🧪Empirical & Molecular Formula, Calculating Composition, Determining formulas
Molecular Formula
The molecular formula is the exact formula of a compound/chemical with the subscripts present as it is found.
Example:
Ethane: C2H6
This is also the most useful formula for chemists to use, as the molecular formula gives the exact unique identity for the sampled subject
Empirical Formula
Empirical formula is like the simplified version of the molecular formula, and they contain the same proportions and therefor percentage compositions. This formula is used to calculate compositions typically as it is easier due to the simplified subcripts.
Example:
Ethane: CH3
This is calculated through dividing all of the exponents within a given chemical/compound by the greatest common factor.
For this example, the greatest common factor (GCF) of ethane’s molecular formula (C2H6), using 2 and 6, is 2. So Carbon: 2/2=1 and Hydrogen: 6/2=3.
Calculating Composition
Calculating the composition of the matter (elements) within a chemical or compound provides us with a percentage of the space taken up by the individual elements. However, because different elements have different molar masses, we must take their weight into account. Because empirical and molecular formulas are proportional, you can use either to calculate the elemental composition.
We do the by: # of elements present (molar mass) / total molar mass of compound then converting the decimal into a percent
Or:
(# of atoms of element × molar mass of element)
----------------------------------------------——————-
total molar mass of compound
Example:
Ethanes Empirical composition is CH3
Carbon mass 12.011/(12.011+1.008+1.008)=0.80 → (convert to a percent)
means carbons composition = 80%
100-80=20 so Hydrogen = 20% composition of CH3
Determining molecular formula from compounds
Type 1: Given empirical formula & g/mol mass
When given the empirical formula and molar mass (g/mol), divide the given mass by the combined molar mass of all elements (including subscripts!) to get the GCF of the molecular formula to get to the empirical formula. Once you have this, you multiply the given formulas subscripts by the factor to get the molecular formula!
Example:
Empirical formula is NaO, mass is 78g/mol
78g / (22.99+15.999) = 2
(NaO)2
Molecular formula for NaO = Na2O2
Type 2: Given percentages & grams of sample
Find each mass of eah individual element in the compound by multiplying the total compounds molar mass by the decimal form form of the elements composition to get the weight in grams.
Divide this total mass present of each element in the total compound by the molar mass of said element to get the subscript number for each element. This gives you the compounds molecular formula.
Simplify the molecular formula like previously to find empirical formula if needed.