What is Equivalent Weight?
Equivalent weight in chemistry is the most common term used and one of the basic concepts of chemistry in the physical chemistry part. An equivalent weight which is also known as gram equivalent can be defined as is the mass of one equivalent, that is the mass of a given substance that will combine with or displace a fixed quantity of another substance. Thus, in other words, gram equivalent or the equivalent weight of a substance is the mass of the substance that can displace 1.008 grams of hydrogen or 8.0 grams of oxygen or 35.5 grams of chlorine. Thus to find out the equivalent weight, the atomic weight of the substance is divided by its valence. As an example, the equivalent weight of the oxygen will be equal to 16.0 g / 2 = 8.0 g.
In the acid-base reaction, the equivalent mass of an acid or base is always equal to the amount of mass that supplies or reacts with the one mole of hydrogen ion (H+). similarly, for the redox reaction, the equivalent weight of the substance is the mass that supplies or reacts with one gram mole of electrons (e-) produced in the redox reaction. It has a dimension of the unit mass that is unlike that of the atomic mass that is dimensionless in nature. The equivalent weight can be determined by the experiment and it can be determined from the molar mass of the substance. In addition, the equivalent weight can be determined by dividing the molecular mass by the number of positive or negative electrical charges that result from the dissolution of the compound.
Here, we have covered the important topics related to the equivalent weight of metal. What is the equivalent weight definition in chemistry? The amount of substances that completely react with each other in the reaction is called equivalent weight in chemistry. While answering the question, what is equivalent weight? you should keep in mind that its definition depends on the two factors; the molar mass and valency factor of the compound.
Equivalent Weight of Acid and base
Equivalent weight = molecular weight / X
In the above formula X represents the valency factor.
For Acids:
Taking an example of sulfuric acid as follows:-
H2SO4 + 2OH- → 2H2O + SO42-
The equivalent weight of the acid can be determined by determining the individual molecular weight of each of the elements from the periodic table and firstly adding them together. This will give us the molecular weight of the acid.
2(1) + (32) + 4(16) = 98.0.
The acid is seen to be donating two protons as the sulfate ion is seen to acquire negative charges. Therefore the equivalent weight of the acid would be
98.0/2 = 49.0.
In the case of hydrochloric acid (HCl)
HCl → H+ + Cl-
The number of hydrogen ions or hydronium ions released by hydrochloric acid is one.
So, the valency factor will be one.
The molecular weight of hydrochloric acid = 36.45
As we know, Equivalent weight = molecular weight / X
The equivalent weight of hydrochloric acid = 36.45 / 1 = 36.45
For Bases
The reasoning for the base is the same. For example, ammonium hydroxide can accept a proton in solution to become an ammonium ion.
NH4OH + H+ → H2O + NH4+
The molecular weight of the hydroxide will be as follows.
(14) + (4)(1) + (16) + 1 = 35.0.
Here, since only one proton is accepted, thus the equivalent weight is equal to
35.0/1 = 35.0.
For the base, X (valency factor) is the acidity
Acidity- Acidity is the number of hydroxyl ions or hydroxide ions released by a base.
In the case of calcium hydroxide base Ca(OH)2
Ca(OH)2 →Ca+ + 2 OH-
The number of hydroxyl ions released by the calcium hydroxide base is 2. Therefore, its valency factor or X value will be two.
The molecular weight of the calcium hydroxide base is 74.
As we know, Equivalent weight = molecular weight / X
The equivalent weight of calcium hydroxide base= 74 / 2 = 37
In the case of aluminium hydroxide base Ca(OH)2
Al(OH)3 → Al+3 + 3 OH-
The number of hydroxyl ions released by the aluminium hydroxide base is 3. Therefore, its valency factor or X value will be three.
The molecular weight of the aluminium hydroxide base is 78 g/mol.
As we know, Equivalent weight = molecular weight / X
The equivalent weight of calcium hydroxide base= 78 / 3 = 26.
Equivalent Weight of the Metal in Salt or Compounds
1. Aluminium Chloride
Step by Step Calculation for Finding the Equivalent Weight of Aluminium Salts
For the metals, X (valency factor) is the total positive charge on the positive ion (cation).
In the case of aluminium chloride salt Al(Cl)3
AlCl3 → Al3+ + 3Cl-
The number of positive charges on aluminium cation is three. Therefore, its valency factor or X value will be three.
The molecular weight of the aluminium chloride base is 133.34 g/mol.
As we know, Equivalent weight = molecular weight / X
The equivalent weight of aluminium chloride salt= 133.34 / 3 = 44.44.
2. Silver Carbonate (Ag₂CO₃)
Step by step calculation for finding the equivalent weight of silver salts
For the salts, X (valency factor) is the total positive charge on the positive ion (cation).
In the case of silver carbonate salt Ag2CO3
Ag2CO3 → 2Ag+ + CO3-
The total positive charge of the silver cation is two. Therefore, its valency factor or X value will be two.
The molecular weight of the silver carbonate salt is 275.75 g/mol.
As we know, Equivalent weight = molecular weight / X
The equivalent weight of silver carbonate salt= 275.75 / 2 = 137.87.
Did You Know?
Equivalent weight is used in the calculation of normality. The normality of a solution is defined as the number of gram equivalents of the solute present per litre of the solution. It is represented by the symbol, N.
Normality = gram equivalent of the solute/volume of the solution in litres
The number of gram equivalents of the solute is calculated as follows:
No. of gram equivalents = mass of solute in grams / equivalent mass of the solute.
Summary
The equivalent masses of acids, bases, and salts are calculated as follows:
Equivalent mass of an acid = molecular mass of the acid/basicity.
Equivalent mass of a base = molecular mass of the base/acidity.
Equivalent mass of a salt = molecular mass of the salt/total positive valency of metal atoms.
FAQs on Equivalent Weight
1. What is meant by Equivalent Weight in chemistry?
The equivalent weight of a substance is defined as the mass of that substance which will combine with or displace a fixed quantity of another substance. This fixed quantity is typically 1.008 parts by mass of hydrogen, 8 parts by mass of oxygen, or 35.5 parts by mass of chlorine. It is a measure of the reacting capacity of a substance in a particular reaction.
2. What is the general formula to calculate the equivalent weight of a substance?
The equivalent weight of a substance is calculated using a general formula that connects its molar mass to its reactive capacity in a specific reaction. The formula is:
Equivalent Weight = Molar Mass / n-factor
Here, the n-factor (or valency factor) represents the total number of electrons gained or lost, the number of H⁺ ions donated or accepted, or the total charge on the cation/anion, depending on the type of substance and reaction.
3. How do you calculate the equivalent weight for an acid and a base?
The calculation for acids and bases depends on their ability to donate or accept protons (H⁺ ions).
- For an Acid: The n-factor is its basicity, which is the number of replaceable H⁺ ions it can donate per molecule. For example, for H₂SO₄, the basicity is 2, so its equivalent weight is Molar Mass / 2.
- For a Base: The n-factor is its acidity, which is the number of replaceable OH⁻ ions it can furnish per molecule. For example, for Ca(OH)₂, the acidity is 2, so its equivalent weight is Molar Mass / 2.
4. How is the equivalent weight of a salt like Sodium Carbonate (Na₂CO₃) determined?
For a salt, the n-factor is the total magnitude of the positive or negative charge on the ions produced upon dissociation. For Sodium Carbonate (Na₂CO₃), it dissociates into 2Na⁺ and CO₃²⁻. The total positive charge is +2 (from two Na⁺ ions). Therefore, the n-factor for Na₂CO₃ is 2. Its equivalent weight would be its Molar Mass divided by 2.
5. Is the equivalent weight of a substance always the same as its molecular weight?
No, they are not the same, except in specific cases. Molecular weight is the mass of one mole of a substance and is a fixed value. Equivalent weight, however, depends on the n-factor, which can change depending on the chemical reaction. For example, in a reaction where a substance's n-factor is 1 (like HCl donating one H⁺), its equivalent weight equals its molecular weight. But for H₂SO₄ (n-factor=2), the equivalent weight is half its molecular weight.
6. Why is the concept of equivalent weight important in chemistry?
The concept of equivalent weight is crucial for several practical applications in chemistry, especially in stoichiometry and analytical chemistry. Its main importance lies in:
- Normality Calculations: It is fundamental for calculating the Normality of a solution, which expresses concentration in terms of gram equivalents per litre.
- Titrations: It simplifies calculations in acid-base and redox titrations, as one equivalent of a substance reacts exactly with one equivalent of another.
- Electrochemistry: It is used in Faraday's laws of electrolysis, which relate the amount of substance deposited at an electrode to the quantity of electric charge passed.
7. How can the equivalent weight of an oxidising or reducing agent change depending on the reaction?
The equivalent weight of an oxidising or reducing agent is highly dependent on the reaction medium (acidic, basic, or neutral) because the number of electrons transferred (the n-factor) can vary. For example, potassium permanganate (KMnO₄) is a strong oxidising agent:
- In an acidic medium, Mn goes from +7 to +2 oxidation state (a gain of 5 electrons). So, n-factor = 5.
- In a neutral or weakly alkaline medium, Mn goes from +7 to +4 (a gain of 3 electrons). So, n-factor = 3.
