Ch.3 Molecules and Compounds Types of Chemical Formulas
Percent Composition
Determining Formulas

Formulas:

Formulas Formula - shorthand way of expressing the number of atoms of each type in a molecule
Sucrose - C12H22O11
Ethanol - C2H6O
Dimethyl ether - C2H6O

Formulas:

Formulas Empirical formula - each element is written once
Structural formula - provides information on how the atoms are connected

Determining Empirical Formulas:

Determining Empirical Formulas Strategy
%composition A ----> g of A -----> moles of A
%composition B ----> g of B -----> moles of B
find the mole ratio = A/B
Ratio gives the formula

What is the empirical formula for hydrazine?:

What is the empirical formula for hydrazine? Hydrazine contains 87.42% N and 12.58% H
Strategy
% composition N ----> g of N
in a 100g sample
87.42g of N and 12.58 g H
g of N -----> moles of N use the molar mass

Empirical Formula:

Empirical Formula 12.58% H
% composition H ----> g of H
12.58g of H
g of H-----> moles of H

Empirical Formula:

Empirical Formula 6.241 mol N
12.48 mol H
find the mole ratio = A/B = 12.48mol H/6.241 mol N
mole ratio = 2 mol H/ mol N
Ratio gives the formula
NH2

Molecular formula:

Molecular formula Indicates the number of atoms in each molecule, not just the ratio
You need the molar mass as well as the percent composition to determine molecular formula
What if NH2 really represented the hydrazine molecule?
Molar mass = 14.007 + 1.0079 +1.0079 = 16g/mole

Molecular formula:

Molecular formula Experimental data shows that the molar mass for hydrazine is 32.0g/mole
Molecular formula = N2H4

Fractional mole ratios Mole ratio = 15.76 mol H / 7.004 mol C = 2.25
mole ratio = 2 1/4 = 9/4
Nine H atoms for every four C atoms
Empirical formula is C4H9
If C4H9 then molar mass = 9x1 + 4x12 = 57g/mole
but molar mass is actually 114g/mole
Molecular formula is C8H18

Word Problems:

Word Problems Sn metal + I2 ------> SnxIy
Mass of tin (Sn) in the crucible originally = 1.056g
Mass of iodine reacted = 1.947g
Mass of tin (Sn) left over after the reaction= 0.601g
Mass of tin (Sn) in the original mixture = 1.056g
Mass of tin (Sn) left over after the reaction= -0.601g
Mass of tin (Sn) consumed in the reaction = .455g

Convert grams to moles:

Convert grams to moles

Mole Ratio and Formulas:

Mole Ratio and Formulas Mole ratio = 15.34x10-3 mol I / 3.83 x 10-3 mol Sn = 4
Mole ratio = 4 mol I / mol Sn
Empirical Formula is SnI4
If SnI4 then molar mass = 118.71 + 4x126.9 = 626.31g/mole
and the molar mass is actually 626.31g /mole
Molecular formula is SnI4
The molecular formula can sometimes be the same as the empirical formula

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