Formula Mass:



Formula Mass:

➢ compounds are represented by formulas

o formulas show the type and number of atoms presents in the compound.

o the mass of the smallest unit of the compound is the formula mass.

➢ formula mass is the sum of the atomic masses of all the atoms present.

o formula mass is used for ionic and network solids

▪ network solids are networks of atoms like carbon forming 3 dimensional network structures.

• examples include – diamonds, graphite and “bucky-ball”.

• bucky-ball is buckminsterfullerene – it contains 60 carbon atoms bonded in a pattern similar to a soccer ball.

NOTE:

➢ Molecular mass – is used to represent the mass of a unit for a compound. “Formula mass” is the preferred term because ionic compounds do not form molecules.

➢ GRAM FORMULA MASS:

o Gram formula mass is the formula mass of a substance expressed in grams instead of atomic mass units (amu)

o Gram molecular mass is the gram formula mass for molecular substances.

THE MOLE: “The chemist’s dozen”

➢ A mole of any element is defined as the number of atoms of that element equal to the number of atoms in exactly 12.0 grams of C-12

➢ A mole is the number of atoms of an element whose mass in grams is numerically equal to

6.02 x 1023.

▪ 6.02 x 1023 H atoms in 1 g of hydrogen

▪ 6.02 x 1023 O atoms in 16 g of oxygen

▪ 6.02 x 1023 Pb atoms in 207 g of lead

➢ the mole establishes the relationship between the amu and the gram.

➢ the number of atoms in 1 mole of an element = 6.02 x 1023

➢ the number of molecules in 1 mole of a compound = 6.02 x 1023

For example:

1 dozen = 12 things

1 pair = 2 things

1 gross = 144 things

1 mole = 6.02 x 1023 things

Mole is therefore used to describe numbers of atoms or molecules.

6.02 x 1023 is known as “Avogadro’s #”

To find the mass of one mole of a substance – determine its gram formula mass.

mole = mol

How do you convert grams to moles:

➢ moles = number of grams x 1 mol

gram formula mass

For example:

How many moles are = to 4.75 g of NaOH?

step 1: what do you know – the formula mass of NaOH

NaOH = 1 atom of Na = 1 x 23.0 amu = 23.0 amu

1 atom of O = 1 x 16.0 amu = 16.0 amu

1 atom of H = 1 x 1.0 amu = 1.0 amu

formula mass NaOH = 40.0 amu

gram formula mass NaOH = 40.0 grams

Moles = number of grams x 1 mol

gram formula mass

Moles = 4.75 g x 1 mol

40.0g

Moles of NaOH = 0.119 mol

More Practice –

How many moles are = to the following:

a. 39 g of LiF

b. 15 g lithium

c. 22 grams argon

d. 9.8 grams of calcium chloride

➢ Molar Mass = the weight of one mole of a chemical compound. expressed in grams/mole

▪ we can equate the number of moles to a mass = molar mass.

▪ to find molar mass, need to know the formula –

• multiply # of atoms of each element times the atomic masses.

• add them together = molar mass

molar mass of H2SO4

2 H 2 x 1.0 = 2.0

1 S 1 x 32.1 = 32.1

4 O 4 x 16.0 = 64.0

97.1 g/ml

molar mass of H2SO4 = 97.1 g/mol

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