Molecular FormulasSection 11.4

CaffeineCaffeine is known medically as trimethylxanthine, and the chemical formula is C8H10N4O2

It’s important to know that caffeine is an addictive drug.Among its many actions, it operates using the same mechanisms that amphetamines,cocaine, and heroine use to

stimulate the brain. Relatively speaking, caffeine's effects are milder than amphetamines, cocaine and heroin, but it is manipulating the same channels in the brain, and that is one of the things that gives caffeine its addictive qualities.

Molecular FormulasThe final step in identifying an unknown compound is determining the molecular formula, which shows the actual number of atoms in each molecule of the substance. To determine the molecular formula, the molar mass of the compound must be determined through experimentation.

For example, the molar mass of acetylene (C2H2) is 26 g/mol

The empirical formula for acetylene is CH and the molar mass of the empirical formula is 13 g/mol

Therefore, if the molar mass of the molecular formula for acetylene is divided by the molar mass of the empirical formula for acetylene

26 g/mol ÷ 13 g/mol = 2

The resulting value of “2” shows that the molecular mass must contain twice as the number of carbon atoms and twice the number of hydrogen atoms as

the empirical formula. C2H2

How to Find Molecular Formulas

1. calculate the empirical formula

2. use the equation : (empirical formula mass)x = molar mass

3. find value for x:    x = molar mass/empirical formula mass

4. multiply each subscript in empirical formula by value for x

How to Find Molecular Formulas

Example: Given that the empirical formula of a compound is CH and the molar mass is 104 g/mol, calculate the molecular formula.

mass of C  =  12.0 g/mol mass of H  =   1.01 g/mol

empirical formula mass  =   13.0 g/mol

to find the number of CH units in the compound:

# CH units  =   (104 g/mol)/(1 mol/13.0 g)  =   8.00

Molecular formula  =   8(CH) or C8H8

Steps to Determine Molecular Formulas

1.Express percent by mass in grams.

2.Determine the number of moles of each element.

3.Write the empirical formula. Be sure to examine the mole ratio for whole numbers.

4.Determine the integer that relates the empirical and molecular formulas by dividing the experimental molar mass by the mass of the empirical formula.

5.Multiply the subscripts by the answer to step # 4.

6.Write the molecular formula.

Molecular Formula Practice I1. Analysis of a chemical used in photographic developing fluid indicates a chemical composition of 65.45% C, 5.45% H, and 29.09% O. The molar mass is found to be 110 g/mol. Determine the molecular formula.

2. A compound was found to contain 49.98 g carbon and 10.47 g hydrogen. The molar mass of the compound is 58.12 g/mol. Determine the molecular mass of the formula.

3. A colorless liquid composed of 46.68% nitrogen and 53.32% oxygen has a molar mass of 60.01 g/mol. What is the molecular formula?

Hydrates

A hydrate is a compound that has a specific number of water molecules bound to its atoms. To analyze hydrates, the water of hydration must be driven off by heating the compound. When the water of hydration is driven off, an anhydrous solid is produced. The anhydrous form of a hydrate (dessicator) is used to absorb water. Anhydrous compounds can be packaged with materials that need to be kept moisture free.

Example: Hydrated Silica Hydrated silica is the abrasive used in gel toothpastes, or in the clear portion of striped toothpastes. It is often used in opaque toothpastes as well, as a secondary abrasive. When dried in an oven, hydrated silica looses the water, and becomes a dessicant, a substance that attracts water from the air. You find little packets of "silica gel" crystals in containers whose contents will be damaged by condensing moisture, such as vitamin bottles, consumer electronics, pepperoni, or leather products.

Steps to Determining Anhydrous Formulas

1. Determine the number of moles of the compound.

2. Determine the number of moles of water.

3. Divide by the smallest number of moles.

4. Determine the ratio.

Examples of Hydrate Problems

1. What is the mole ratio of the hydrate CuSO4 ·____ H2O, if the anhydrous mass of the CuSO4 is 25.42 g and the mass of the water is 14.3 g?

Examples of Hydrate Problems Continued

2. What is the mole ratio of the hydrate Na2CO3 · ____ H2O, if the original mass of the hydrated Na2CO3 was 43.69 g and the mass of the anhydrous Na2CO3 is 16.19 g?

Naming HydratesPrefix Molecules

H20Formula Name

Mono- 1 (NH4)2C2O4 · H20 Ammonium oxalate monohydrate

Di- 2 CaCl2 · 2H20 Calcium chloride dihydrate

Tri- 3 NaC2H3O2 · 3H20 Sodium acetate trihydrate

Tetra- 4 FePO4 · 4H20 Iron (III) phosphate tetrahydrate

Penta- 5 CuSO4 · 5H20 Copper (II) sulfate pentahydrate

Hexa- 6 CoCl2 · 6H20 Cobalt (II) chloride hexahydrate

Hepta- 7 MgSO4 · 7H20 Magnesium sulfate heptahydrate

Octa- 8 Ba(OH)2 · 8H20 Barium hydroxide octahydrate

Deca- 10 Na2CO3 · 10H20 Sodium carbonate decahydrate