7-4 Determining Chemical Formulas
7-4 Determining Chemical Formulas
7-4 Determining Chemical Formulas
Find the Empirical formula for a compound with…
Empirical Formula – chemical formula of a compound with
subscripts reduced to lowest whole number mole ratios
Na → 32.38%
C6H12O6
Simplifies to:
Simplifies to:
CH2
CH2O
O → 44.99%
Assume a sample of 100.0g and convert into moles.
Examples of Empirical Formulas…
C4H8
S → 22.65%
32.38 g Na
1 mole Na
= 1.41 moles Na
23.0 g Na
22.65 g S
1 mole S
= 0.706 moles S
32.1 g S
44.99 g O
1 mole O
= 2.81 moles O
16.0 g O
1
7-4 Determining Chemical Formulas
7-4 Determining Chemical Formulas
Continued…
Find the Empirical formula for …
A 10.150 g sample of a compound made only of phosphorus
and oxygen was measured to have 4.433 g of phosphorus.
Divide each amount by the smallest amount to get the
lowest whole number ratios
1.41 moles Na
2
Continued…
0.706 moles S
≈ 2 moles Na
0.706
≈ 1 mole S
0.706
2.81 moles O
Find the mass of oxygen present…
10.150 g Compound - 4.433 g P = 5.717 g O
≈ 4 moles O
4.433 g P
1 mole P
31.0 g P
5.717 g O
1 mole O
0.706
The empirical formula is Na2SO4
16.0 g O
= 0.143 moles P
= 0.357 moles O
3
7-4 Determining Chemical Formulas
7-4 Determining Chemical Formulas
Molecular Formulas
Continued…
A correct molecular formula may be the multiple of an
empirical formula. We can use “x” as a multiplier.
Divide each amount by the smallest amount to get the
lowest whole number ratios
0.143 moles P
≈ 1 moles P
4
Continued…
0.357 moles S
0.143
≈ 2.5 mole O
0.143
molecular formula mass
=x
empirical formula mass
(empirical formula)x = molecular formula
PO2.5 must have whole number subscripts…
Find the molecular formula…
PO2.5 → (PO2.5)2 = P2O5
A compound has an empirical formula of P2O5 but has a
molar mass of 284.0 g/mol. (Molar mass of P2O5 = 142.0 g/mol)
The empirical formula is P2O5
5
284.0 g/mol
142.0 g/mol = 2
→
(P2O5)2 = P4O10
6
1