Val (unit)

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A val , also equivalent or gram equivalent ( unit symbol : val) is an outdated unit of the amount of substance . It stands for the amount of a substance that one mole of hydrogen is able to bind or replace in compounds and is therefore calculated using the following numerical equation :

In words: the amount of substance in Val is equal to the amount of substance in moles times the stoichiometric value  , because every particle is counted -fold.

In DIN 32625, the symbol is  named and referred to as equivalent number  .

The numerical value that results from this for an amount of a substance given in Val depends on its use in the chemical reaction under consideration. One speaks of equivalence whenever the substances involved in the reaction react completely with one another, i.e. their amounts of substance are equivalent to one another with regard to the respective reaction equation.

Equivalent concentration (normality) is a molar concentration where the amount of mol is given in Val (obsolete) or in moles using equivalent particles.

Equivalent particle

The Val has been officially inadmissible since January 1, 1978 and was replaced on that day by the SI unit Mol .

The unit symbols cmol or mol c are often used when a substance amount is to be specified. However, this is a violation of German unit law , the recommendations of many specialist societies and DIN 32625; cmol is the unit symbol of the centimole. Instead, the previous concept of equivalents without additions to the unit symbol mol is carried out in accordance with the SI and standard .

The background is that the particles on which the mole is based must be precisely specified according to the mole definition. The Val can therefore be replaced by the mole by using its imaginary fraction instead of the real particle - for example K 2 Cr 2 O 7 : the equivalent particle 1/6 K 2 Cr 2 O 7 , where . can take on different values ​​for one and the same particle in different chemical reactions.

Val and Mol

The difference between Val and Mol, which only emerges when looking at the following chemical reaction:

  • In moles when using molecules as a basis as specified particles: Two moles of sodium hydroxide molecules react with one mole of sulfuric acid molecules to form one mole of sodium sulfate molecules and two moles of water molecules.
  • In val: two val sodium hydroxide react with two val sulfuric acid to form two val sodium sulfate and two mol water (with water it is not possible to speak of val, since water is neutral in itself).
  • In moles based on equivalent particles as specified particles: Two moles of sodium hydroxide equivalents react with two moles of sulfuric acid equivalents ½ H 2 SO 4 to form two moles of sodium sulfate equivalents ½ Na 2 SO 4 and two moles of water equivalents.
  • The reaction equation can also be interpreted in such a way that the symbols stand for individual particles, not for amounts of substance.

Note: According to the definition of the mole in the SI , the underlying particles must be specified.

Equivalent mass

The substance-related mass , which corresponds to one val (in the case of the mole, referred to as molar mass ) is called equivalent mass or, earlier, equivalent weight ( unit : g / val).

The equivalent mass is:

  1. for chemical elements: the atomic mass divided by the valency, i.e. the mass of the element that a gram atom of hydrogen can bind or replace in compounds. Example: oxygen 16/2 = 8; The valency does not only depend on the element, but also on the chemical reaction under consideration.
  2. for compounds: the molecular weight divided by the number of equivalents of the substance with which the compound reacts. Example: Hydrogen chloride with an equivalent weight of 36.5 g / eq and sulfuric acid with 98/2 g = 49 g / eq (HCl has only one, H 2 SO 4, however, has two replaceable hydrogen atoms).
  3. for ions: ion mass divided by the number of free charges of the ion, whereby ions with different valency levels like iron can also have different equivalent masses.
  4. for radicals : sum of all atomic masses that make up the radical, divided by its valency.

The equivalent mass can - like the molar mass - be used to indicate mass; To avoid confusion, the term gram-val or gram equivalent was used in the past. In a chemical reaction, this equivalent mass corresponds to the mass of the substance that would be completely reacted with the equivalent mass of another substance (for example one gram equivalent equal to 49 g sulfuric acid).

Decimal parts of the Val

1 meq = 0.001 eq

Val / liter and Val-%

1 eq / l corresponds to a 1- normal solution. A 1-molar (1 mol / l) hydrochloric acid is 1-normal, a 0.5-molar (0.5 mol / l) sulfuric acid (z = 2) is also 1-normal. Both have 1 eq / liter.

For Val-%, the reference value (100%) is the sum of all Val values ​​of all cations (or anions) present. A Val-% is therefore a hundredth of this sum. The sum of all Val values ​​of cations must be exactly the same as that of the anions (electroneutrality of a solution!).

Individual evidence

  1. Entry on equivalent entity . In: IUPAC Compendium of Chemical Terminology (the “Gold Book”) . doi : 10.1351 / goldbook.E02192 Version: 2.3.
  2. ^ Brockhaus ABC Chemie , VEB FA Brockhaus Verlag Leipzig 1965, p. 501.
  3. ^ Brockhaus ABC Chemie, VEB FA Brockhaus Verlag Leipzig 1965, entry: Mineralquellen, composition of the Marienquelle of Bad Elster.