calculations

• Q- a compound is found to contain the following
  by mass 69.58 Ba, 6.090 C, 24.32 O. What is
  the simplest (i.e. empirical) formula?
• Step 1 imagine that you have 100 g of the
  substance. Thus, will become mass in grams.
  E.g. 69.58 Ba becomes 69.58 g Ba. (Some
  questions will give grams right off, instead of
  )
• Step 2 calculate the of moles (mol g
  g/mol)
• Step 3 express moles as the simplest ratio by
  dividing through by the lowest number.
• Step 4 write the simplest formula from mol
  ratios.

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Simplest formula sample problem

• Q- 69.58 Ba, 6.090 C, 24.32 O.
• What is the empirical (a.k.a. simplest) formula?
• 1 69.58 g Ba, 6.090 g C, 24.32 g O
• 2 Ba 69.58 g 137.33 g/mol 0.50666 mol Ba
• C 6.090 g 12.01 g/mol 0.50708 mol C
• O 24.32 g 16.00 g/mol 1.520 mol O
• 3

O
C
Ba
1.520
0.50708
0.50666
1.520/ 0.50666 3.000
0.50708/ 0.50666 1.001
0.50666/ 0.50666 1
4 the simplest formula is BaCO3
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Mole ratios and simplest formula

• Given the following mole ratios for the
  hypothetical compound AxBy, what would x and y be
  if the mol ratio of A and B were
• A 1 mol, B 2.98 mol
• A 1.337 mol, B 1 mol
• A 2.34 mol, B 1 mol
• A 1 mol, B 1.48 mol

AB3
A4B3
A7B3
A2B3

• A compound consists of 29.1 Na, 40.5 S, and
  30.4 O. Determine the simplest formula.
• A compound is composed of 7.20 g carbon, 1.20 g
  hydrogen, and 9.60 g oxygen. Find the empirical
  formula for this compound
• 3. - 6. Try questions 3 - 6 on page 189.

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Question 1

• 1 Assume 100 g 29.1 g Na, 40.5 g S, 30.4 g O
• 2 Na 29.1 g 22.99 g/mol 1.266 mol Na
• S 40.5 g 32.06 g/mol 1.263 mol S
• O 30.4 g 16.00 g/mol 1.90 mol O
• 3

O
S
Na
1.90
1.263
1.266
1.90/ 1.263 1.50
1.263/ 1.263 1
1.266/ 1.263 1.00
4 the simplest formula is Na2S2O3
For instructor prepare molecular models
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Question 2

• 1 7.20 g C, 1.20 g H, 9.60 g O
• 2 C 7.20 g 12.01 g/mol 0.5995 mol C
• H 1.20 g 1.01 g/mol 1.188 mol H
• O 9.6 g 16.00 g/mol 0.60 mol O
• 3

O
H
C
0.60
1.188
0.5995
0.60/ 0.5995 1.0
1.188/ 0.5995 1.98
0.5995/ 0.5995 1
4 the simplest formula is CH2O
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Question 3

• 1 Assume 100 g 28.9 g K, 23.7 g S, 47.7 g O
• 2 C 7.20 g 12.01 g/mol 0.5995 mol C
• H 1.20 g 1.01 g/mol 1.188 mol H
• O 9.6 g 16.00 g/mol 0.60 mol O
• 3

O
H
C
0.60
1.188
0.5995
0.60/ 0.5995 1.0
1.188/ 0.5995 1.98
0.5995/ 0.5995 1
4 the simplest formula is CH2O
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Molecular formula calculations

• There is one additional step to solving for a
  molecular formula. First you need the molar mass
  of the compound. E.g. in Q2, the molecular
  formula can be determined if we know that the
  molar mass of the compound is 150 g/mol.
• First, determine molar mass of the simplest
  formula. For CH2O it is 30 g/mol (12216).
• Divide the molar mass of the compound by this to
  get a factor 150 g/mol 30 g/mol 5
• Multiply each subscript in the formula by this
  factor C5H10O5 is the molecular formula.
  (models)
• Q- For OF, give the molecular formula if the
  compound is 70 g/mol

O2F2 70 35 2
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• Combustion analysis gives the following
• 26.7 C, 2.2 hydrogen, 71.1 oxygen.
• If the molecular mass of the compound is 90
  g/mol, determine its molecular formula.
• What information must be known to determine a)
  the empirical formula of a substance? b)
  the molecular formula of a substance?
• A compounds empirical formula is CH, and it
  weighs 104 g/mol. Give the molecular formula.
• A substance is decomposed and found to consist of
  53.2 C, 11.2 H, and 35.6 O by mass. Calculate
  the molecular formula of the unknown if its molar
  mass is 90 g/mol.

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Question 7

• 1 Assume 100 g total. Thus
• 26.7 g C, 2.2 g H, and 71.1 g O
• 2 C 26.7 g 12.01 g/mol 2.223 mol C
• H 2.2 g 1.01 g/mol 2.18 mol H
• O 71.1 g 16.00 g/mol 4.444 mol O
• 3

2.223
2.18
4.444
2.223/2.18 1.02
2.18/ 2.18 1
4.444/2.18 2.04
4 the simplest formula is CHO2 5 factor
90/452. Molecular formula C2H2O4
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Question 8, 9

• For the empirical formula we need to know the
  moles of each element in the compound (which can
  be derived from grams or ).
• For the molecular formula we need the above
  information the molar mass of the compound
• Molar mass of CH 13 g/mol
• Factor 104 g/mol 13 g/mol 8
• Molecular formula is C8H8

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Question 10

• 1 Assume 100 g total. Thus
• 53.2 g C, 11.2 g H, and 35.6 g O
• 2 C 53.2 g 12.01 g/mol 4.430 mol C
• H 11.2 g 1.01 g/mol 11.09 mol H
• O 35.6 g 16.00 g/mol 2.225 mol O
• 3

4.430
11.09
2.225
4.43/2.225 1.99
11.09/2.225 4.98
2.225/2.225 1
4 the simplest formula is C2H5O 5 factor
90/452. Molecular formula C4H10O2
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Assignment

• Calculate the percentage composition of each
  substance a) SiH4, b) FeSO4
• Calculate the simplest formulas for the compounds
  whose compositions are listed
• a) carbon, 15.8 sulfur, 84.2
• b) silver,70.1 nitrogen,9.1 oxygen,20.8
• c) K, 26.6 Cr, 35.4, O, 38.0
• The simplest formula for glucose is CH2O and its
  molar mass is 180 g/mol. What is its molecular
  formula?

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• Determine the molecular formula for each compound
  below from the information listed.
• substance simplest formula molar mass(g/mol)
• a) octane C4H9 114
• b) ethanol C2H6O 46
• c) naphthalene C5H4 128
• d) melamine CH2N2 126
• The percentage composition and approximate molar
  masses of some compounds are listed below.
  Calculate the molecular formula of each
• percentage composition molar mass(g/mol)
• 64.9 C, 13.5 H, 21.6 O 74
• 39.9 C, 6.7 H, 53.4 O 60
• 40.3 B, 52.2 N, 7.5 H 80

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• 1 a) Si 87.43 (28.09/32.13 x 100), H 12.57
• b) Fe 36.77 (55.85/151.91 x 100),
  S 21.10 (32.06/151.91 x 100), O 42.13

2 a) Assume 100 g. Thus 15.8 g C, 84.2 g
S. C 15.8 g 12.01 g/mol 1.315 mol C S
84.2 g 32.06 g/mol 2.626 mol S
the simplest formula is CS2
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2 b) Ag 70.1 g 107.87 g/mol 0.6499 mol Ag
N 9.1 g 14.01 g/mol 0.6495 mol
N O 20.8 g 16.00 g/mol 1.30 mol O
2 c) K 26.6 g 39.10 g/mol 0.6803 mol K
Cr 35.4 g 52.00 g/mol 0.6808 mol
Cr O 38.0 g 16.00 g/mol 2.375 mol O
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• 3 C6H12O6 (CH2O 30 g/mol, 180/30 6)

4 a) C8H18 (C4H9 57 g/mol, 114/57
2) b) C2H6O (C2H6O 46 g/mol, 46/46
1) c) C10H8 (C5H4 64 g/mol, 128/64
2) d) C3H6N6 (CH2N2 54 g/mol, 126/42 3)
5 a) C 64.9 g 12.01 g/mol 5.404 mol C H
13.5 g 1.01 g/mol 13.37 mol H O 21.6
g 16.00 g/mol 1.35 mol O
C4H10O (C4H10O 74 g/mol, 74/74 1)
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5 b) C 39.9 g 12.01 g/mol 3.322 mol C H
6.7 g 1.01 g/mol 6.63 mol H O 53.4
g 16.00 g/mol 3.338 mol O
C2H4O2 (CH2O 30 g/mol, 60/30 2)
5 c)
B3N3H6 (BNH2 26.84 g/mol, 80/26.84 2.98)
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