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

A tabela periódica dá a massa atômica média de cada átomo. Contudo, as amostras macroscópicas contêm números extremamente grandes de átomos. Desta forma, é essencial utilizar uma contagem unitária chamada de número de Avogadro.O número de Avogadro, indicado pelo símbolo NA, é normalmente arredondado a 6, 022 X 10^23. O seu valor é igual ao número de átomos em exatamente 12 gramas do isótopo puro de carbono-12. O termo mol, abreviado mol, é utilizado para descrever o número de Avogadro, da mesma forma que o termos familiar, dúzia, é utilizado para descrever a quantidade de 12.No entanto, uma dúzia de artigos diferentes podem pesar diferentemente apesar de terem o mesmo número de artigos. Da mesma forma, os mols de elementos diferentes têm massas diferentes mas contêm sempre o mesmo número de átomos. A massa em gramas de um mol de um elemento é chamada de massa molar, indicada em gramas por mol.O seu valor é numericamente equivalente à massa atômica do elemento. Assim, a definição de mol permite-nos contar partículas pesando-as. A massa molar e o número de Avogadro podem ser utilizados como fatores de conversão.Isto pode ser utilizado para calcular o número de átomos numa barra de 5 gramas de ouro. A massa molar média do ouro é de 196, 96 gramas por mol. Assim, para converter a massa em mols, divide-se a massa da barra pela massa molar média.Depois multiplica-se o número de mols pelo número de Avogadro para obter o número total de átomos na amostra. Portanto, esta barra de ouro contém 1, 529 X 10^22 átomos.

massa molar

A identidade de uma substância é definida não só pelos tipos de átomos ou iões que contém, mas também pela quantidade de cada tipo de átomo ou ião. Por exemplo, água, H2O, e peróxido de hidrogénio, H2O2, são semelhantes porque as suas respectivas moléculas são compostas por átomos de hidrogénio e oxigénio. No entanto, como uma molécula de peróxido de hidrogénio contém dois átomos de oxigénio, ao contrário da molécula de água, que tem apenas uma, as duas substâncias apresentam propriedades muito diferentes.

Átomos e moléculas são extremamente pequenos. Portanto, para medir as suas quantidades macroscópicas, é necessária uma unidade científica padrão. O mole é uma unidade de quantidade semelhante a unidades familiares como par, dúzia, bruto, etc. Fornece uma medição específica do número de átomos ou moléculas em uma amostra de matéria. A conotação em Latim para a palavra “mole” é “massa grande” ou “volumoso”, o que é consistente com o seu uso como nome para esta unidade. O mole fornece uma ligação entre uma propriedade macroscópica de fácil medição, massa volumosa, e uma propriedade fundamental extremamente importante, número de átomos, moléculas, e assim por diante.

Um mole de uma substância é a quantidade em que existem 6,02214076 × 1023 entidades discretas (átomos ou moléculas). Este grande número, convenientemente arredondado para 6,022 × 1023, é uma constante fundamental conhecida como número de Avogadro (NA) ou constante de Avogadro em honra do cientista Italiano Amedeo Avogadro. Esta constante é adequadamente relatada com uma unidade explícita de “por mole”.

Consistente com a sua definição de unidade de quantidade, 1 mole de qualquer elemento contém o mesmo número de átomos que 1 mole de qualquer outro elemento. As massas de 1 mole de diferentes elementos, no entanto, são diferentes, uma vez que as massas dos átomos individuais são drasticamente diferentes. A massa molar de um elemento (ou composto) é a massa em gramas de 1 mole dessa substância, propriedade expressa em unidades de gramas por mole (g/mol).

A massa molar de qualquer substância é numericamente equivalente ao seu peso atómico ou molar em amu. De acordo com a definição de amu, um único átomo de carbono pesa 12 amu (a sua massa atómica é de 12 amu). Um mole de carbono pesa 12 g (12 g C = 1 mol átomos C = 6,022 × 1023 átomos C) e a massa molar de carbono é 12 g/mol. Esta relação é aplicável a todos os elementos, uma vez que as suas massas atómicas são medidas em relação à substância de referência da amu, carbono-12. Estendendo este princípio, a massa molar de um composto em gramas é da mesma forma numericamente equivalente à sua massa molar em amu. Por exemplo, o hélio tem uma massa atómica de 4,002 amu e uma massa molar de 4,002 g/mol.

Embora a massa atómica e a massa molar sejam numericamente equivalentes, tenha em mente que são muito diferentes em termos de escala. Para apreciar a enormidade do mole, considere uma pequena gota de água que pesa cerca de 0,03 g. Embora isto represente apenas uma pequena fração de 1 mole de água (~18 g), contém mais moléculas de água do que se pode imaginar. Se as moléculas fossem distribuídas igualmente entre os cerca de sete mil milhões de pessoas na terra, cada pessoa receberia mais de 100 mil milhões de moléculas de água.

O mole define a relação entre a massa e o número de átomos. Isto permite que o número de átomos seja calculado com base em formas adequadas do factor de conversão: 1 mole de átomos = 6,022 × 1023 átomos. Para converter entre a massa de um elemento (em gramas) e o número de moles, a massa molar do elemento (g/mol) é utilizada como factor de conversão.

Texto 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>

Massa Molar

A identidade de uma substância é definida não apenas pelos tipos de átomos ou íons que ela contém, mas pela quantidade de cada tipo de átomo ou íon. Por exemplo...

Education

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Química

Atoms and Elements

Átomos e Elementos

KEY TERMS AND CONCEPTS

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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.

Mole, Mass, and Avogadro’s Number | JoVE K12

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Structure And Properties Of Matter

Quantifying matter

Interactions Of Matter

Stoichiometry

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CH.2.a

CH.4

CH.4.a

Physical Science

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Arizona Science Standards

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HS+C.P1

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CHEM.8

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CHEM.11

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Florida Department of Education

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Mole, Mass, and Avogadro’s Number

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