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

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

Los compuestos químicos generalmente se describen usando una fórmula empírica o molecular. Estas fórmulas proporcionan información sobre la cantidad de átomos diferentes de los elementos involucrados. Pero, cómo se establecen estas fórmulas?El análisis experimental, como la descomposición de compuestos, se utiliza para estimar las masas relativas de los elementos constituyentes del compuesto. Estas masas relativas se utilizan posteriormente para calcular el número de moles de cada elemento para determinar la fórmula de un compuesto químico. Por ejemplo, se determina experimentalmente que una muestra de un compuesto contiene 43, 64 gramos de fósforo y 56, 36 gramos de oxígeno.Usando masas molares como factores de conversión, las masas relativas de los datos experimentales se convierten en 1, 41 moles de fósforo y 3, 52 moles de oxígeno. Estos valores molares, cuando se asignan como subíndices provisionales a los elementos, producen una pseudofórmula del compuesto. Dividir los valores molares por el valor molar más pequeño proporciona las relaciones molares de aproximadamente 2, 5 moles de oxígeno por 1 mol de fósforo, que se relacionan directamente con la proporción real de los elementos en el compuesto.Si uno de los cocientes sigue siendo un decimal, entonces todos los números se multiplican por el número de conteo más pequeño que da una proporción de números enteros más pequeña de subíndices, generando la fórmula empírica de P2O5. La fórmula molecular de los compuestos se puede determinar a partir de su fórmula empírica y la masa molar o el peso molecular. Por ejemplo, el compuesto químico con la fórmula empírica de P2O5 se mide experimentalmente para que tenga una masa molar de 283, 89 gramos por mol.Su fórmula molecular es un número entero múltiplo de su fórmula empírica, mientras que su masa molar es un número entero múltiplo de su fórmula empírica masa. La relación de la masa molar y la masa de la fórmula empírica da como resultado el número de unidades de fórmula. Al multiplicar la fórmula empírica por el número de unidades de la fórmula se obtiene la fórmula molecular.Por lo tanto, a partir de la fórmula molecular P4O10, el compuesto se identifica como decaóxido de tetrafosforo, o más comúnmente conocido por su nombre de la fórmula empírica es decir pentóxido de fósforo.

determinación experimental de la fórmula química

La composición elemental de un compuesto define su identidad química y las fórmulas químicas son la manera más concisa de representar esta composición elemental. Cuando se desconoce la fórmula de un compuesto, la medición de la masa de sus elementos constitutivos suele ser el primer paso para determinar experimentalmente la fórmula.
 
 <h4>Determinación de fórmulas empíricas</h4>
 
 El enfoque más común para determinar la fórmula química de un compuesto es medir primero las masas de sus elementos constitutivos. Sin embargo, las fórmulas químicas representan los números relativos y no las masas de átomos en la sustancia. Por lo tanto, cualquier dato derivado experimentalmente que involucre la masa debe ser usado para obtener los números correspondientes de átomos en el compuesto. Esto se logra usando masas molares para convertir la masa de cada elemento a su número de moles. Estas cantidades molares se utilizan para calcular proporciones de números enteros que se pueden utilizar para obtener la fórmula empírica de la sustancia.
 
 Considere una muestra de un compuesto determinado que contiene 1,71 gramos de carbono y 0,287 gramos de hidrógeno. Los números de átomos correspondientes son 0,142 moles de carbono y 0,284 moles de hidrógeno. Por lo tanto, este compuesto puede ser representado por la fórmula C0,142H0,284. Por convención, las fórmulas contienen subíndices de números enteros, que se pueden lograr dividiendo cada subíndice por el subíndice más pequeño (0,142). La fórmula empírica para este compuesto es por lo tanto CH2. Los subíndices de “1” no se escriben, sino que se asumen si no hay ningún otro número presente. Esta puede ser o no la fórmula molecular del compuesto; sin embargo, se necesita información adicional para hacer esa determinación.
 
 Como segundo ejemplo, se determina que una muestra de un compuesto contiene 5,31 gramos de cloro y 8,40 gramos de oxígeno. El mismo enfoque da como resultado una fórmula empírica provisional de ClO3,5. En este caso, dividir por el subíndice más pequeño aún deja un decimal en la fórmula empírica. Para convertir esto en un número entero, multiplique cada uno de los subíndices por dos, manteniendo la misma proporción de átomos y produciendo Cl2O7 como la fórmula empírica final.
 
 <h4>Obtener fórmulas empíricas a partir de la composición porcentual</h4>
 
 En los casos en que la composición porcentual de un compuesto está disponible, se utiliza para calcular las masas de los elementos presentes en el compuesto. Dado que la escala de porcentajes es de 100, es conveniente calcular la masa de los elementos presentes en una muestra de 100 gramos de peso. Las masas obtenidas se utilizan para obtener la fórmula empírica.
 
 Por ejemplo, supongamos que un compuesto gaseoso contiene 27,29% de C y 72,71% de O. Los porcentajes de masa, por lo tanto, se expresan como fracciones:
 
 <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-01v2.png" style="height: 35px;" />
 
 <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-02v2.png" style="height: 35px;" />
 
 La masa de carbono, 27,29 g, corresponde a 2,272 moles de carbono, y la masa de oxígeno, 72,71 g, corresponde a 4,544 moles de oxígeno. Por consiguiente, la fórmula representativa es C2,272O4,544. Al dividir cada subíndice por 2,272 se obtiene la fórmula empírica: CO2. 
 
 <h4>Obtención de fórmulas moleculares</h4>
 
 Determinar el número absoluto de átomos que componen una sola molécula de un compuesto covalente requiere conocimiento tanto de su fórmula empírica como de su masa molecular o masa molar. Estas cantidades pueden determinarse experimentalmente por diversas técnicas de medición. La masa molecular, por ejemplo, a menudo se deriva del espectro de masa del compuesto.
 
 Las fórmulas moleculares se obtienen comparando la masa molar o la masa molecular del compuesto con su masa de fórmula empírica. Como su nombre lo indica, una masa de fórmula empírica es la suma de las masas atómicas promedio de todos los átomos representados en una fórmula empírica. Si la masa molar conocida de una sustancia se divide por la masa de fórmula empírica, se obtiene el número de unidades de fórmula empírica por molécula (n). 
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-03v2.png" style="height: 40px;" />
 
 La fórmula molecular se obtiene multiplicando cada subíndice de la fórmula empírica porn, como muestra la fórmula empírica genérica AxBy:
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-04v2.png" style="height: 17px;" />
 
 Por ejemplo, se determina que la fórmula empírica de un compuesto covalente es de CH2O, y que su masa de fórmula empírica es de aproximadamente 30 uma. Si se determina que la masa molecular del compuesto es de 180 uma, esto indica que las moléculas de este compuesto contienen seis veces el número de átomos representados en la fórmula empírica. 
 
 <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;" />
 
 Las moléculas de este compuesto son representadas por una fórmula molecular con subíndices que son seis veces mayores que los de la fórmula empírica: (CH2O)6 = C6H12O6.
 
 Este texto ha sido adaptado de <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Química 2e, Sección 3.2: Determinando fórmulas empíricas y moleculares.</a>

Determinación Experimental de la Fórmula Química

La composición elemental de un compuesto define su identidad química, y las fórmulas químicas son la forma más concisa de representar esta composición elemental. ...

Education

Educación

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Molecules, Compounds, and Chemical Equations

Moléculas, compuestos y ecuaciones químicas

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KEY TERMS AND CONCEPTS

TÉRMINOS Y CONCEPTOS CLAVE

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

Matter is made of elements, and elements are built from atoms. An atom is the smallest unit of an element that still keeps that element’s properties. A silver coin, for example, is made of silver atoms. Each atom keeps the unique properties of silver. Elements are pure substances made of identical atoms. They cannot be broken down into simpler substances by chemical changes. Most elements are atomic elements, which use single atoms as their basic unit. Carbon and sodium are examples of atomic...

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formula mass and mole concepts of compounds

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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...

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...

chemical equations

Reading Chemical Equations and Balancing Them

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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Ohio’s Learning Standards and Model Curriculum

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CH.3.a

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PS.Chm.3.1

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Analyze the structure of atoms and isotopes.

Finding Empirical and Molecular Formulas

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