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 numbersExample: 2 L of H2 reacts with 1 L of O2 to

produce 2L of H2O

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 22.41410 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

1VP

V T V n

By adding a constant R we get:

Or PV= nRT

1V R T nP

1V T nP

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

mRTPVM

mRTMPV

MPDRT

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 62.58 g H 9.63 g O 27.79 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

General Chemistry: Chapter 3Prentice-Hall © 2002 Slide 17 of 37

Step 2: Convert masses to amounts in moles.

OmolOgOmolOgn

HmolHgHmolHgn

CmolCgCmolCgn

O

H

C

737.1999.15

179.27

55.9008.1

163.9

210.5011.12

158.62

Step 3: Write a tentative formula.

Step 4: Convert to small whole numbers.

C5.21H9.55O1.74

C2.99H5.49O

Example 3-5

General Chemistry: Chapter 3Prentice-Hall © 2002 Slide 18 of 37

Step 5: Convert to a small whole number ratio.

Multiply x2 to get C5.98H10.98O2

The empirical formula is C6H11O2

Step 6: Determine the molecular formula.

Empirical formula mass is 115 u.Molecular formula mass is 230 u.

The molecular formula is C12H22O4

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

molesofsolutemolaritylitersofsolution

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

Symbol is m