Name: chemical formula mass and percent composition
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 56 s
Channel: JoVE

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.3

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Formula Mass and Mole Concepts of Compounds

La più piccola unità rappresentativa di un composto è la molecola rappresentata da una formula chimica. Le masse atomiche di tutti gli atomi nella formula chimica, se aggiunte, danno la massa molecolare del composto, misurata in unità di massa atomica. Per esempio, per trovare la massa molecolare del saccarosio o dello zucchero da tavola, la massa atomica di ciascun elemento viene moltiplicata per il numero di atomi dell'elemento corrispondente.

La somma totale corrisponde alla massa molecolare del saccarosio. La massa di tutte le molecole presenti nella mole di un composto chimico è chiamata massa molare, misurata in grammi per mole. La massa molare di un composto è numericamente uguale alla sua massa molecolare Per esempio, il saccarosio, con una massa molecolare di 342, 3 unità di massa atomica, avrà una massa molare di 342, 3 grammi per mole.

Conoscere il numero effettivo di molecole o particelle presenti in una sostanza chimica è fondamentale per comprenderne la composizione, le interazioni e le combinazioni. Tuttavia, in un dato campione di un composto, il conteggio del numero effettivo di molecole con mezzi ordinari non è pratico. Utilizzando la relazione tra massa molare, mole e numero di Avogadro, è possibile calcolare le molecole totali presenti.

Secondo questo concetto, la massa molare di un composto è equivalente ad una mole del composto, costituendo 6, 022 x 10^23 particelle. Pertanto, 342, 3 grammi di saccarosio contengono 6, 022 x 10^23 molecole di saccarosio. Tuttavia, se un campione di saccarosio, per esempio, pesa 150 grammi, il numero di molecole può essere determinato mediante il calcolo utilizzando un metodo di moltiplicazione incrociata.

Il risultato mostra che 150 grammi di saccarosio costituiscono 2, 63 x 10^23 molecole di saccarosio. La percentuale di ogni elemento in un composto può essere espressa utilizzando un valore percentuale in massa. Per il calcolo, la massa dell'elemento in un mole del composto viene divisa per la massa totale di una mole del composto, e convertita in percentuale.

In pratica, per calcolare la percentuale di massa di idrogeno nel saccarosio, viene inclusa nella formula la massa totale degli atomi di idrogeno e la massa totale delle molecole di saccarosio. Il valore calcolato mostra che il saccarosio contiene il 6, 4 percento della massa dell'idrogeno.

concetti di massa molare e mole dei composti

Massa molare dei composti covalenti

Le formule chimiche rappresentano la composizione elementare delle sostanze. Per i composti covalenti, la formula rappresenta i numeri e i tipi di atomi che compongono una singola molecola della sostanza; pertanto, la massa molare può essere correttamente definita massa molecolare. La formula molecolare del cloroformio (CHCl_3), un composto covalente, indica che una singola molecola contiene un atomo di carbonio, un atomo di idrogeno e tre atomi di cloro. La massa molecolare media di una molecola di cloroformio è uguale alla somma delle masse atomiche medie di questi atomi:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;" />

Massa molare dei composti ionici

I composti ionici sono composti da cationi e anioni discreti che si combinano in rapporti per produrre materia macroscopica elettricamente neutra. La massa molare per un composto ionico viene calcolata allo stesso modo della massa molare per i composti covalenti: sommando le masse atomiche medie di tutti gli atomi nella formula del composto. Tuttavia, la formula di un composto ionico non rappresenta la composizione di una molecola distinta, quindi potrebbe non essere correttamente definita “massa molecolare”.

Ad esempio, il comune sale da cucina o cloruro di sodio (NaCl) è un composto ionico composto da cationi di sodio (Na^+) e anioni cloruro (Cl^−) combinati in un rapporto 1:1. La massa molare per questo composto viene calcolata sommando le masse atomiche medie dei suoi elementi costitutivi:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;" />

Nel calcolare la massa molare di un composto ionico, sono state utilizzate le masse medie degli atomi neutri di sodio e cloro, anziché le masse dei cationi sodio e degli anioni cloruro. Anche se un catione di sodio ha una massa leggermente più piccola di un atomo di sodio (poiché gli manca un elettrone), questa differenza sarà compensata dal fatto che un anione cloruro è leggermente più massiccio di un atomo di cloro (a causa dell'elettrone in più). Inoltre, la massa di un elettrone è trascurabilmente piccola rispetto alla massa di un tipico atomo.

Composizione percentuale in massa

La composizione elementare di un composto definisce la sua identità chimica e le formule chimiche sono il modo più conciso per rappresentare questa composizione elementare. La percentuale in massa di ciascun elemento nel composto è chiamata percentuale in massa di quel particolare elemento. La composizione percentuale può essere calcolata dividendo la massa di ciascun elemento per la massa complessiva del composto e quindi convertendola in percentuale.

La composizione percentuale è utile per valutare l'abbondanza relativa di un dato elemento in diversi composti di formule note. Purché sia nota la formula molecolare o empirica del composto in questione, la composizione percentuale può essere derivata dalle masse atomiche o molari degli elementi del composto.

Ad esempio, una molecola di acido nitrico (HNO_3) contiene un atomo di azoto che pesa 14,01 amu, un atomo di idrogeno che pesa 1,008 amu e tre atomi di ossigeno che pesano (3 × 16,00 amu) = 48,00 amu. La massa molare dell'acido nitrico è quindi (14,01 amu + 1,008 amu + 48,00 amu) = 63,02 amu, e la sua composizione percentuale è:

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-03.png" style="height: 40px;" />

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-04.png" style="height: 40px;" />

<img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-05.png" style="height: 40px;" />

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> e da <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Chemistry 2e, Section 3.2: Determining Empirical and Molecular Formulas.</a>

Concetti di massa molare e mole dei composti

Formula Massa di composti covalenti

Le formule chimiche rappresentano la composizione elementare delle sostanze. Per i composti covalenti, la formula rappresenta il ...

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Molecules, Compounds, and Chemical Equations

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TERMINI E CONCETTI CHIAVE

chemical formula mass and percent composition

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance.

For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses.

Ionic compounds are different because they contain cations and anions that combine in ratios that make the bulk material electrically neutral. Their formula mass is found the same way, by adding the average atomic masses of the atoms in the formula. Sodium chloride, or table salt, is one example. Its formula, NaCl, shows sodium and chloride in a 1:1 ratio. For ionic compounds, the term molecular mass is not the best choice because the formula does not describe a single molecule.

When calculating formula mass for an ionic compound, neutral atom masses are used instead of ion masses. A sodium cation has slightly less mass than a sodium atom because it has lost an electron. A chloride anion has slightly more mass than a chlorine atom because it has gained an electron. The difference is small, and the mass of an electron is negligibly small compared with the mass of a typical atom.

Percent composition tells how much of a compound’s mass comes from each element. It is found by dividing the mass of each element by the total mass of the compound and converting the result to a percentage. This is useful for comparing the relative amount of an element in compounds with known formulas.

Nitric acid, HNO3, shows how the calculation works. One molecule contains one nitrogen atom with a mass of 14.01 amu, one hydrogen atom with a mass of 1.008 amu, and three oxygen atoms with a total mass of 48.00 amu. Its formula mass is 63.02 amu. From that total, the percent composition of each element can be determined.

Chemical Formula Mass and Percent Composition

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formula mass and mole concepts of compounds

Chemical formula mass and percent composition help describe the makeup of compounds. A chemical formula shows which atoms are present and how many of each atom appear in a substance. For covalent compounds, the formula shows the atoms in one molecule. That is why formula mass can also be called molecular mass. Chloroform, CHCl3, is one example. Its formula shows one carbon atom, one hydrogen atom, and three chlorine atoms. The molecular mass is the sum of their average atomic masses. Ionic...

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Learn how to calculate formula mass for covalent and ionic compounds, then use it to find percent composition with examples like chloroform, salt, and nitric acid.

Chemical Formula Mass and Percent Composition | JoVE K12

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Chemical Formula Mass and Percent Composition

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