Name: chemical formula mass and percent composition
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 56 s
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K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.3

formula-mass-and-mole-concepts-of-compounds

Formula Mass and Mole Concepts of Compounds

A unidade representativa mais pequena de um composto é a molécula representada por uma fórmula química. As massas atómicas de todos os átomos na fórmula química, se adicionados, indicam a massa molecular do composto, medida numa unidade de massa atômica. Por exemplo, para saber qual a massa molecular da sacarose ou açúcar, a massa atômica de cada elemento é multiplicada pelo número de átomos do elemento correspondente.O valor total corresponde à massa molecular da sacarose. A massa de todas as moléculas presente num mol de um composto químico é denominado de massa molar, medida em gramas por mol. A massa molar de um composto é numericamente igual à sua massa molecular Por exemplo, a sacarose com uma massa molecular de 342, 3 de unidade de massa atómica, terá uma massa molar de 342, 3 gramas por mol.Saber o número atual de moléculas ou partículas presentes numa substância química é fundamental para compreender a sua composição, interações e combinações. Contudo, numa dada amostra de um composto, contar o número atual de moléculas por meios normaisé inviável. Ao aplicar a relação entre a massa molar, o mol e o número de Avogadro, pode ser calculado do total de moléculas presentes.De acordo com o conceito, a massa molar de um composto é equivalente a um mol de um composto, constituindo 6, 022 x 10^23 partículas. Desta forma, 342, 3 gramas de sacarose contêm 6, 022 x 10^23 moléculas de sacarose. Contudo, se uma amostra de sacarose pesar 150 gramas, por exemplo, então o número de moléculas pode ser determinado por cálculo utilizando um método de multiplicação cruzada.O resultado mostra que 150 gramas de sacarose constituem 2, 63 x 10^23 moléculas de sacarose. A percentagem de cada elemento num composto pode ser expressa utilizando um valor de percentagem de massa. Para calcular a massa de um elemento num mole de um composto é dividido pela massa total de um mole do composto e convertida numa percentagem.Na prática, para calcular a percentagem de massa de hidrogênio na sacarose, a massa total dos átomos de hidrogênio e a massa total das moléculas de sacarose são incluídas na fórmula. O valor calculado mostra que a sacarose contém 6, 4 por cento de massa de hidrogênio.

conceitos de massa molar e moles de compostos

<h4>Massa Molar de Compostos Covalentes</h4>

As fórmulas químicas representam a composição elementar das substâncias. Para os compostos covalentes, a fórmula representa os números e tipos de átomos que compõem uma única molécula da substância; assim, a massa molar pode ser correctamente referida como massa molecular. A massa molecular do clorofórmio (CHCl3), um composto covalente, indica que uma única molécula contém um átomo de carbono, um átomo de hidrogénio, e três átomos de cloro. A massa molecular média de uma molécula de clorofórmio é igual à soma das massas atómicas médias destes átomos:

<img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;" />

<h4>Massa Molar de Compostos Iónicos</h4>

Os compostos iónicos são compostos por catiões e aniões discretos que são combinados em relações de forma a produzir massa volumosa eletricamente neutra. A massa molar para um composto iónico é calculada da mesma forma que a massa molar para compostos covalentes: somando as massas atómicas médias de todos os átomos na fórmula do composto. No entanto, a fórmula de um composto iónico não representa a composição de uma molécula discreta, portanto pode não ser corretamente referida como ‘massa molecular’.

Por exemplo, o sal de mesa comum ou cloreto de sódio (NaCl) é um composto iónico composto por catiões de sódio (Na+) e aniões cloreto (Cl−) combinados em uma razão de 1:1. A massa molar para este composto é calculada adicionando as massas atómicas médias dos seus elementos constitutivos:

<img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;" />

Ao computar a massa molar de um composto iónico, são utilizadas as massas médias de átomos neutros de sódio e cloro, em vez das massas para catiões de sódio e aniões cloreto. Mesmo que um catião de sódio tenha uma massa ligeiramente menor do que um átomo de sódio (uma vez que está a faltar um eletrão), esta diferença será compensada pelo facto de que um anião cloreto é ligeiramente mais pesado do que um átomo de cloro (devido ao eletrão extra). Além disso, a massa de um eletrão é pequena, de forma desprezível, em relação à massa de um átomo típico.

<h4>Composição Percentual de Massa</h4>

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. A percentagem de massa de cada elemento no composto é chamada de percentagem de massa desse elemento específico. A composição percentual pode ser calculada dividindo a massa de cada elemento pela massa total do composto e, em seguida, converter para uma percentagem.

A composição percentual é útil para avaliar a abundância relativa de um determinado elemento em diferentes compostos de fórmulas conhecidas. Desde que a fórmula molecular ou empírica do composto em questão seja conhecida, a composição percentual pode ser derivada das massas atómicas ou molares dos elementos do composto. 

Por exemplo, uma molécula de ácido nítrico (HNO3) contém um átomo de azoto pesando 14,01 amu, um átomo de hidrogénio pesando 1,008 amu, e três átomos de oxigénio pesando (3 × 16,00 amu) = 48,00 amu. A massa molar do ácido nítrico é, portanto (14,01 amu + 1,008 amu + 48,00 amu) = 63,02 amu, e sua composição percentual é:

<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;" />

Este texto é adaptado de <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 <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>

Conceitos de Massa Molar e Moles de Compostos

Massa de fórmula de compostos covalentes

As fórmulas químicas representam a composição elementar das substâncias. Para compostos covalentes, a fórmula representa o ...

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Química

Molecules, Compounds, and Chemical Equations

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KEY TERMS AND CONCEPTS

TERMOS-CHAVE E CONCEITOS

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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Finding Empirical and Molecular Formulas

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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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Structure And Properties Of Matter

Quantifying matter

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Matter and Its Interactions

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Chemical Formula Mass and Percent Composition

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