# How to find the relative mass

The universal weight reference in chemistry is the relative atomic weight u. It is equivalent to 1 12 of the mass of the 6 12 C isotope. This is a very small number, 1. In order to avoid calculating with this number, the symbol u is used instead. In principle, any number would do, but given that u already introduces a number with a negative exponential, why not take a number that gets rid of the exponential all together?

So we define this number to be. The number we have introduced here is referred to as the Avogadro constant. So if we take this number of molecules and calculate the overall weight of this mass, the result would amount to a round number. Sometimes, especially in biology and biochemistry, molecular weights are expressed in Dalton Da. The Avogadro constant is often confused with the Loschmidt number N L that describes for an ideal gas the number of atoms or molecules contained in a given volume see also section 6. Molecular Weights. Now, calculating the molecular weight of a molecule is simply a question of adding up the values given for the relative atomic weights of the individual atoms that a molecule is composed of.

These values are given, e. Now how does this help us? Remember, the relative atomic mass is given as 1 12 of the mass of the 6 12 C isotope in atomic units u.

### Key Concepts

Therefore, the molecular weight can be simply read from the periodic table. Using the molecular weight, we can formulate the dependence of the amount of substance measured in number of molecules, i. This allows us to convert from a given mass m of a substance to the number of molecules that are contained in this quantity using the molecular weight M. In general, chemistry works with substance amounts n rather then with mass m. This is due to the fact that chemical reactions involve individual molecules and if a chemical reaction requires two different molecules, it makes sense to supply a quantity that will ensure that there is a sufficient and balanced number of both molecules.

As an example, in order to create carbon dioxide, one needs two molar parts of oxygen and one molar part of carbon. This correct balancing of substance amount is referred to as stoichiometry. Uneven Atom Mass. As stated in section 6. This is common, e. As an example, for neon the given atomic mass is The given atomic weight is an averaged value.

Other prominent examples of this are lithium, boron, or chlorine. Educt, Reactant, Product, Reagent. All of chemistry relies on the concerted rearrangement of atoms, the removal of bonds, or the reintroduction of new bonds. This is done by chemical reactions. A chemical reaction is referred to as the process that changes the atom arrangement and the bonds of a given molecule supplied to the reaction, creating a molecule of a different layout and most likely different chemical and physical properties.

The material supplied to a chemical reaction as starting material is generally referred to as educt. The term reactant is used synonymously for educt. A chemical reaction often requires more than one educt to proceed. The rearranged molecule resulting from the reaction is referred to as product. If referring to either educt or product, the term reagent is used. As stated, chemical reactions consume educts to form products. The nature of the chemical reaction at hand demands that certain quantities of educts are available and that a given quantity of product is created from them.

The mathematical relationship between the amount of product and the amount of required educts is referred to as stoichiometry. We have already discussed the example of the creation of carbon dioxide from carbon and oxygen. Another example is the creation of magnesium oxide from magnesium and oxygen.

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The reaction is. The stoichiometry of this reaction implies that two molar parts of magnesium must be provided and one molar part of oxygen gas. If less than the required amount is given, which is referred to as under-stoichiometry , product is only produced until all the amount of the under-stoichiometrically provided educt is consumed. Again, the limiting factor will be the educt provided under-stoichiometrically. We have already seen that in some cases, a molecule contains more than one atom that may participate in a reaction.

Let us return to the example of magnesium oxide that is created by burning magnesium see Rct 6. We could also have written. Equivalent factors are thus calculated by taking the reciprocal of the number of reactions one molecule can perform. Equivalents are important, e. Often substances are dissolved in liquids for and during chemical reactions. The properties of these solutions change depending on the amount of substance. A good example are electrolytes which are usually made by dissolving substances which dissociate into their respective ions. There is more than one way to display concentrations.

Usually if a specific substance is referred to and the mixture contains more than one an index will be given to the respective symbol, e.

## Relative atomic mass

In general the solvent in which the substance is dissolved must be indicated, e. If this is not the case, water is usually assumed to be the solvent. Molar Concentration. The molar concentration is defined as the amount of substance per volume of the solvent. Very often molar concentrations are given with a capital M, e. Mass Concentration.

The mass concentration is defined as the mass of a substance per volume of the solvent.

## Relative Atomic Mass Calculations Chemistry Tutorial

The molality is defined as the amount of substance per mass of the solvent. Molality refers only to the mass used and is therefore practical because it does not depend on thermodynamic conditions. The volume of the solvent is dependent on the temperature, the mass is not. Mass Fraction.

In a mixture, the mass fraction is defined as the mass of a given substance per overall mass of the mixture. This means that g of this acid will contain 9 g hydrogen chloride and 91 g water. Mole Fraction. In a mixture, the mole fraction or molar fraction is defined as the amount of a given substance per overall amount of the mixture.

Equivalent Molar Concentration. The equivalent concentration is defined as the fraction of the molar concentration and the equivalence factor. In order to achieve complete neutralization of an acid, the equivalent concentrations of acid and base have to be identical.

## Atomic theory: - Relative mass determination

An example would be, if we want to neutralize a 0. In order to neutralize completely, we have to use the same volume of a 0.

Relative Atomic Mass

Equivalent Amount of Substance. We have already used the equivalent amount of substance indirectly in the last example. It is given as the fraction of the amount of substance with the equivalence factor. The chemical and physical behaviour of different isotopes of the same element are virtually identical; however, there are very small differences that can be exploited to allow artificial concentration of a particular isotope.

• Relative Atomic Mass.
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