Name: finding empirical and molecular formulas
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 37 s
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K12 Standards

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North Carolina K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.3

experimental-determination-of-chemical-formula

Experimental Determination of Chemical Formula

I composti chimici vengono solitamente descritti utilizzando una formula empirica o molecolare. Queste formule forniscono informazioni sulla quantità dei diversi atomi degli elementi coinvolti. Ma come sono stabilite queste formule?

L'analisi sperimentale, come la decomposizione dei composti, viene utilizzata per stimare le masse relative degli elementi del composto. Queste masse relative vengono, quindi, utilizzate per calcolare il numero di moli di ciascun elemento al fine di determinare la formula di un composto chimico. Per esempio, il campione di un composto viene determinato sperimentalmente contenere 43, 64 grammi di fosforo e 56, 36 grammi di ossigeno.

Usando le masse molari come fattori di conversione, le masse relative ottenute dai dati sperimentali vengono convertite in 1, 41 moli per il fosforo e 3, 52 moli per l'ossigeno. Questi valori molari, quando assegnati come pedici provvisori agli elementi, producono una pseudo-formula del composto. Dividendo i valori molari per il valore molare più piccolo si ottengono i rapporti molari di circa 2, 5 moli di ossigeno e 1 mole di fosforo, che si riferiscono direttamente alla proporzione effettiva di elementi nel composto.

Se uno dei quozienti è ancora un decimale, tutti i numeri vengono moltiplicati per il numero di calcolo più piccolo che fornisce il più piccolo rapporto di numeri interi dei pedici, generando la formula empirica di P2O5. La formula molecolare dei composti può essere determinata dalla loro formula empirica e dalla massa molare, o dal peso molecolare. Per esempio, il composto chimico, con la formula empirica di P2O5, viene misurato sperimentalmente avere una massa molare di 283, 89 grammi per mole.

La sua formula molecolare è un multiplo intero della sua formula empirica, mentre la sua massa molare è un multiplo intero della sua massa della formula empirica. Il rapporto tra la massa molare e la massa della formula empirica fornisce il numero di unità di formula. Moltiplicando la formula empirica per il numero di unità di formula si ottiene la formula molecolare.

Dunque, dalla formula molecolare P4O10, il composto è identificato come decaossido di tetrafosforo, o più comunemente noto con il nome della sua formula empirica come pentossido di fosforo.

determinazione sperimentale della formula chimica

La composizione elementare di un composto definisce la sua identit chimica e le formule chimiche sono il modo pi conciso per rappresentare questa composizione elementare. Quando la formula di un composto sconosciuta, misurare la massa dei suoi elementi costitutivi spesso il primo passo per determinare sperimentalmente la formula.

Determinazione di formule empiriche

L’approccio pi comune per determinare la formula chimica di un composto innanzitutto misurare le masse dei suoi elementi costitutivi. Tuttavia, le formule chimiche rappresentano i numeri relativi e non le masse degli atomi nella sostanza. Pertanto, qualsiasi dato derivato sperimentalmente che coinvolga la massa devessere utilizzato per ottenere il numero corrispondente di atomi nel composto. Ci si ottiene utilizzando le masse molari per convertire la massa di ciascun elemento nel suo numero di moli. Queste quantit molari vengono utilizzate per calcolare i rapporti di numeri interi che possono essere utilizzati per derivare la formula empirica della sostanza.

Consideriamo un campione di un composto che contiene 1,71 grammi di carbonio e 0,287 grammi di idrogeno. I corrispondenti numeri di atomi sono 0,142 moli di carbonio e 0,284 moli di idrogeno. Pertanto, questo composto pu essere rappresentato dalla formula C_0.142H_0.284. Per convenzione, le formule contengono pedici interi, che possono essere ottenuti dividendo ciascun pedice per il pedice pi piccolo (0,142). La formula empirica per questo composto quindi CH_2. I pedici di “1? non vengono scritti ma piuttosto assunti se non presente nessun altro numero. Questa pu essere o meno la formula molecolare del composto; tuttavia, per effettuare tale determinazione sono necessarie ulteriori informazioni.

Come secondo esempio, si determina che un campione di un composto contiene 5,31 grammi di cloro e 8,40 grammi di ossigeno. Lo stesso approccio produce una formula empirica provvisoria di ClO_3.5. In questo caso, la divisione per il pedice pi piccolo lascia comunque un decimale nella formula empirica. Per convertirlo in un numero intero, bisogna moltiplicare ognuno dei pedici per due, mantenendo lo stesso rapporto atomico e ottenendo Cl_2O_7 come formula empirica finale.

Derivazione di formule empiriche dalla composizione percentuale

Nei casi in cui disponibile la composizione percentuale di un composto, viene utilizzata per calcolare le masse degli elementi presenti nel composto. Poich la scala percentuale 100, conveniente calcolare la massa degli elementi presenti in un campione del peso di 100 grammi. Le masse ottenute vengono utilizzate per derivare la formula empirica.

Ad esempio, supponiamo che un composto gassoso contenga il 27,29% di C e il 72,71% di O. Le percentuali in massa, quindi, sono espresse come frazioni:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-01v2.png" style="height: 35px;" />

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-02v2.png" style="height: 35px;" />

La massa di carbonio, 27,29 g, corrisponde a 2,272 moli di carbonio, e la massa di ossigeno, 72,71 g, corrisponde a 4,544 moli di ossigeno. La formula rappresentativa , quindi, C_2.272O_4.544. Dividendo ogni pedice per 2.272 si ottiene la formula empirica: CO_2.

Derivazione di formule molecolari

Determinare il numero assoluto di atomi che compongono una singola molecola di un composto covalente richiede la conoscenza sia della sua formula empirica che della sua massa molecolare o massa molare. Queste quantit possono essere determinate sperimentalmente attraverso varie tecniche di misurazione. La massa molecolare, ad esempio, spesso derivata dallo spettro di massa del composto.

Le formule molecolari vengono derivate confrontando la massa molare o la massa molecolare del composto con la massa della sua formula empirica. Come suggerisce il nome, una formula empirica massa la somma delle masse atomiche medie di tutti gli atomi rappresentati in una formula empirica. Se la massa molare nota di una sostanza viene divisa per la massa della formula empirica, si ottiene il numero di unit della formula empirica per molecola (n).

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-03v2.png" style="height: 40px;" />

La formula molecolare si ottiene quindi moltiplicando ciascun pedice della formula empirica per n, come mostrato dalla formula empirica generica A_xB_y:

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-04v2.png" style="height: 17px;" />

Ad esempio, si determina che la formula empirica di un composto covalente CH_2O e la sua massa formula empirica di circa 30 amu. Se si determina che la massa molecolare del composto 180 amu, ci indica che le molecole di questo composto contengono sei volte il numero di atomi rappresentati nella formula empirica.

<img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;" />

Le molecole di questo composto sono poi rappresentate da una formula molecolare con pedici sei volte maggiori di quelli della formula empirica: (CH_2O)_6 = C_6H_12O_6.

Questo testo adattato 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>

Determinazione sperimentale della formula chimica

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. ...

Education

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Chimica

Molecules, Compounds, and Chemical Equations

Molecole, composti ed equazioni chimiche

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KEY TERMS AND CONCEPTS

TERMINI E CONCETTI CHIAVE

finding empirical and molecular formulas

Chemical formulas show the elemental makeup of a compound. When a formula is unknown, scientists often start by measuring the masses of the elements in the sample. Those mass data are then used to find the simplest whole-number ratio of atoms, which gives the empirical formula.

To get from mass to atom ratio, the masses of the elements are first converted to moles using molar masses. Chemical formulas describe numbers of atoms, not masses, so this step is necessary. In one example, 1.71 grams of carbon and 0.287 grams of hydrogen become 0.142 moles of carbon and 0.284 moles of hydrogen. Dividing by the smallest value gives the empirical formula CH2, with the 1 omitted by convention.

Sometimes the numbers do not become whole right away. A sample containing 5.31 grams of chlorine and 8.40 grams of oxygen gives a tentative ratio of ClO3.5. Multiplying both subscripts by two keeps the same atom ratio and gives the whole-number empirical formula Cl2O7.

Percent composition can also be used to find an empirical formula. Because percentages are based on 100, it is convenient to imagine a 100-gram sample. For a gas that is 27.29% carbon and 72.71% oxygen, those masses correspond to 2.272 moles of carbon and 4.544 moles of oxygen. Dividing by the smaller amount gives the empirical formula CO2.

A molecular formula gives the actual number of atoms in one molecule of a covalent compound. To find it, both the empirical formula and the molecular mass or molar mass are needed. The empirical formula mass is the sum of the average atomic masses in the empirical formula, and dividing the molar mass by that value gives the number of empirical units per molecule, n. If the empirical formula is CH2O and the molecular mass is 180 amu, then n equals 6, so the molecular formula is C6H12O6.

Finding Empirical and Molecular Formulas

molecules and compounds

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experimental determination of chemical formula

Chemical formulas show the elemental makeup of a compound. When a formula is unknown, scientists often start by measuring the masses of the elements in the sample. Those mass data are then used to find the simplest whole-number ratio of atoms, which gives the empirical formula. To get from mass to atom ratio, the masses of the elements are first converted to moles using molar masses. Chemical formulas describe numbers of atoms, not masses, so this step is necessary. In one example, 1.71 grams...

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Chemical equations show the identities and relative amounts of substances in a chemical reaction. The substances that react are called reactants, and their formulas are written on the left side of the equation. The substances formed are called products, and their formulas are written on the right side. A plus sign (+) separates formulas on the same side, and an arrow (→) shows the change from reactants to products. The numbers in a chemical equation are shown with coefficients. A coefficient...

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Learn how to use mass, percent composition, and molar mass to determine empirical formulas and molecular formulas from experimental data.

Finding Empirical and Molecular Formulas | JoVE K12

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Finding Empirical and Molecular Formulas

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