Empirical formula
- Contain symbols for element, and subscripts that shows the smallest whole number ratio
of different elements
- Ex: BH3
Molecular formula
- Contain symbols for element, and actual formula of molecular compound
- Ex: B2H6
Relationship between Empirical formula and Molecular formula
- Whole number * (empirical formula) = molecular formula
- 2 * BH3 (empirical formula) = B2H6 (molecular formula)
- If a compound’s molecular formula cannot be simplfied any more then the empirical
formula is the same as the molecular formula.
Calculation of Empirical Formulas
- How to Calculate Empirical Formula from percentage Composition
- Percentage composition each element => Empirical Formula
a. Step 1 calculate the amount of each element in the sample
- (Mass of sample) * (percentage composition of elements)
- 100g * 78.1% (B) =
78.1g (B)
- 100g * 21.9% (H) =
21.9g (H)
b. Step 2 calculate the number of moles in mass composition of each element
- (Mass of composition) / (molar mass of each element)
- 78.1g (B) / 10.81g (B) * 1 mol B (for unit conversion) = 7.22 mol B
- 21.9g (H) / 1.01g (H) * 1 mol H (for unit conversion) = 21.7 mol H
- This gives mole ratio
- 7.22 mol B : 21.7 mol H
c. Step 3 calculate the smallest whole number ratio
- (Number of moles) / (smallest number in the existing ratio)
- 7.22 mol B / 7.22 mol B = 1 mol B
- 21.7 mol H / 7.22 mol B = 3.01 mol H
- This gives Empirical formula
- 1 mol B : 3.01 mol H => BH3
Calculation of Molecular Formulas
- How to Calculate Molecular Formula from empirical formula and molar mass of
Molecular Formula
- Empirical formula * x = molecular formula
- Goal: finding X
-
Step 1 Calculate the empirical formula mass
- Mass of P2O5
- 2* 30.97 (P) (Mass of P) + 5 * 16.00 (O) (Mass of O) = 141.94
amu
-
Step 2 Calculate molecular formula
(Molecular formula Mass) / (Mass of Empirical formula)
- 283.89 amu (Molecular formula Mass) / 141.94 (Empirical formula
mass)
- X=2
-
Step 3 Calculate molecular formula
- X * empirical formula
- 2 * P2O5 (empirical formula)
- P4O10
Example question:
1. Empirical formula of a compound of Carbon, Hydrogen and Oxygen is C2H6O.
Experimentation shows that the molar mass of this compound is 138.21 g/mol.
What is the compound’s molecular formula?
2. Prof. Kyungtae took an experiment to find new substances. He found that
Quantitative analysis shows that a compound is composed of 2.01% of Hydrogen,
65.3% of Oxygen and 32.5% of Sulfur. Find the empirical formula of this compound.
3. There are imaginary compounds A and B. Compound A is composed of 2.1% of
Boron, 0.4% of Hydrogen and 97.5% of Iodine. Compound B is composed of 2.2% of
Boron and 97.8% of Bromine. Mad scientist is trying to mix both compounds in new
ways, which is just adding the numbers of elements. (For example if you want to mix
H2O and CH4 then the mixture becomes CH5O) Then what is the empirical formula
of the new mixture.
답
1. C6H18O3
2. H2SO4
3. BHI2Br3