The equivalent weight (PE) of a substance is one that participates in a chemical reaction, and is used as the basis for a titration. Depending on the type of reaction, it can be defined in one way or another.
For acid-base reactions, PE is the weight in grams of the substance required to supply or react with one mole of H+ (1.008 g); for redox reactions, the weight in grams of the substance that is required to supply or react with one mole of electrons.
For precipitation or complex formation reactions, the weight of substance required to supply or react with one mole of a monovalent cation, 1/2 mole of a divalent cation, 1/3 mole of a trivalent cation . And so on.
Although it may seem a bit complicated at first, some substances always behave chemically in the same way; therefore, it is not difficult to learn the PE values given the cases.
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John Dalton (1808) proposed the equivalent weight of hydrogen as a unit of mass. However, a number of objections to this approach arose. For example, it was noted that most elements did not react directly with hydrogen to form simple compounds (XH)..
Furthermore, elements with various oxidation states, for example permanganate, have more than one equivalent weight. This made it difficult to accept the equivalent weight as a unit of mass..
The presentation by Dimitri Mendeleev (1869) of his periodic table, in which the chemical properties of the elements were related to the ordered order of their atomic weights, constituted a strong argument from those who objected to the use of the equivalent weight as a unit of mass.
Actually, there is no need to use the term "equivalent", since any stoichiometric calculation can be done in terms of moles. However, this term is often used and should not be ignored..
For convenience, the term "equivalent" was introduced: an equivalent of any acid reacts with an equivalent of any base; one equivalent of any oxidizing agent reacts with one equivalent of any reducing agent, etc..
The use of PE in elements and chemical compounds has been replaced by the use of its molar mass. The main reason is the existence of elements and compounds with more than an equivalent weight.
For example, iron (Fe), an element with an atomic weight of 55.85 g / mol, has two valences: +2 and +3. Therefore, it has two equivalent weights: when it works with valence +2 its equivalent weight is 27.93 g / eq; while, when using the valence +3, its equivalent weight is 18.67 g / eq.
Of course, it is not possible to speak of the existence of an equivalent weight of Fe, but the existence of an atomic weight of Fe can be pointed out..
Phosphoric acid has a molecular weight of 98 g / mol. This acid when dissociated into H+ + HtwoPO4-, has an equivalent weight of 98 g / eq, since it releases 1 mole of H+. If phosphoric acid dissociates into H+ + HPO4two-, its equivalent weight is (98 g.mol-1) / (2eq / mol-1) = 49 g / eq. In this dissociation the H3PO4 releases 2 moles of H+.
Although it is not titratable in an aqueous medium, the H3PO4 can dissociate in 3 H+ + PO43-. In this case, the equivalent weight is (98 g.mol-1) / (3 mol eq.-1) = 32.7 g / eq. The H3PO4 delivers in this case 3 moles of H+.
So, phosphoric acid has up to 3 equivalent weights. But this is not an isolated case, so for example, sulfuric acid has two equivalent weights and carbonic acid also has.
-To reduce the errors that can be made during the action of weighing substances, in analytical chemistry the use of a substance of greater equivalent weight is preferred. For example, in the titration of a sodium hydroxide solution with acids of different equivalent weights. The use of the acid with the highest equivalent weight is recommended.
-When using the mass of a solid acid that can react with sodium hydroxide, you have the option of choosing between three solid acids: oxalic acid dihydrate, potassium acid phthalate and potassium hydrogeniodate, with equivalent weights respectively. of 63.04 g / eq, 204.22 g / eq and 389 g / eq.
In this case, it is preferred to use potassium hydrogeniodate acid in the titration of sodium hydroxide, since as it has a greater equivalent weight, the relative error made when weighing it is less..
Equivalent weight is defined in its own way in this substance analysis technique. Here, it is the mass of precipitate that corresponds to one gram of the analyte. This is the element or compound of interest in the study or analysis that is being carried out.
In gravimetry it is common to quote the results of the analyzes as a fraction of the mass of the analyte, often expressed as a percentage..
The equivalence factor is explained as a numerical factor by which the mass of the precipitate must be multiplied to obtain the mass of the analyte, usually expressed in grams..
For example, in the gravimetric determination of nickel, the precipitate that contains it is bis (nickel dimethylglyoximate) with a molar mass of 288.915 g / mol. The molar mass of nickel is 58.6934 g / mol.
The molar mass of the precipitate divided by the molar mass of nickel produces the following result:
288.915 g.mol-1/ 58.6934 g.mol-1 = 4.9224. This means that 4.9224 g of the compound is equivalent to 1 g of nickel; or in other words, 4.9224 g of the precipitate contains 1 g of nickel.
The equivalence factor is calculated by dividing the molar mass of nickel by the molar mass of the precipitate that contains it: 58.693 g.mol-1/ 288.915 g.mol-1 = 0.203151. This tells us that per gram of the nickel-containing precipitate there are 0.203151 g of nickel.
In polymer chemistry, the equivalent weight of a polymerization reagent is the mass of the polymer that has one equivalent of reactivity.
It is particularly important in the case of ion exchange polymers: one equivalent of an ion exchange polymer can exchange one mole of monocharged ions; but only half a mole of doubly charged ions.
It is common to express the reactivity of a polymer as the inverse of the equivalent weight, which is expressed in units of mmol / g or meq / g.
It is obtained by dividing its atomic weight by its valence:
Peq = Pa / v
There are items that have only one equivalent weight and items that can have 2 or more.
Atomic weight = 40 g / mol
Valencia = +2
Peq = 40 g.mol-1/2eq.mol-1
20 g / eq
Atomic weight = 27 g / mol
Valencia = +3
Peq = 27 g.mol-1/ 3 eq.mol-1
9 g / eq
Atomic weight = 58.71 g / mol
Valencia = +2 and +3
Nickel has two equivalent weights corresponding to when it reacts with valence +2 and when it reacts with valence +3.
Peq = 58.71 g.mol-1/ 2 eq. Mol-1
29.35 g / eq
Peq = 58.71 g.mol-1/ 3 eq.mol-1
19.57 g / eq
One way to calculate the equivalent weight of an oxide is by dividing its molecular weight by the valence product of the metal and the subscript of the metal..
Peq = Pm / V S
Pm = molecular weight of oxide.
V = valence of metal
S = subscript of the metal
The product V S is called the total or net charge of the cation.
Molecular weight = Al (2 x 27 g / mol) + O (3 x 16 g / mol)
102 g / mol
Valencia = +3
Subscript = 2
Peq AltwoOR3 = Pm / V · S
Peq AltwoOR3 = 102 g.mol-1/ 3 eqmol-1. two
17 g / eq
There is another way to solve this problem based on stoichiometry. In 102 g of aluminum oxide there are 54 grams of aluminum and 48 grams of oxygen.
Peq del Al = Atomic weight / Valencia
27 g mol-1/ 3 eq.mol-1
9 g / eq
Based on the equivalent weight of aluminum (9 g / eq), it is calculated that in 54 g of aluminum there are 6 equivalents of aluminum.
Then from the properties of the equivalents: 6 equivalents of aluminum will react with 6 equivalents of oxygen to give 6 equivalents of aluminum oxide.
In 102 g. of aluminum oxide there are 6 equivalents.
Therefore:
Al PeqtwoOR3 = 102 g / 6 eq
17 g / eq
The equivalent weight is obtained by dividing its molecular weight by the number of oxyhydryl groups (OH).
Molecular weight = 90 g / mol
OH number = 2
Small Fe (OH)two = 90 g.mol-1/ 2 eq. Mol-1
45 g / eq
In general, it is obtained by dividing its molecular weight by the number of hydrogen it gives up or releases. However, polyrotonic acids can dissociate or release their H in various ways, so they can have more than one equivalent weight..
Equivalent weight HCl = molecular weight / number of hydrogen
Small HCl = g.mol-1/ 1 eq.mol-1
36.5 g / eq
Sulfuric acid (HtwoSW4) can be dissociated in two ways:
HtwoSW4 => H+ + HSO4-
HtwoSW4 => 2 H+ + SW4two-
When he releases an H+ your PE is:
Molecular weight = 98 g / mol
Peq = 98 g.mol-1/ 1 eq.mol-1
98 g / small
And when it releases 2H+:
Molecular weight = 98 g / mol
Peq = 98 g.mol-1/ 2 eq. Mol-1
49 g / eq
For the same reason, phosphoric acid (H3PO4) of molecular weight 98 g / mol, it can have up to three equivalent weights: 98 g / eq, 49 g / eq and 32.67 g / eq.
And finally, the equivalent weight of a salt can be calculated by dividing its molecular weight by the product of the valence of the metal by the subscript of the metal..
PE = PM / V S
Molecular weight = 400 g / mol
Iron valence = +3 eq / mol
Iron subscript = 2
Peq = 400 g.mol-1/ 3 eq.mol-1 x 2
66.67 g / eq
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