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Simplest formula calculations

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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 69.58g 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.

mol (reduced)

Simplest formula: sample problemQ- 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:

Ba

C

O

0.50666

0.50708

1.520

0.50666/ 0.50666 = 1

0.50708/ 0.50666 = 1.001

1.520/ 0.50666 = 3.000

4: the simplest formula is BaCO3

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
- A compound consists of 28.9 % K, 23.7 % S, and 47.7 % O. Determine the simplest formula.

mol (reduced)

Question 11: 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:

Na

S

O

1.266

1.263

1.90

1.266/ 1.263 = 1.00

1.263/ 1.263 = 1

1.90/ 1.263 = 1.50

4: the simplest formula is Na2S2O3

For instructor: prepare molecular models

mol (reduced)

Question 21: 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:

C

H

O

0.5995

1.188

0.60

0.5995/ 0.5995 = 1

1.188/ 0.5995 = 1.98

0.60/ 0.5995 = 1.0

4: the simplest formula is CH2O

mol (reduced)

Question 31: 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:

C

H

O

0.5995

1.188

0.60

0.5995/ 0.5995 = 1

1.188/ 0.5995 = 1.98

0.60/ 0.5995 = 1.0

4: the simplest formula is CH2O

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 (12+2+16).
- 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

O2F270 ¸ 35 = 2

- 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 compound’s 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.

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 = 90/45=2. Molecular formula: C2H2O4

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

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 = 90/45=2. Molecular formula: C4H10O2

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?

- Determine the molecular formula for each compound below from the information listed.
substancesimplest formulamolar 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 compositionmolar 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

C the information listed.

S

Mol

1.315

2.626

Mol reduced

1.315/1.315

= 1

2.626/1.315

= 2.00

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

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%

the simplest formula is CS2

2 b) Ag: 70.1 g the information listed.¸ 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

AgNO2

Ag

N

O

Mol

0.6499

0.6495

1.30

Mol reduced

.6499/.6495

= 1.0

.6495/.6495

= 1

1.30/.6495

= 2.00

K2Cr2O7

K

Cr

O

Mol

0.6803

0.6808

2.375

Mol reduced

.6803/.6803

= 1

.6808/.6803= 1.00

2.375/.6495

= 3.49

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

C the information listed.4H10O

C

H

O

Mol

5.404

13.37

1.35

Mol reduced

5.404/1.35

= 4.00

13.37/1.35

= 9.90

1.35/1.35

= 1

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)

3 C6H12O6(CH2O = 30 g/mol, 180/30 = 6)

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)

5 b) C: 39.9 g the information listed.¸ 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

CH2O

C

H

O

Mol

3.322

6.63

3.338

Mol reduced

3.322/3.322

= 1

6.63/3.322

= 2.0

3.338/3.322

= 1.00

B

N

H

Mol

3.728

3.726

7.43

Mol reduced

3.728/3.726

= 1.00

3.726/3.726

= 1

7.43/3.726

= 2.0

C2H4O2 (CH2O = 30 g/mol, 60/30 = 2)

5 c)

B3N3H6 (BNH2 = 26.84 g/mol, 80/26.84= 2.98)

For more lessons, visit www.chalkbored.com

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