Name: chemical formula mass and percent composition
Uploaded: 2020-09-03T13:38:26.000Z
Duration: 2 min 56 s
Channel: JoVE

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Chemistry

Matter and its Interactions

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Formula Mass and Mole Concepts of Compounds

De kleinste representatieve eenheid van een verbinding is het molecuul weergegeven door een chemische formule. De atoommassa's van alle atomen in de chemische formule, indien toegevoegd, geven de molecuulmassa van de verbinding, gemeten in atomaire massa-eenheid. Om bijvoorbeeld de molecuulmassa van sucrose of tafelsuiker te vinden, wordt de atoommassa van elk element vermenigvuldigd met het aantal atomen van het overeenkomstige element.De totale som geeft de molecuulmassa van sucrose. De massa van alle moleculen die aanwezig zijn in één mol van een chemische verbinding, wordt de molaire massa genoemd, gemeten in gram per mol. De molaire massa van een verbinding is numeriek gelijk aan de moleculaire massa Bijvoorbeeld, sucrose, met een molecuulmassa van 342, 3 atomaire massa-eenheid, heeft een molaire massa van 342, 3 gram per mol.Het feitelijke aantal moleculen of deeltjes weten dat aanwezig is in een chemische stof, is van fundamenteel belang om de samenstelling, interacties en combinaties ervan te begrijpen. In een bepaald monster van een verbinding is het tellen van het werkelijke aantal moleculen met gewone middelen echter onpraktisch. Door gebruik te maken van de relatie tussen molaire massa, mol en het getal van Avogadro, kan het totale aantal aanwezige moleculen worden berekend.Volgens het concept is de molaire massa van een verbinding gelijk aan één mol van de verbinding, wat neerkomt op 6, 022 x 10^23 deeltjes. Daarom bevat 342, 3 gram sucrose 6, 022 x 10^23 sucrosemoleculen. Als een sacharosemonster bijvoorbeeld 150 gram weegt, kan het aantal moleculen worden bepaald door berekening met behulp van de methode van kruislings vermenigvuldigen.Het resultaat laat zien dat 150 gram sucrose 2, 63 x 10^23 sucrosemoleculen vormt. Het percentage van elk element in een verbinding kan worden uitgedrukt met behulp van een massapercentagewaarde. Om te berekenen, wordt de massa van het element in één mol van de verbinding gedeeld door de totale massa van één mol van de verbinding en omgezet in een percentage.Om in de praktijk het massapercentage waterstof in sucrose te berekenen, wordt de totale massa van waterstofatomen en de totale massa van sucrosemoleculen in de formule opgenomen. De berekende waarde laat zien dat sucrose 6, 4 massaprocent waterstof bevat.

formulemassa- en molconcept van verbindingen

<h4>Formulemassa van covalente bindingen</h4> Chemische formules geven de elementaire samenstelling van stoffen weer. Bij covalente bindingen geeft de formule het aantal en het type atomen waaruit één molecuul van de stof aan. Daarom wordt de formulemassa in dit geval ook wel de molecuulmassa genoemd. De moleculaire formule van chloroform (CHCl3), een covalente binding, geeft aan dat een enkel molecuul één koolstofatoom, één waterstofatoom en drie chlooratomen bevat. De gemiddelde molecuulmassa van een chloroformmolecuul is gelijk aan de som van de gemiddelde atoommassa's van deze atomen: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;"> <h4>Formulemassa van ionische bindingen</h4> Ionische bindingen bestaan uit afzonderlijke kationen en anionen in verhoudingen die elektrische neutraliteit behouden. De formulemassa voor een ionische binding wordt op dezelfde manier berekend als de formulemassa voor covalente bindingen: door de gemiddelde atoommassa's van alle atomen in de formule van de binding op te tellen. De formule voor een ionische binding geeft echter niet de samenstelling van een afzonderlijk molecuul weer, dus er kan niet correct naar worden verwezen als de ‘molecuulmassa’. Gewoon tafelzout of natriumchloride (NaCl) is bijvoorbeeld een ionische binding die bestaat uit natriumkationen (Na+) en chloride-anionen (Cl−) gecombineerd in een verhouding van 1:1. De formulemassa voor deze binding wordt berekend door de gemiddelde atoommassa's van de samenstellende elementen op te tellen: <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;"> Bij het berekenen van de formulemassa van een ionische binding werden de gemiddelde massa's van neutrale natrium- en chlooratomen gebruikt, in plaats van de massa's van natriumkationen en chloride-anionen. Hoewel een natriumkation een iets kleinere massa heeft dan een natriumatoom (omdat het een elektron mist), wordt dit verschil gecompenseerd door het feit dat een chloride-anion iets massiever is dan een chlooratoom (vanwege het extra elektron). Bovendien is de massa van een elektron verwaarloosbaar klein ten opzichte van de massa van een typisch atoom. <h4>Massapercentage samenstelling</h4> De elementaire samenstelling van een binding definieert de chemische identiteit ervan, en chemische formules zijn de meest beknopte manier om deze elementaire samenstelling weer te geven. Het massapercentage van elk element in de binding wordt het massapercentage van dat specifieke element genoemd. Het percentage samenstelling kan worden berekend door de massa van elk element te delen door de totale massa van de binding en vervolgens om te zetten in een percentage. Percentage samenstelling is nuttig voor het evalueren van de relatieve overvloed van een bepaald element in verschillende bindingen met bekende formules. Zolang de moleculaire of empirische formule van de binding in kwestie bekend is, kan het percentage samenstelling worden afgeleid van de atoom- of molmassa's van de elementen van de binding. Eén molecuul salpeterzuur (HNO3) bevat bijvoorbeeld één stikstofatoom met een gewicht van 14,01 amu, één waterstofatoom met een gewicht van 1,008 amu en drie zuurstofatomen met een gewicht (3 x 16,00 amu) = 48,00 amu. De formulemassa van salpeterzuur is daarom (14,01 amu + 1,008 amu + 48,00 amu) = 63,02 amu, en de procentuele samenstelling is: <img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-03.png" style="height: 40px;"> <img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-04.png" style="height: 40px;"> <img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-05.png" style="height: 40px;"> Deze tekst is aangepast van <a href="https://openstax.org/books/chemistry-2e/pages/3-1-formula-mass-and-the-mole-concept">Openstax, Chemistry 2e, Sectie 3.1: Formulemassa en het mole-concept</a> en <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Chemistry 2e, Sectie 3.2: Bepalen van empirische en moleculaire formules.</a>

Formulemassa- en molconcept van verbindingen

Formula Mass of Covalent Compounds

Chemical formulas represent the elemental makeup of substances. For covalent compounds, the formula represents the ...

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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 compounds are different because they contain cations and anions that combine in ratios that make the bulk material electrically neutral. Their formula mass is found the same way, by adding the average atomic masses of the atoms in the formula. Sodium chloride, or table salt, is one example. Its formula, NaCl, shows sodium and chloride in a 1:1 ratio. For ionic compounds, the term molecular mass is not the best choice because the formula does not describe a single molecule.

When calculating formula mass for an ionic compound, neutral atom masses are used instead of ion masses. A sodium cation has slightly less mass than a sodium atom because it has lost an electron. A chloride anion has slightly more mass than a chlorine atom because it has gained an electron. The difference is small, and the mass of an electron is negligibly small compared with the mass of a typical atom.

Percent composition tells how much of a compound’s mass comes from each element. It is found by dividing the mass of each element by the total mass of the compound and converting the result to a percentage. This is useful for comparing the relative amount of an element in compounds with known formulas.

Nitric acid, HNO3, shows how the calculation works. One molecule contains one nitrogen atom with a mass of 14.01 amu, one hydrogen atom with a mass of 1.008 amu, and three oxygen atoms with a total mass of 48.00 amu. Its formula mass is 63.02 amu. From that total, the percent composition of each element can be determined.

Chemical Formula Mass and Percent Composition

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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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Organic molecules are carbon-based compounds found in living things and in many everyday products. Most organic compounds have carbon as the main element, bonded to hydrogen and other carbon atoms. Some carbon-containing substances, such as carbonates, cyanides, and simple oxides like CO and CO2, are not classified as organic. Carbon can form up to four strong bonds, which helps explain why organic compounds can make chains and rings of many sizes and shapes. These molecules appear in...

formula mass and mole concepts of compounds

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...

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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...

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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 calculate formula mass for covalent and ionic compounds, then use it to find percent composition with examples like chloroform, salt, and nitric acid.

Chemical Formula Mass and Percent Composition | JoVE K12

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The Mole Concept and Avogadro’s Number

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Analyze the structure of atoms and isotopes.

Chemical Formula Mass and Percent Composition

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