Name: finding empirical and molecular formulas
Uploaded: 2020-09-03T13:38:26.000Z
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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

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Experimental Determination of Chemical Formula

Les composés chimiques sont généralement décrits à l'aide d'une formule empirique ou moléculaire. Ces formules fournissent des informations sur la quantité des différents atomes d'éléments impliqués. Mais comment ces formules sont-elles établies?L'analyse expérimentale, comme la décomposition des composés, est utilisée pour estimer les masses relatives des éléments constitutifs du composé. Ces masses relatives sont ensuite utilisées pour calculer le nombre de moles de chaque élément afin de déterminer la formule d'un composé chimique. Par exemple, un échantillon d'un composé est déterminée expérimentalement pour contenir 43.64 grammes de phosphore et de 56.36 grammes d'oxygène.En utilisant les masses molaires comme facteurs de conversion, les masses relatives des données expérimentales sont converties en 1, 41 mole pour le phosphore et 3, 52 mole pour l'oxygène. Ces valeurs molaires, lorsqu'elles sont attribuées comme indices provisoires aux éléments, donnent une pseudo-formule du composé. Plonger les valeurs moles par la plus petite valeur mole fournit les rapports moles d'environ 2, 5 mol d'oxygène à 1 mol de phosphore, qui se rapportent directement à la proportion réelle d'éléments dans le composé.Si l'un des quotients est toujours une décimale, alors tous les nombres sont multipliés par le plus petit nombre de compte qui donne le plus petit rapport de nombres entiers d'indices, générant la formule empirique de P2O5. La formule moléculaire des composés peut être déterminée à partir de leur formule empirique et de la masse molaire ou du poids moléculaire. Par exemple, le composé chimique avec la formule empirique de P2O5 est mesuré expérimentalement d'avoir une masse molaire de 283, 89 grammes par mol.Sa formule moléculaire est un multiple entier de sa formule empirique, tandis que sa masse molaire est un multiple entier de la masse de sa formule empirique. Le rapport de la masse molaire et de la masse de formule empirique donne le nombre d'unités de formule. En multipliant la formule empirique par le nombre d'unités de formule, on obtient la formule moléculaire.Par conséquent, à partir de la formule moléculaire P4O10, le composé est identifié comme décaoxyde de tétraphosphore, ou plus communément connu par son nom de formule empirique comme pentoxyde de phosphore.

détermination expérimentale de la formule chimique

La composition élémentaire d'un composé définit son identité chimique, et les formules chimiques sont la façon la plus concise de représenter cette composition élémentaire. Lorsque la formule d’un composé est inconnue, la mesure de la masse de ses éléments constitutifs est souvent la première étape dans la détermination expérimentale de la formule. 
 
 <h4>Détermination des formules empiriques </h4>
 
 L’approche la plus courante pour déterminer la formule chimique d’un composé consiste d’abord à mesurer les masses de ses éléments constitutifs. Cependant, les formules chimiques représentent les nombres relatifs et non les masses d'atomes dans la substance. Par conséquent, toute donnée déduite expérimentalement impliquant la masse doit être utilisée pour obtenir les nombres correspondants d'atomes dans le composé. Ceci est réalisé en utilisant des masses molaires pour convertir la masse de chaque élément en son nombre de moles. Ces quantités molaires sont utilisées pour calculer les rapports de nombres entiers, qui peuvent être utilisés pour déduire la formule empirique de la substance. 
 
 Prenons l'exemple d'un échantillon d'un composé contenant 1,71 gramme de carbone et 0,287 gramme d'hydrogène. Les nombres correspondants d'atomes sont 0,142 mole de carbone et 0,284 mole d'hydrogène. Ainsi, ce composé peut être représenté par la formule C0,142H0,284. Par convention, les formules contiennent des indices avec des nombres entiers, qui peuvent être obtenus en divisant chaque indice par le plus petit indice (0,142). La formule empirique de ce composé est donc CH2. Les indices de “ 1 ” ne sont pas écrits mais plutôt supposés si aucun autre nombre n'est présent. Il peut s'agir ou non de la formule moléculaire du composé ; toutefois, des informations supplémentaires sont nécessaires pour le déterminer. 
 
 Comme deuxième exemple, il a été déterminé qu'un échantillon d'un composé contient 5,31 grammes de chlore et 8,40 grammes d'oxygène. La même approche donne une formule empirique provisoire de ClO3,5. Dans ce cas, diviser par le plus petit indice laisse toujours une virgule dans la formule empirique. Pour convertir ceci en un nombre entier, on doit multiplier chacun des indices par deux, en conservant le même rapport d'atomes pour donner Cl2O7 comme formule empirique finale. 
 
 <h4>Déduire les formules empiriques à partir de la composition en pourcentage </h4>
 
 Dans les cas où la composition en pourcentage d'un composé est disponible, elle est utilisée pour calculer les masses d'éléments présents dans le composé. Comme l'échelle des pourcentages est de 100, il est pratique de calculer la masse des éléments présents dans un échantillon pesant 100 grammes. Les masses obtenues sont utilisées pour déduire la formule empirique. 
 
 Par exemple, supposons qu'un composé gazeux contient 27,29 % de C et 72,71 % de O. les pourcentages massiques, par conséquent, sont exprimés en fractions : 
 
 <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 masse de carbone de 27,29 g correspond à 2,272 moles de carbone, et la masse d'oxygène de 72,71 g correspond à 4,544 moles d'oxygène. La formule représentative est donc C2,272O4,544. Diviser chaque indice par 2,272 fournit la formule empirique : CO2.  
 
 <h4>Déduction des formules moléculaires </h4>
 
 <Déterminer les nombres absolus d'atomes qui composent une seule molécule d'un composé covalent nécessite de connaître sa formule empirique et sa masse moléculaire, ou masse molaire. Ces quantités peuvent être déterminées expérimentalement par diverses techniques de mesure. La masse moléculaire, par exemple, est souvent déduite du spectre de masse du composé. 
 
 Les formules moléculaires sont obtenues en comparant la masse molaire ou la masse moléculaire du composé à sa masse de formule empirique. Comme son nom l'indique, une masse de formule empirique est la somme des masses atomiques moyennes de tous les atomes représentés dans une formule empirique. Si la masse molaire connue d'une substance est divisée par la masse de formule empirique, elle produit le nombre d'unités de formule empirique par molécule (n).  
 
 <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-03v2.png" style="height: 40px;" />
 
 La formule moléculaire est ensuite obtenue en multipliant chaque indice de la formule empirique par n, comme le montre la formule empirique générale AxBy : 
 
 <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-04v2.png" style="height: 17px;" />
 
 Par exemple, il a été déterminé que la formule empirique d'un composé covalent est CH2O et que la masse de sa formule empirique est d'environ 30 uma. Si la masse moléculaire du composé est de 180 uma, cela indique que les molécules de ce composé contiennent six fois le nombre d'atomes représentés dans la formule empirique.  
 
 <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;" />
 
 Les molécules de ce composé sont alors représentées par une formule moléculaire avec des indices six fois plus grands que ceux de la formule empirique : (CH2O)6 = C6H12O6. 
 
 Ce texte est adapté de <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Chimie 2e, Section 3.2 : Détermination des formules empiriques et moléculaires.</a>

Détermination expérimentale de la formule chimique

La composition élémentaire d’un composé définit son identité chimique, et les formules chimiques sont la façon la plus concise de représenter cette composition élémentaire. ...

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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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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...

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Finding Empirical and Molecular Formulas

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