Name: finding empirical and molecular formulas
Uploaded: 2020-09-03T13:38:26.000Z
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Chemistry

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Experimental Determination of Chemical Formula

Химические соединения обычно описываются с использованием эмпирической или молекулярной формулы. Эти формулы предоставляют информацию о количестве атомов различных элементов. Но как создаются эти формулы?Для оценки относительных масс составляющих элементов в соединении, используется экспериментальный анализ, также как и разложение соединений. Эти относительные массы затем используются для расчета количества молей каждого элемента для определения формулы химического соединения. Например, экспериментально определено, что образец соединения содержит 43, 64 грамма фосфора и 56, 36 грамма кислорода.Используя молярные массы в качестве коэффициентов пересчета, относительные массы из экспериментальных данных переводятся в 1, 41 моль для фосфора и 3, 52 моль для кислорода. Эти мольные значения, приписанные к элементам в виде предварительных индексов, дают так называемую псевдформулу соединения. Деление мольных значений на наименьшее мольное значение дает молярные отношения примерно 2, 5 моля кислорода к 1 молю фосфора, которые напрямую связаны с фактической долей элементов в соединении.Если одно из частных по-прежнему является дробным, то все числа умножаются на наименьшее счетное число, которое дает наименьшее целочисленное отношение индексов, создавая эмпирическую формулу P2O5. Молекулярная формула соединений может быть определена по их эмпирической формуле, либо по молярной массе, или по молекулярной массе. Например, химическое соединение с эмпирической формулой P2O5, согласно экспериментальным измерениям, имеет молярную массу 283, 89 грамма на моль.Его молекулярная формула является целым числом, кратным его эмпирической формуле, в то время как его молярная масса является целым числом, кратным массе его эмпирической формулы. Соотношение молярной массы и массы, полученной по эмпирической формуле, дает количество формульных единиц. Умножение эмпирической формулы на количество формульных единиц дает молекулярную формулу.Следовательно, соединение с молекулярной формулой P4O10 идентифицируется как декаоксид тетрафосфора или как пятиокись фосфора, более широко известное по его эмпирической формуле.

Экспериментальное определение химической формулы

Элементный состав вещества определяет его химическую идентичность, а химические формулы являются наиболее точным способом представления этого элементного состава. Когда формула вещества неизвестна, измерение массы его составных элементов часто является первым шагом в экспериментальном определении формулы. 
 
 <h4>Определение эмпирических формул </h4>
 
 Наиболее распространенный подход к определению химической формулы вещества заключается в том, чтобы сначала измерить массы составляющих его элементов. Однако химические формулы представляют собой относительные числа, а не массы атомов в веществе. Поэтому для получения соответствующих чисел атомов в составе соединения необходимо использовать любые экспериментально полученные данные, связанные с массой. Это достигается с помощью молярных масс для преобразования массы каждого элемента в количество молей. Количества молей используются для вычисления целых чисел, которые могут быть утилизированы для вычисления эмпирической формулы вещества. 
 
 Рассмотрим образец соединения, содержащего 1.71 грамма углерода и 0.287 грамма водорода. Соответствующие числа атомов — 0.142 моля углерода и 0.284 моля водорода. Таким образом, это соединение может быть представлено формулой C0.142H0.284. В соответствии с условными формулами используются целые числа подстрочных индексов, которые можно получить, разделив каждый подстрочный индекс на наименьший индекс (0.142). Таким образом, эмпирическая формула для этого соединения CH2. Подстроки “1” не написаны, а скорее предположены, если нет другого числа. Это может быть или не быть молекулярной формулой соединения; однако для этого требуется дополнительная информация. 
 
 В качестве второго примера можно привести образец соединения, содержащий 5.31 грамма хлора и 8.40 грамма кислорода. Такой же подход дает предварительную эмпирическую формулу ClO3.5. В этом случае деление на наименьший индекс все еще оставляет десятичный знак в эмпирической формуле. Чтобы преобразовать это в целое число, умножьте каждый индекс на два, сохранив тот же коэффициент атома и уступая Cl2О7 конечной эмпирической формуле. 
 
 <h4>Получение эмпирических формул из процентного состава </h4>
 
 В случаях, когда доступен процентный состав вещества, он используется для расчета массы элементов, присутствующих в составе вещества. Так как шкала процентов составляет 100, удобно рассчитать массу элементов, присутствующих в образце весом 100 грамм. Полученные массы используются для получения эмпирической формулы. 
 
 Например, предположим, что газообразное соединение содержит 27.29% C и 72.71% O. таким образом, процентное содержание массы выражается в виде фракций: 
 
 <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;"" />
 
 Масса углерода, 27.29 грамма, соответствует 2.272 моля углерода, а масса кислорода, 72.71 грамма, соответствует 4.544 моля кислорода. Поэтому репрезентативной формулой является C2.272О4.544. Разделение каждого подстрочного индекса на 2.272 дает эмпирическую формулу: CO2.  
 
 <h4>Создание молекулярных формул </h4>
 
 Определение абсолютных чисел атомов, составляющих одну молекулу ковалентного соединения, требует знания как его эмпирической формулы, так и его молекулярной массы или молярной массы. Эти количества могут быть определены экспериментально различными методами измерения. Молекулярная масса, например, часто выводится из масс-спектра соединения. 
 
 Молекулярные формулы получены путем сравнения молярной массы или молекулярной массы соединения с его эмпирической массой формулы. Как следует из названия, масса эмпирической формулы — это сумма средних атомных масс всех атомов, представленных в эмпирической формуле. Если известная молярная масса вещества делится на массу эмпирической формулы, то она дает количество единиц эмпирической формулы на молекулу (n).  
 
 <img alt=""Eq3"" src=""https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-03v2.png"" style=""height: 40px;"" />
 
 Затем молекулярная формула получается путем умножения каждого индекса в эмпирической формуле на n, как показано на общей эмпирической формуле AxBy:
 
 <img alt=""Eq4"" src=""https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-04v2.png"" style=""height: 17px;"" />
 
 Например, эмпирическая формула ковалентного соединения определена как CH2O, а его эмпирическая формула массы составляет приблизительно 30 amu. Если молекулярная масса соединения определяется как 180 amu, это означает, что молекулы этого соединения содержат в шесть раз больше атомов, представленных в эмпирической формуле.  
 
 <img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;" />
 
 Молекулы этого соединения затем представлены молекулярной формулой с подстроками, которые в шесть раз больше, чем в эмпирической формуле: (CH2)6 = C6Н12О6. 
 
 Этот текст адаптирован из <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Химия 2е изд., раздел 3.2: Определение эмпирических и молекулярных формул.</a>

Элементный состав соединения определяет его химическую идентичность, и химические формулы являются наиболее кратким способом представления этого элементного состава. ...

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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

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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.

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Finding Empirical and Molecular Formulas

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