El Átomo

"Los conceptos y principios fundamentales de la ciencia son invenciones libres del espiritu humano"
--Albert Einstein--

miércoles, 20 de enero de 2010

Fórmula Empírica y Molecular

Fórmula Empírica y Molecular.

La fórmula empírica es una expresión o forma que representa la proporción más simple en la que están presentes los átomos que forman un compuesto químico, en ella se relacionan los símbolos de los elementos con un número entero correspondiente a cada símbolo. Puede coincidir o no con la fórmula molecular, que indica el número de átomos presentes en la molécula.

Determinación experimental de fórmulas empíricas.

1. El análisis químico indica el numero de gramos de cada elemente presentes en una determinada cantidad de un compuesto.

2. Las cantidades de gramos se convierten a moles de cada elemento, recuerde que para convertir moles a gramos es necesario utilizar el peso atómico del elemento.

3. Seguidamente se debe realizar una comparación entre las moles obtenidas a fin de determinar la de menor valor.

4. Por último las moles de los demás elementos deben ser divididas por el número de moles de menor valor escogido en el numeral anterior. El cociente de esta división debe ser un numero entero, en caso contrario debe aproximarse (siempre y cuando sea posible) al valor entero más cercano o por ensayo y error determinar un numero entero que multiplique a todos los cocientes y los convierta en valores enteros. Una vez realizado este procedimiento se escribe la formula empírica con lo cocientes determinados para cada elemento.

Ejemplo 3.

La alicina es el compuesto responsable del olor característico del ajo. Un análisis de dicho compuesto muestra la siguiente composición porcentual en masa: C: 44,4%; H: 6,21%; S: 39,5%; O: 9,86%. Calcule su fórmula empírica.

Solución:

Se inicia suponiendo que hay 100 gramos de compuesto en este caso alicina, de tal manera que:

Una vez obtenidos los moles de cada elemento se procede a realizar la división de cada uno de ellos entre las moles de menor valor que en este caso corresponden al oxígeno a fin de hallar los cocientes para escribir de la formula empírica:



Formula empírica: C6H10S2O

Determinación de Formulas Moleculares

Para calcular la fórmula molecular, o real, se requiere conocer la masa molecular aproximada del compuesto, además de su formula empírica. Conociendo que el peso molecular de un compuesto debe ser un múltiplo entero (n) del peso molecular de la fórmula empírica. Por tanto,

(Peso Molecular de la Formula Empírica)*n = (Peso Molecular de la Fórmula Molecular)

Ejemplo 4.

Determinar la formula molecular de la alicina (ejemplo 3), sabiendo que el peso molecular de su formula molecular es 120g/mol.

Peso molecular de la Formula empírica:

C = 12g x 3 = 36g/mol

H = 1g x 10 = 10g/mol

S = 32g x 2 = 64g/mol

O = 16g x 1 = 16g/mol

Peso Molecular = 120g/mol


En este caso, Formula Molecular = Formula Empírica

25 comentarios:

Anónimo dijo...

hola como estás, la verdad que es espectacular la explicación, te felicito, pero quiero hacer una puntualización, cuando estás calculando la formula molecular, multiplicas el peso atómico del carbono por 3, y en realidad es por 6, esto cambiaría el restado y no sería 120g sino que sería 162g, igualmente la formula molecular es la misma que la empírica... demás está excelente, muchos profesores debería aplicar la técnica que tu usaste.. TE FELICITO !!!

Unknown dijo...

n = Pm compuesto/ Pm formula empirica
Lo que multiplica a la incógnita pasa a dividir.

Unknown dijo...

36+10+64+16= 126

Unknown dijo...

Really great blog, it's very helpful and has great knowledgeable information.
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Unknown dijo...

SE DEBE CORREGIR EL ERROR DE LA MASA MOLECULAR QUE ES 162g/mol-g

Tarita dijo...

Sii...hay un error...el peso de la formula empírica da 162 ...al dividir 120 entre 162 da 0.7...esta cifra se aproxima a 1?

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