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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