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Molecular Equation

A chemical reaction includes the conversion of one or more molecules to other molecules by combination, decomposition or other processes. Chemical reactions are represented by simple molecular formulas. However the reactions actually take place by means of ions.

When reactions take place in aqueous medium that is, in presence of water, the atoms in the molecule either gain or lose electrons to form ions. These ions then combine with other ions to form resultant products. Generally the reactions are not shown as taking place by means of ions. However when done so, it is called total ionic equation.

On the basis of type of conversion, chemical reaction can be classified in six types.
  • Combination reaction
  • Synthesis reaction
  • Decomposition reaction
  • Displacement reaction
  • Redox reaction
  • Neutralization relation
Any chemical reaction can be represents by using chemical formulas of substances involve in reaction, reactant and product. The symbolic representation of chemical reaction is known as chemical equation.

Chemical equation shows the number of reactant and product involve in reaction as well as physical state of substances. A balance chemical equation is based on the law of conservation of mass. Thus both side of chemical equation must have same quantity of all atoms.

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Molecular Equation Definition

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"A chemical equation for reaction which involves only complete formulas of substances existed as molecules is known as molecular equation." It is a balanced chemical equation which shows ionic compounds as their neutral formulas rather than as ions.

For example reaction between silver nitrate and sodium chloride in aqueous solution react to form silver chloride and sodium nitrate. The molecular equation of this reaction shows the chemical formulas of substances with their physical states.

The molecular equation would be

AgNO3(aq) + NaCl(aq) $\to $ AgCl(s) + NaNO3(aq)

Other examples of molecular equations are as follow

CaCl2(aq) + 2AgNO3(aq) $\to $ Ca(NO3)2(aq) + 2AgCl(s)
FeCl3 (aq) + 3AgNO3 (aq)
$\to $ 3AgCl (s) + Fe(NO3)3 (aq)
2KOH(aq) + Mg(NO3)2(aq)
$\to $ Mg(OH)2(s) + 2KNO3(aq)
2HCl (aq) + Na2CO3 (aq)
$\to $ H2O(l) + CO2(g) + 2NaCl (aq)
Na2CO3(aq)+ 2HNO3(aq)
$\to $ 2NaNO3(aq)+ H2O(l) + CO2(g)
KNO3(aq) + HCl(aq)
$\to $ KCl(aq)+ HNO3(aq)
3Ba(OH)2 (aq) + Fe2(SO4)3 (aq)
$\to $ 2Fe(OH)3 (s) + 3BaSO4 (aq)
2NaOH (aq) + CO2 (g)
$\to $ Na2CO3 (aq) + H2O (l)
2Na3PO4 (aq) + 3CaCl2 (aq)
$\to $ 6NaCl(aq) + Ca3(PO4)2(s)

Molecular Equation for HCl and NaOH

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HCl is a strong acid and NaOH is a strong base, thus reaction of strong acid and base results the formation of salt (sodium chloride) and water. Reactions is a type of neutralization reaction and the molecular equation of given reaction must be as follow.

HCl(aq) +NaOH(aq) NaCl(s) + H2O(l)

Molecular Mass Formula

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  1. Like atoms, molecules are also very small in size and therefore their actual masses cannot be determined by direct weighing.
  2. One of the best ways to determine the mass of molecule is to use the relative mass of molecule with respect to the mass of carbon atom (C-12). This relative mass of molecule is known as molecular mass.
  3. Hence molecular mass of a substance can be defined as the average relative mass of a molecule of the substance as compared with mass of an atom of carbon (C-12) taken as 12 u.
  4. Molecular mass of substance indicates the number of times of a molecule is heavier than 1/12th of mass of carbon (C-12) atom. For example relative molecular mass of water (H2O) is 18.
  5. Thus a molecule of water is 18 times heavier than 1/12th of mass of a carbon atom (C-12). Relative molecular mass is a dimensionless quantity and symbolized by MM.
  6. Molecular mass of a substance can be calculated by addition the atomic mass of all the atoms present in a molecule of the substance.

For example the molecular mass equation for ammonia can be written as;

Molecular mass of ammonia (NH3) = atomic mass of nitrogen atom + 3 (atomic mass of hydrogen atom)
Atomic mass of nitrogen atom = 14 u
Atomic mass of hydrogen atom = 1 u
Thus molecular mass of ammonia = 14 + 3(1) = 17 u
Similarly for the molecular mass of magnesium hydroxide, Mg(OH)2
  • Atomic mass magnesium = 24.3 u
  • Atomic mass hydrogen = 1.0 u
  • Atomic mass of oxygen = 16.0 u
The molecular mass equation for magnesium hydroxide, Mg(OH)2 is

Molecular mass of Mg(OH)2 = (1 x atomic mass of Mg) + (2 x atomic mass of O) + (2 x atomic mass of H)

Molecular mass of Mg(OH)2 = 24.3 + (2 x 16.0) + (2 x 1.0) = 24.3 + 32.0 + 2.0 = 58.3 u

The mass of molecule can also be representing as molar mass. It may be defined as that much quantity of the substance whose mass in gram is numerically equal to its molecular mass. Or the mass of one mole of substance is defined as molar mass. It is also called as one gram molecule of the substance. The molar mass represents by ‘m’ or ‘MM’.

Molar mass = $\frac{Mass\ of\ substance}{number\ of\ moles}$


Number of g-molecule = $\frac{Mass\ in\ gram\ [Wg]}{Gram\ molecular\ mass\ [MM]}$

Molecular Weight Formula

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The molecular weight of molecule is the sum of the weights of its constituent atoms. The unit of molecular weight is Daltons which is equals to one-twelfth the weight of an atom of 12C. Molecular weight can be expressed in terms of Avogadro's number that is 6 x 1023.

The number of atom or molecules or particles in one mole is called as Avogadro's number represents as N0. The weight of N0 molecules is equals to the molecular weight of a molecule. It can be calculated from chemical formula of molecule and the atomic weights. For example, the molecular formula of sodium chloride is NaCl. The atomic weight of sodium is 22.990 gram and for Chlorine, it is 35.453 gram.

Thus molecular weight of sodium chloride is

Mw(NaCl) = 22.990 + 35.453 = 58.443 gram

Since one mole is a quantity of N0 atoms or molecules, therefore, the molecular
weight is the weight of one mole of molecules. The molecular weight equation can be written as

W = n - Mw

Mw - molecular weight of a molecule,
n - number of moles
W - total weight of the molecules

For example, the weight of 4 moles of sodium chloride would be 4 x 58.44 = 233.76 gram.

Let’s calculate the molecular weight of water and number of moles of water in 100 gram of it. The molecular formula of the water molecule is H2O, hence two atoms of

Hydrogen and one of Oxygen.
Atomic weight of Hydrogen: 1.00794 gram
Atomic weight of Oxygen: 15.9994 gram.
Hence the molecular weight of water will be

Mw(H2O) = 2 x1.00794 + 15.9994 = 18.015 gram

The number of moles of water in 100 gram of water will be

n = W / Mw = 100 / 18.015 = 5.5509 moles
Molecular Weights of Various Compounds

Molecular Formula Equation

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The molecular formula of a compound may be defined as the formula which gives the actual number of the atoms of various elements present in the molecule of a compound. For example the molecular formula of ethane can be represents as C2H6. Thus a molecule of ethane contains two atoms of carbon and six atoms of hydrogen. For the determination of molecular formula, first we have to determine the empirical formula form the percentage composition.

The empirical formula of a compound can defined as the formula which gives the simplest whole number of atoms of the various elements present in the molecule of compound. The empirical formula of ethane will be CH2.

The relation between both formulas can be written as

n = Molecular formula/Empirical formula

Where n = 1, 2, 3, 4……….

The empirical formula and molecular formula for some common compounds are as follow.
Empirical Formula and Molecular Formula for some Compounds

For writing the molecular formula.
  1. First calculate the empirical formula from percentage composition.
  2. Find out the empirical formula mass by adding the atomic masses of all atoms present in the empirical formula of the compound.
  3. Divide the molecular mass by the empirical mass and find out the value of ‘n’.
  4. Multiply the empirical formula of the compound with ‘ n ’ to get molecular formula.

Let’s take an example, A compound is 54.24% carbon, 9.05% hydrogen, and 36.71% oxygen by mass. It has a molecular weight of 88 g/mol. What is the molecular formula for this compound?

1. Find the empirical formula of given compound

Convert the mass of each element to moles.
(54.24 g C) (1 mole / 12.00 g C) = 4.52 mole C
(9.05 g H) (1 mole /1.008 g H) = 9.05 mole H
(36.71 g O) (1 mole / 16.00 g O) = 2.29 mole O
Find the simple ratio of each element
(2.29 mole O) / (2.29) = 1 mole O
(4.52 mole C) / (2.29) = 1.97 mole C
(9.05 mole H) / (2.29) = 3.95 mole H

Write the empirical formula by using the mole ratio: the empirical formula becomes C2H4O

2. Find the mass of the empirical unit. 12 x 2 +1 x 4 + 16 x 1= 44 g/mol

3. Figure out the value of ‘n’: (88 g/mol) / (44 g/mol) = 2 empirical units per molecular unit.

4. Since there are two empirical units in a molecular unit, therefore the molecular formula is C2H4O x 2 = C4H8O2
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