QuestionsCategory: ChemistryChemistry: Empirical Formula and Molecular Formula
akinlabi asked 2 years ago
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I really need to understand this topic more, i tend to make pretty mistakes most of the time.

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9jaedu.com replied 2 years ago

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9jaedu.com answered 2 years ago
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Brief explanation on what You asked:
A molecule is composed of similar or different atoms which are bonded together through chemical bonds. The atoms are bonded with each other with the help of their valence electrons. The bonding of atoms can be ionic or covalent in nature. Any molecule can be represented with the help of chemical formula. A chemical formula represents the constituent atoms with their number.

We know that an atom can represented with the help of atomic symbols such as atomic symbol for calcium is Ca, and for potassium it is ‘K’. The constituent atoms in the molecule must be written in the chemical formula of it. The number of atoms must be written as subscript after atomic symbol. For example, the chemical formula of carbon dioxide is CO2CO2. It represents that there are one carbon atom and two oxygen atoms in one molecule of carbon dioxide molecule.

The chemical formula of a molecule can be written as molecular formula or empirical formula or structural formula. The molecular formula denotes the total number of atoms in a molecule whereas structural formula of a molecule represents the structure of the molecule. Let’s discuss molecular formula of some other molecules.
NOW THE DEFINITIONS:

Molecular Formula Definition
The molecular formula of a substance expresses the actual number of atoms of the element present in its molecule. It may be the same as the empirical formula or an exact multiple of it.

Empirical Formula Definition

The empirical formula of a compound is the simplest formula which expresses its percentage composition. It is the ratio of the different elements present in a chemical compound. Empirical formula does not show the exact number of elements present.
For example, molecular formula of Benzene is C6H6. This shows us that the molecule contains six carbon atoms and six hydrogen atoms. The molecular formula of benzene is different from its empirical formula which is CH.
 
Empirical formula shows the simplest whole number ratio of the atoms in a molecule.
The ‘molecule’ referred to here, could be an ionic compound or a covalent compound. One more example can be given of Ethyne, which has a molecular formula of C2H2.

This molecule also has an empirical formula of CH. So, more than one compound can have the same empirical formula, but not a molecular formula.

We can calculate the molecular formula from the empirical formula if we know the relative molecular mass or molar mass of the substance.
 

Molecular Formula Examples

Examples of some molecular formulas are:

NH3 : Ammonia
CaCO3 : Calcium Carbonate
HNO2 : Nitrous acid

NaNO2 : Sodium Nitrite
CH3COOH : Acetic acid

Molecular Formula and Empirical Formula Examples

Substance
Empirical Formula
Molecular Formula

Methane
CH4
CH4

Ethylene
CH2
C2H4 or (CH2)2

Glucose
CH2O
C6H12O6

Diborane
BH3
(BH3)2 or B2H6

How to Find Molecular Formula?

Molecular formula of a compound is calculated by adding the atomic masses of all the elements present in the compound. We need to take care of the number of each of these elements present in a chemical compound. Also, the brackets need to be taken care of. The atomic masses of all the compounds are obtained from the periodic table.

Solved Examples
Question 1: Find the molecular formula for NaCl.
Solution:

The compound contains Sodium: Na 
Atomic mass of Sodium = 22.98 grams/mole.
Atomic mass of chlorine = 35.45 grams/mole.
Molecular mass of Sodium Chloride = 22.98 + 35.45 = 58.43 grams/mole.

Question 2: Find the molecular formula for CaSO4
Solution:

In calcium sulphate, there are one mole of Calcium, One mole of Sulfur and 4 moles of Oxygen. Let us calculate the molecular mass of this compound:
Atomic mass of Calcium = 40.078 grams/mole.
Atomic mass of Sulfur = 32.066 grams /mole.
Atomic mass of Oxygen = 16 grams / mol. There are four moles of Oxygen. So, 
            4 x 16 of oxygen = 64 
Adding the atomic masses of all elements:  
Ca + S + 4 X O = 40.078 + 32.066 + 64 = 136.144 grams/mole.
When a compound has a complex formula, we need to take care of the brackets too, while calculating the molecular formula.

Question 3: Find the molecular formula for Al2(SO4)3
Solution:

Aluminium Sulphate has: Aluminium – 2 moles.
In the brackets: Sulfur – one mole x 3 = 3 moles of Sulfur.
                            Oxygen – 4 moles x 3 = 12 moles of oxygen.
 
Now, adding molar mass of all the elements + the numbers of these elements present:
Molecular formula of Aluminium Sulphate 
               = 2 x Al + 3 x (S + 4 x O)
               = 2 x Al + 3 x S + 12 x O
               = 2 x 26.98 + 3 x 32.066 + 12 x 16
               = 342.15grams / mole.

Question 4: Calculate the molecular formula for CH3COOH
Solution:

This compound, acetic acid contains: 2 moles of Carbon, 2 moles of Oxygen and 4 moles of Hydrogen.
To calculate the molecular mass: 
Molecular mass of Acetic acid = 2 x 12.01 + 2 x 16 + 4 x 1.008
= 60.05grams/mole.

Question 5: Calculate the molecular mass of K2Cr2O7
Solution:

Molecular mass of Potassium dichromate = 2x K + 2 x Cr + 7 x O
                                                             = 2 x 39.09 + 2 x 51.99 + 7 x 16
                                                             = 294.185grams / mole.
 
Molecular formula of a compound can be calculated from its empirical formula too.

Molecular Formula From Empirical Formula

Solved Examples
Question 1: The molar mass of a compound, by molecular mass calculation experiment was found to be 44. Analysis of its oxide showed that 2.2 grams of it contained 1.4 grams of Nitrogen atoms, and 0.8 grams of Oxygen atoms. What will be the molecular mass of the compound?
Solution:
 
First step would be to find out the number of moles of Nitrogen and oxygen atoms:
 
Number of moles of Nitrogen atoms = Mass of Nitrogen in gramsMolar mass of N2Mass of Nitrogen in gramsMolar mass of N2


= 1.4 grams14 grams.mol11.4 grams14 grams.mol−1


= 0.1 mole.
 
Number of moles of Oxygen = Mass of OxygenMolar mass of O2Mass of OxygenMolar mass of O2


= 0.8 grams16 grams.Mol10.8 grams16 grams.Mol−1


= 0.05 mole
 
Moles of Nitrogen are twice that of Oxygen. Therefore, ratio of moles of nitrogen to oxygen is 2:1. Thus, the empirical formula can be given as: N2O.
 
To calculate the molecular formula, we need to use the molecular mass data given.
Molecular mass of the compound = 44 grams/ mole.
The empirical formula mass is = 2 x N + O = 28 + 16 = 44 g/mol.
This is matching with the molecular formula mass or the molar mass. So, in this case, empirical formula is the molecular formula.
 

Question 2: A chloride of sulfur was found to have a relative molecular mass of 135. A 5.4 grams sample was also found to contain 2.84 grams of chlorine. What is the molecular formula of chloride?
Solution:
 
Given data: Relative molar mass = 135.
Amount of chlorine present = 2.84 grams.
Since this is a chlorine compound, the mass here refers to the mass of ‘Cl’ and not ‘Cl2’.
Mass of sulfur from the sample = Mass of total sample – mass of chlorine present.
= 5.4 grams – 2.84 grams = 2.56 grams.
 
So, 2.56 grams of Sulfur is present in the sample.
Number of moles of Sulfur atoms = Mass of sulfurAtomic mass of SMass of sulfurAtomic mass of S


= 2.56g32g.mol12.56g32g.mol−1


= 0.08 moles.
 
Number of Moles of Chlorine is =
Mass of chlorineAtomic mass of Cl.Mass of chlorineAtomic mass of Cl.


= 2.84 g35.5 g.mol12.84 g35.5 g.mol−1


= 0.08 moles.
 
So, there are same moles of Chlorine and sulfur in this compound.
Empirical formula would be SCl.
We know that the molecular mass given is = 135 g/mol.
Empirical formula mass = S + Cl = 67.5 g/mol.
The factor for multiplication of empirical formula would be: 135 / 67. 5 = 2
Therefore, molecular formula = 2 x (SCl) = S2Cl2

WE HOPE YOU WILL FIND THIS HELPFUL AKINLABI, 
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