Name: mole, mass, and avogadro’s number
Uploaded: 2020-09-03T13:38:25.000Z
Duration: 1 min 54 s
Channel: JoVE

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.2.1

Molar Mass

La tavola periodica fornisce la massa atomica media di ogni atomo. Tuttavia, i campioni macroscopici contengono un numero alquanto elevato di atomi. Pertanto, è essenziale utilizzare un'unità di conteggio, chiamata numero di Avogadro.

Il numero di Avogadro, espresso con il simbolo NA, è solitamente arrotondato a 6, 022 X 10^23. Il suo valore è uguale al numero di atomi in esattamente 12 grammi dell'isotopo puro carbonio-12. Il termine mole, abbreviato con mol, è usato per descrivere il numero di Avogadro, proprio come il termine più familiare, dozzina, è usato per descrivere un numero di 12.

Tuttavia, una dozzina di articoli può pesare in modo diverso pur avendo lo stesso numero di articoli. Allo stesso modo, le moli di elementi diversi, hanno masse diverse ma contengono sempre lo stesso numero di atomi. La massa in grammi della mole di un elemento è chiamata massa molare, espressa in grammi per mole.

Il suo valore è numericamente equivalente alla massa atomica dell'elemento. Quindi, la definizione di mole ci consente di contare le particelle pesandole. La massa molare e il numero di Avogadro possono essere utilizzati come fattori di conversione.

Questo può essere utilizzato per calcolare il numero di atomi in un lingotto d'oro da 5 grammi. La massa molare media dell'oro è 196, 96 grammi per mol. Dunque, per convertire la massa in moli, dividete la massa del lingotto per la massa molare media.

Quindi, moltiplicate il numero di moli per il numero di Avogadro per ottenere il numero di atomi totali nel campione. Quindi, questo lingotto d'oro contiene 1, 529 x 10^22 atomi.

massa molare

L'identità di una sostanza è definita non solo dai tipi di atomi o ioni che contiene ma dalla quantità di ciascun tipo di atomo o ione. Ad esempio, l'acqua, H_2O, e il perossido di idrogeno, H_2O_2, sono simili in quanto le loro rispettive molecole sono composte da atomi di idrogeno e ossigeno. Tuttavia, poiché una molecola di perossido di idrogeno contiene due atomi di ossigeno, a differenza della molecola di acqua, che ne ha solo uno, le due sostanze presentano proprietà molto diverse.

Gli atomi e le molecole sono estremamente piccoli. Pertanto, per misurare le loro quantità macroscopiche, è necessaria un'unità scientifica standard. La mole è un'unità di quantità simile alle unità familiari come coppia, dozzina, lordo, ecc. Fornisce una misura specifica del numero di atomi o molecole in un campione di materia. La connotazione latina della parola "mole" è “grande massa" o "massa", il che è coerente con il suo utilizzo come nome di questa unità. La massa fornisce un collegamento tra una proprietà macroscopica facilmente misurabile, la massa complessiva, e una proprietà fondamentale estremamente importante, il numero di atomi, molecole e così via.

La mole di una sostanza è quella quantità in cui sono presenti 6,02214076 × 10^23 entità discrete (atomi o molecole). Questo grande numero, opportunamente arrotondato a 6.022 × 10^23, è una costante fondamentale nota come numero di Avogadro (N_A) o costante di Avogadro in onore dello scienziato italiano Amedeo Avogadro. Questa costante è correttamente riportata con un'unità esplicita “per mole”.

Coerentemente con la sua definizione di unità di quantità, 1 mole di qualsiasi elemento contiene lo stesso numero di atomi di 1 mole di qualsiasi altro elemento. Le masse di 1 mole di elementi diversi, tuttavia, sono diverse, poiché le masse dei singoli atomi sono drasticamente diverse. La massa molare di un elemento (o composto) è la massa in grammi di 1 mole di quella sostanza, proprietà espressa in grammi per mole (g/mol).

La massa molare di qualsiasi sostanza è numericamente equivalente al suo peso atomico o formula in amu. Secondo la definizione di amu, un singolo atomo di carbonio pesa 12 amu (la sua massa atomica è 12 amu). Una mole di carbonio pesa 12 g (12 g C = 1 mol di atomi di C = 6,022 × 10^23 atomi di C) e la massa molare del carbonio è 12 g/mol. Questa relazione vale per tutti gli elementi poiché le loro masse atomiche sono misurate rispetto a quelle della sostanza di riferimento dell'amu, il carbonio-12. Estendendo questo principio, anche la massa molare di un composto in grammi è numericamente equivalente alla sua massa molare in amu. Ad esempio, l'elio ha una massa atomica di 4.002 amu e una massa molare di 4.002 g/mol.

Sebbene la massa atomica e la massa molare siano numericamente equivalenti, bisogna tenere presente che sono molto diverse in termini di scala. Per apprezzare l'enormità della mole si consideri una piccola goccia d'acqua del peso di circa 0,03 g. Sebbene rappresenti solo una piccola frazione di 1 mole di acqua (~ 18 g), contiene pi molecole d'acqua di quanto si possa immaginare. Se le molecole fossero distribuite equamente tra i circa sette miliardi di persone sulla terra, ogni persona riceverebbe pi di 100 miliardi di molecole di acqua.

La mole definisce la relazione tra la massa e il numero di atomi. Ciò consente di calcolare il numero di atomi in base all'utilizzo di forme adeguate del fattore di conversione: 1 mole di atomi = 6,022 × 10^23 atomi. Per convertire tra la massa di un elemento (in grammi) e il numero di moli, la massa molare dell'elemento (g/mol) viene utilizzata come fattore di conversione.

Questo testo è adattato da <a href="https://openstax.org/books/chemistry-2e/pages/3-1-formula-mass-and-the-mole-concept">Openstax Chemistry 2e, Section 3.1: Formula Mass and the Mole Concept.</a>

Massa molare

L'identità di una sostanza è definita non solo dai tipi di atomi o ioni che contiene, ma anche dalla quantità di ciascun tipo di atomo o ione. Per esempio...

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mole, mass, and avogadro’s number

The mole links mass and the number of particles in a substance. It gives chemists a standard way to count atoms and molecules in samples that are far too small to see. It also helps show why substances with the same kinds of atoms can still have very different properties, like water, H2O, and hydrogen peroxide, H2O2.

The mole is an amount unit, like a pair or a dozen. One mole contains 6.02214076 × 10^23 discrete entities, such as atoms or molecules. This number is usually rounded to 6.022 × 10^23 and is called Avogadro’s number, or the Avogadro constant, named for Amedeo Avogadro. The constant is written with the unit per mole.

One mole of any element contains the same number of atoms as one mole of any other element. The masses are different, though, because atoms have different individual masses. That is why one mole of one element does not weigh the same as one mole of another element.

Molar mass is the mass in grams of one mole of a substance. It is reported in grams per mole, or g/mol. The molar mass of an element or compound is numerically equal to its atomic mass or formula mass in amu. For example, carbon has an atomic mass of 12 amu, so 1 mole of carbon has a mass of 12 g and a molar mass of 12 g/mol.

This same relationship works for other elements and for compounds. Helium, for example, has an atomic mass of 4.002 amu and a molar mass of 4.002 g/mol. Although the numbers match, atomic mass and molar mass are very different in scale.

A tiny drop of water can still contain an enormous number of molecules. A drop that weighs about 0.03 g is only a small part of 1 mole of water, which is about 18 g. Even so, it holds more water molecules than can easily be imagined. If those molecules were shared equally among the roughly seven billion people on Earth, each person would still get more than 100 billion molecules of water.

The mole also lets scientists convert between mass and amount. Using the relationship 1 mole of atoms = 6.022 × 10^23 atoms, the number of atoms can be calculated from a sample. The molar mass of an element is also used as a conversion factor to move between grams and moles.

Mole, Mass, and Avogadro’s Number

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molar mass

The mole links mass and the number of particles in a substance. It gives chemists a standard way to count atoms and molecules in samples that are far too small to see. It also helps show why substances with the same kinds of atoms can still have very different properties, like water, H2O, and hydrogen peroxide, H2O2. The mole is an amount unit, like a pair or a dozen. One mole contains 6.02214076 × 10^23 discrete entities, such as atoms or molecules. This number is usually rounded to 6.022 ×...

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Learn how the mole connects mass, atoms, and molecules through Avogadro’s number and molar mass, with clear examples for high school chemistry.

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CH.4

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Mole, Mass, and Avogadro’s Number

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