Chemistry Chapter 7: Section 3 and 4

Molar Mass

The mass of one mole of a pure substance. (Equal to the mass in grams of one mole, or approximately 6.022 x 10²³ particles; written in g/mol)

Percentage Composition

The percentage by mass of each element in a compound.

Calculating the Formula Mass

This is found by adding the average atomic masses of all atoms represented in the formula. (Written in amu)

Formula For Finding the Molecular Formula

x = molecular formula (or mass) / empirical formula (or mass) x = m/e

Using Empirical Formula to Determine Molecular Formula

The relationship between a compound's empirical formula and its molecular formula can be written as follows. x(empirical formula) = molecular formula The number represented by x is a whole-number multiple indicating the factor by which the subscripts in the empirical formula must be multiplied to obtain the molecular formula. (The value of x is sometimes 1.) The formula masses have a similar relationship. x(empirical formula mass) = molecular formula mass To determine the molecular formula of a compound, you must know the compound's formula mass. Dividing the experimental formula mass by the empirical formula mass gives the value of x. Recall that a compound's molecular formula mass is numerically equal to its molar mass, so a compound's molecular formula can also be found given the compound's empirical formula and its molar mass.

Formula Mass

The sum of the average atomic masses of all atoms represented in the formula of any molecule, formula unit, or ion. (Written in amu's)

Calculating the Molar Mass

This is calculated by summing the masses of the elements present in a mole of the molecules or formula units that make up the compound. (Written in g/mol)

Calculating the Empirical Formula

To determine a compound's empirical formula from its percentage composition, begin by converting percentage composition to a mass composition. Assume that you have a 100.0 g sample of the compound. Then calculate the amount of each element in the sample. Next, the mass composition of each element is converted to a composition in moles by dividing by the appropriate molar mass. To find the ratio of smallest whole numbers, divide each number of moles by the smallest number in the existing ratio.

Calculating the Percentage Composition

To find the mass percentage of an element in a compound, one can divide the mass of the element in a sample of the compound by the total mass of the sample, then multiply this value by 100.

Converting Moles to Mass

molar mass / 1 mol

Converting Mass to Moles

1 mol / molar mass

Converting Moles to Particles

6.022 x 10²³ / 1 mol

Formula Mass vs. Molar Mass

A compound's molar mass is numerically equal to its formula mass. Formula mass is written in the unit "amu," but molar mass is written in the unit g/mol.

Empirical Formula

Consists of the symbols for the elements combined in a compound, with subscripts showing the smallest whole-number mole ratio of the different atoms in the compound.

Chemical Formula

Indicates the relative numbers of atoms of each kind in a chemical compound by using atomic symbols and numerical subscripts.

Molecular Formula

Shows the types and numbers of atoms combined in a single molecule of a molecular compound.

Empirical Formula vs. Molecular Formula

The empirical formula contains the smallest possible whole numbers that describe the atomic ratio. The molecular formula is the actual formula of a molecular compound.

Converting Particles to Moles

1 mol / 6.022 x 10²³