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

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Experimental Determination of Chemical Formula

Os compostos químicos normalmente são descritos por utilizarem uma fórmula empírica ou molecular. Estas fórmulas fornecem informação sobre a quantidade de átomos diferentes dos elementos envolvidos. Mas como são estabelecidas estas fórmulas?A análise experimental, como a decomposição de compostos, é utilizada para estimar as massas relativas dos elementos constituintes do composto. Estas massas relativas são depois utilizadas para calcular o número de mols de cada elemento para determinar a fórmula de um composto químico. Por exemplo, a amostra de um composto está experimentalmente determinada a conter 43, 64 gramas de fósforo e 56, 36 gramas de oxigênio.Utilizando as massas molares como fatores de conversão, as massas relativas dos dados experimentais são convertidas em 1, 41 mols para o fósforo e 3, 52 mols para o oxigênio. Estes valores de mole, quando atribuídos como subscritores provisórios aos elementos, produzem uma pseudo-fórmula do composto. Diminuir os valores dos mols pelo valor de mol mais pequeno fornece rácios molares de aproximadamente 2, 5 mole de oxigênio para 1 mole de fósforo, que está diretamente relacionado com a proporção atual de elementos num composto.Se um dos quocientes ainda for decimal, então todos os números são multiplicados pelo menor número de contagem que dá a menor relação de número inteiro de subscritos, gerando a fórmula empírica de P2O5. A fórmula molecular dos compostos pode ser determinada a partir da sua fórmula empírica e da massa molar ou peso molecular. Por exemplo, o composto químico com a fórmula empírica de P2O5 foi experimentalmente medida para ter uma massa molar de 283, 89 gramas por mol.A sua fórmula molecular é um número inteiro múltiplo da sua fórmula empírica, enquanto a sua massa molar é um número inteiro múltiplo da massa da sua fórmula empírica. A proporção de massa molar e massa da fórmula empírica representa o número de unidades de fórmula. Multiplicar a fórmula empírica pelo número das unidades de fórmula origina a fórmula molecular.Desta forma, a partir da fórmula molecular P4O10, o composto é identificado como sendo decaóxido de tetrafósforo, ou mais comummente conhecido pelo nome da sua fórmula empírica como sendo pentóxido de fósforo.

determinação experimental de fórmulas químicas

A composição elementar de um composto define a sua identidade química, e as fórmulas químicas são a maneira mais concisa de representar esta composição elementar. Quando a fórmula de um composto é desconhecida, medir a massa dos seus elementos constitutivos é muitas vezes o primeiro passo para determinar experimentalmente a fórmula.

<h4>Determinação de Fórmulas Empíricas</h4>

A abordagem mais comum para a determinação da fórmula química de um composto é medir primeiro as massas dos seus elementos constituintes. No entanto, as fórmulas químicas representam os números relativos e não as massas de átomos na substância. Portanto, qualquer dado derivado experimentalmente envolvendo massa deve ser usado para obter os números correspondentes de átomos no composto. Isto é feito usando massas molares para converter a massa de cada elemento para o seu número de moles. Essas quantidades molares são usadas para calcular as razões de número inteiro que podem ser usadas para derivar a fórmula empírica da substância.

Considere uma amostra de um composto que se determinou conter 1,71 gramas de carbono e 0,287 gramas de hidrogénio. Os números correspondentes de átomos são 0,142 moles de carbono e 0,284 moles de hidrogénio. Assim, este composto pode ser representado pela fórmula C0,142H0,284. Por convenção, as fórmulas contêm os subscritos do número inteiro, que podem ser alcançados dividindo cada subscrito pelo menor subscrito (0,142). A fórmula empírica para este composto é, portanto, CH2. Os subscritos de “1” não são escritos, mas sim assumidos se nenhum outro número estiver presente. Esta pode ou não ser a fórmula molecular do composto; no entanto, são necessárias informações adicionais para fazer essa determinação.

Como segundo exemplo, determina-se que uma amostra de um composto contém 5,31 gramas de cloro e 8,40 gramas de oxigénio. A mesma abordagem produz uma fórmula empírica provisória de ClO3,5. Neste caso, a divisão pelo menor subscrito ainda deixa um decimal na fórmula empírica. Para converter isto em um número inteiro, multiplica-se cada um dos subscritos por dois, mantendo a mesma relação de átomos e produzindo Cl2O7 como a fórmula empírica final.

<h4>Derivação de Fórmulas Empíricas a Partir de Composição Percentual</h4>

Nos casos em que a composição percentual de um composto está disponível, ela é usada para calcular as massas dos elementos presentes no composto. Uma vez que a escala das percentagens é de 100, é conveniente calcular a massa dos elementos presentes em uma amostra de 100 gramas. As massas obtidas são usadas para derivar a fórmula empírica.

Por exemplo, suponha que um composto gasoso contém 27,29% C e 72,71% O. As percentagens de massa são, portanto, expressas em frações:

<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;" />

A massa de carbono, 27,29 g, corresponde a 2,272 moles de carbono, e a massa de oxigénio, 72,71 g, corresponde a 4,544 moles de oxigénio. A fórmula representativa é, portanto, C2,272O4,544. Dividindo cada subscrito por 2,272 fornece a fórmula empírica: CO2. 

<h4>Derivação de Fórmulas Moleculares</h4>

Determinar o número absoluto de átomos que compõem uma única molécula de um composto covalente requer o conhecimento da sua fórmula empírica e da sua massa molecular ou massa molar. Estas quantidades podem ser determinadas experimentalmente por várias técnicas de medição. A massa molecular, por exemplo, é muitas vezes derivada do espectro de massa do composto.

As fórmulas moleculares são derivadas comparando a massa molar ou massa molecular do composto com a massa da sua fórmula empírica. Como o nome sugere, massa de fórmula empírica é a soma das massas atómicas médias de todos os átomos representados em uma fórmula empírica. Se a massa molar conhecida de uma substância for dividida pela massa de fórmula empírica, obtém-se o número de unidades de fórmula empírica por molécula (n). 

<img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-03v2.png" style="height: 40px;" />

A fórmula molecular é então obtida multiplicando cada subscrito da fórmula empírica por n, como mostra a fórmula empírica genérica AxBy:

<img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-04v2.png" style="height: 17px;" />

Por exemplo, determina-se que a fórmula empírica de um composto covalente é CH2H, e a sua massa de fórmula empírica é de aproximadamente 30 amu. Se a massa molecular do composto for determinada como 180 amu, isso indica que as moléculas desse composto contêm seis vezes o número de átomos representados na fórmula empírica. 

<img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;" />

As moléculas desse composto são então representadas por uma fórmula molecular com subscritos seis vezes maiores que as da fórmula empírica: (CH2O)6 = C6H12O6.

Este texto é adaptado de <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>

Determinação Experimental de Fórmulas Químicas

A composição elementar de um composto define sua identidade química, e as fórmulas químicas são a maneira mais concisa de representar essa composição elementar. ...

Education

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Moléculas, Compostos e Equações Químicas

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KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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

Atomic Elements, Molecules, and Bonds

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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...

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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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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JoVE Core - Química dá vida a conceitos gerais de química usando vídeos de animação fáceis de entender. Além disso, nos vídeos Cientistas em ação, demonstramos experimentos de pesquisa clássicos e originais realizados atualmente por cientistas em laboratórios em todo o mundo.

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Structure And Properties Of Matter

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Finding Empirical and Molecular Formulas

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