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Amedeo Avogadro

Amedeo Avogadro, a drawing by C. Sentier in 1856

Moles, molarity, and molality

< Pure water and solubility

V Moles and mole fraction
V Molality and molarity
V % w/w and w/v

 

"...one mole (the molar or molecular weight of a substance expressed in grams)..."

W. Ostwald 1902   

Moles and mole fraction

A mole (mol) b is an amount (having both number and resultant mass) of something and is defined as the amount that contains exactly the number of entities of that something as there are 12C atoms in 12 grams of 12C. This number (currently established as 6.022 140 82 ˣ 1023 ˣ mol-1 [2395]) is known as the Avogadro number (see the portrait of Avogadro,above right). Thus, one mole of standard (VSMOW) water is 18.015 265 g of the water.

 

The mole fraction of water (xw) is that (dimensionless) fraction of the molecules (and/or ions) present that are water molecules; that is, xw = moles water/(moles water + moles solute). The mole fraction of a solute is the fraction of all the molecules (and/or ions) present that consist of that solute (that is, xS= moles solute/(moles water + moles solute + moles other materials). At low solute concentrations, xS may be approximated by

 

xS = moles solute/moles water = molality ˣ molar mass of water = mS ˣ MH2O

 

where mS is the molality, see below, and MH2O is the molar mass of water = 0.018015265 kg ˣ mol-1. This can be restated as

ms = ns /(nH2O ˣ MH2O)

where ns and nH2O are the numbers of moles of salt and water in the solution.

Molarity and molality.

These are solution concentrations. Molality (m, with a lower case 'm'; molal concentration) is defined as the number of gram-moles dissolved per kilogram water. Molarity (M, with an upper case 'M'; molar concentration) is defined as the number of gram-moles dissolved per liter of solution. Care should be taken to differentiate these terms. Conversion of molarity to molality and vice versa is often incorrectly described. The formulae are

molarity=1000x molalityxdensity/(1000+molality x relative molecular mass) and molality=1000x molarity/(1000xdensity-molarity x relative molecular mass)

 

Molality compared with molarity, from [70]

 

Molality compared with molarity

 

where ρ is the experimentally determined density in g ˣ mL-1 and MWt is the relative molecular mass of the solute. Allowance must be made for other solutes (and co-ions) present. Also, these equations do not allow for corrections due to any water bound to the solutes.

 

Opposite shown red (for methanol) are the very different values equivalent concentrations may have when expressed as molality or molarity. The lower blue dashed line shows the relationship as the concentrations tends to zero. Molality is always numerically greater than the molarity. Note that % w/w (see below) varies non-linearly with both molarity and molality.

 

Molarity versus molality for CsCl and LiCl

Molarity versus molality for CsCl and LiCl

 

Left shows the molarity/molality relationship for CsCl and LiCl. Mistakenly using molarity in place of molality, or vice versa, can lead to errors of about 2-4% for 1 molar solutions, 4-9% for 2 molar solutions and 10-30% for 5 molar solutions. At lower concentrations, such errors are mostly due to changes in volume of the solution with temperature.

 

Molarity is most often used as it is easy to measure out volumes. However, such solutions are often less accurately prepared and dispensed due to the difficulties in attaining accurate volumes and the changes in volume that occur with temperature variation. Solutions with known molalities, prepared by weighing, are generally more precisely and accurately known, and do not change concentration with temperature or the addition of other solutes (so long as these solutes do not bind any of the 'free' (bulk) water present. Often, molarity is used for chemical reactions where the number of moles of reagent present is simply calculated (moles = molarity ˣ volume in liters), whereas molality should be used when physical or thermodynamic determinations are made, such as the determination of the colligative properties.

 

The SI unit of concentration is mol ˣ m-3 with molarity (M) and molality (m) being recommended for discontinuance by SI. It is expected that molal be replaced by mol ˣ kg-1 and the rarely used term molality be discontinued as it may be confused with the commonly used term molarity. However, these recommendations have not yet been generally accepted.

 

Note that 'moles' here refers to independent species such as molecules or ions. Care should be taken to correct concentration terms when the solution consists of a mixture of solutes where xw = moles water/(moles water + moles solutes), or more exactly xw = moles 'free' water/(moles 'free' water + moles solutes). a When considering colligative properties, these water molecules must be 'free' (see elsewhere), (that is, xw = moles 'free' water/(moles 'free' water + moles solute). In contrast to the molality, bound water makes no difference to the numerical value of the molarity.

Weight per weight and weight per volume

Other concentration units, such as % w/w (g dissolved in100 g solution, % w/v (g dissolved in 100 mL solution) and % v/v (ml mixed with 100-ml) should be avoided wherever possible unless the molecular species present are not established. By-volume dilutions of liquids should never be made as the volumes are generally not additive (i.e. the final volume is not the sum of the two volumes added. By-volume dilutions of by-weight concentrated solutions should never be made as the final concentrations may only be determined using a knowledge of the densities of the solutions concerned and the volumes are generally not additive.

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Footnotes

a In many older textbooks, another (equivalent) way of treating moles 'free' water is as λw times xww.xw) where λw is the activity coefficient of water and xw is the calculated total mole fraction of water (i.e. 'free' water + 'bound' water). the activity coefficient (λw ) is just another way of specifying the 'free' fraction of water. [Back]

 

b The SI definition of the mole is: The mole, symbol mol, is the SI unit of amount of substance of a specified elementary entity, which may be an atom, molecule, ion, electron, any other particle or a specified group of such particles; its magnitude is set by fixing the numerical value of the Avogadro constant to be exactly 6.022 141 29 ˣ 1023when it is expressed in the SI unit mol-1. It is likely that the new definition being developed will include a slightly different value for the Avogadro constant. [Back]

 

 

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This page was established in 2015 and last updated by Martin Chaplin on 14 November, 2017


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