The Mole

Molar Mass Molar mass relates moles to grams The molar mass is the mass in grams of 1 mole of an element or compound  Use the mass numbers from the periodic table, instead of amu’s, the units are grams

Round all masses from the periodic table to the nearest tenth place

The chemical formula gives the ratio of atoms in a compound

The chemical formula can also give the ratio of ions to one another

Molar masses are computed in the typical way

Volume-Mass Relationships of Gases Gay-Lussac’s law of combining volumes of gases states that at constant temperature and pressure, the volumes of gaseous reactants and products can be expressed as ratios of small whole numbers  Example: 2 L of H 2 reacts with 1 L of O 2 to produce 2L of H 2 O

Avogadro’s law states that equal volumes of gases at the same temperature and pressure contain equal numbers of molecules  Also indicates that volume is directly proportional to the amount of gas, at a given temperature and pressure V=kn

According to Avogadro’s law, one mole of any gas will occupy the same volume as any other gas at the same conditions The standard molar volume of gas had been determined to be L (22.4L)

The Ideal Gas Law The ideal gas law is the mathematical relationship among pressure, volume, temperature, and the number of moles of each gas

The ideal gas law is derived by combining Boyle’s law and Charles’s law and the logic of Avogadro’s law By adding a constant R we get: Or PV= nRT

The constant R is known as the ideal gas constant  Its value depend on the units chosen

The ideal gas law can be manipulated to solve for molar mass and density  Given that m = mass and M = molar mass  n=m/M and D=m/V

Empirical formula 1. Choose an arbitrary sample size (100g). 2. Convert masses to amounts in moles. 3. Write a formula. 4. Convert formula to small whole numbers. 5. Multiply all subscripts by a small whole number to make the subscripts integral.

Determining the Empirical and Molecular Formulas of a Compound from Its Mass Percent Composition. Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H and 27.79% O. Its experimentally determined molecular mass is 230 u. What are the empirical and molecular formulas of dibutyl succinate? Step 1: Determine the mass of each element in a 100g sample. C g H 9.63 g O g Dibutyl succinate is an insect repellent used against household ants and roaches. Its composition is 62.58% C, 9.63% H and 27.79% O. Its experimentally determined molecular mass is 230 u. What are the empirical and molecular formulas of dibutyl succinate? Example 3-5

Prentice-Hall © 2002 General Chemistry: Chapter 3 Slide 17 of 37 Step 2: Convert masses to amounts in moles. Step 3: Write a tentative formula. Step 4: Convert to small whole numbers. C 5.21 H 9.55 O 1.74 C 2.99 H 5.49 O Example 3-5

Prentice-Hall © 2002 General Chemistry: Chapter 3 Slide 18 of 37 Step 5: Convert to a small whole number ratio. Multiply x2 to get C 5.98 H O 2 The empirical formula is C 6 H 11 O 2 Step 6: Determine the molecular formula. Empirical formula mass is 115 u. Molecular formula mass is 230 u. The molecular formula is C 12 H 22 O 4 Example 3-5

Concentration & Molarity The ratio of the solute to the solvent is the solution concentration

Molarity is the moles of solute per liter of solution It is abbreviated with an M 5.0 M HCl is read as a 5 molar solution of HCl

Molality is the concentration of a solution expressed in moles of solute per kilogram of solvent Symbol is m