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Empirical Formula of a Compound

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Molecular Formula (MF)

actual formula shows the number of atoms of each element that make up a molecule or a formula unit

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Empirical Formula (EF)

simplest formula of a compound shows the lowest whole number ratio of the elements in a compound can be determined from – the molecular formula or – percentage composition of the compound

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Using Molecular Formula – Examples

Substance Molecular Formula (MF) Empirical Formula (EF) Butane C4H10 C2H5 Glucose C6H12O6 Benzene C6H6 Water H2O

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Using Molecular Formula

– Examples Substance Molecular Formula (MF) Empirical Formula (EF) Butane C4H10 C2H5 Glucose C6H12O6 CH2O Benzene C6H6 CH Water H2O

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Using Percentage Composition – Examples

Calculate the EF of a compound that is 85.6 % carbon and 14.4 % hydrogen. Answer: Assume that you are using a 100 g sample of the compound and complete the next calculations. Use a table for your data.

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Using Percentage Composition – Example #1

Lowest Whole Number Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C H

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Using Percentage Composition – Example #1

Lowest Whole Number Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C 85.6 12.01 H 14.4 1.01 Recall: n = m M

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Using Percentage Composition – Example #1

Lowest Whole Number Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C 85.6 12.01 7.13 H 14.4 1.01 14.3 Recall: n = m M

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Using Percentage Composition – Example #1

Lowest Whole Number Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C 85.6 12.01 7.13 H 14.4 1.01 14.3 Calculate LWN ratio by dividing: molar amount lowest molar amount

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Using Percentage Composition – Example #1

Lowest Whole Number Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C 85.6 12.01 7.13 H 14.4 1.01 14.3 Calculate LWN ratio by dividing: molar amount lowest molar amount

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Using Percentage Composition – Example #1

Lowest Whole Number Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C 85.6 12.01 7.13 7.13 = 1.00 H 14.4 1.01 14.3 14.3 = 2.01 Calculate LWN ratio by dividing: molar amount lowest molar amount

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Using Percentage Composition – Example #1

Element Mass % m (g) Molar Mass M (g/mol) Moles n (mol) LWN Ratio EF C 85.6 12.01 7.13 7.13 = 1.00 H 14.4 1.01 14.3 14.3 = 2.01 H2 Empirical Formula: CH2

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Using Percentage Composition – Examples

2. Calculate the EF of a compound with the following % composition: 32.4 % Na, 22.6 % S and 45.0 % O 3. Calculate the EF of TNT, with the following % composition: 37.0 % C, 2.20 % H, 18.5% N and 42.3 % O Hint: see table 6.2 (p210)

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