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Molecular Formulas

Section 11.4. Molecular Formulas. Caffeine. Caffeine is known medically as trimethylxanthine, and the chemical formula is C 8 H 10 N 4 O 2 It’s important to know that caffeine is an addictive drug.

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Molecular Formulas

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  1. Section 11.4 Molecular Formulas

  2. Caffeine Caffeine 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.

  3. Molecular Formulas The 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

  4. How to Find Molecular Formulas • 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 • multiply each subscript in empirical formula by value for x

  5. 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

  6. 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.

  7. Molecular Formula Practice I 1. 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?

  8. 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.

  9. Steps to Determining Anhydrous Formulas • 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.

  10. 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?

  11. 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?

  12. Naming Hydrates

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