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

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Experimental Determination of Chemical Formula

Chemische verbindingen worden meestal beschreven met behulp van een empirische of moleculaire formule. Deze formules geven informatie over het aantal verschillende atomen van betrokken elementen. Maar hoe komen deze formules tot stand?Experimentele analyse, zoals ontleding van verbindingen, wordt gebruikt om de relatieve massa van samenstellende elementen in de verbinding te schatten. Deze relatieve massa's worden vervolgens gebruikt om het aantal mol van elk element te berekenen om de formule van een chemische verbinding te bepalen. Zo wordt experimenteel bepaald dat een monster van een verbinding 43, 64 gram fosfor en 56, 36 gram zuurstof bevat.Met behulp van molaire massa's als conversiefactoren worden de relatieve massa's uit de experimentele gegevens omgezet in 1, 41 mol voor fosfor en 3, 52 mol voor zuurstof. Deze molwaarden, wanneer ze worden toegekend als voorlopige subscripts aan de elementen, leveren een pseudo-formule van de verbinding op. Door de molwaarden te duiken met de kleinste molwaarde, worden de molverhoudingen van ongeveer 2, 5 mol zuurstof op 1 mol fosfor verkregen, die rechtstreeks verband houden met het werkelijke aandeel van de elementen in de verbinding.Als een van de quotiënten nog steeds een decimaal getal is, worden alle getallen vermenigvuldigd met het kleinste natuurlijke getal dat de kleinste gehele getalverhouding van subscripts geeft, waardoor de empirische formule van P2O5 wordt gegenereerd. De molecuulformule van verbindingen kan worden bepaald uit hun empirische formule en de molaire massa of het molecuulgewicht. De chemische verbinding met de empirische formule van P2O5 wordt bijvoorbeeld experimenteel gemeten om een molaire massa van 283, 89 gram per mol te hebben.Zijn molecuulformule is een veelvoud van zijn empirische formule, terwijl zijn molaire massa een veelvoud is van zijn empirische formule. De verhouding tussen molaire massa en empirische formulemassa levert het aantal formule-eenheden op. Door de empirische formule te vermenigvuldigen met het aantal formule-eenheden, wordt de molecuulformule verkregen.Daarom wordt de verbinding volgens de molecuulformule P4O10 geïdentificeerd als tetrafosfordecaoxide, of beter bekend onder de empirische formulenaam als fosforpentoxide.

experimentele bepaling van chemische formule

De elementaire samenstelling van een verbinding definieert de chemische identiteit ervan, en chemische formules zijn de meest beknopte manier om deze elementaire samenstelling weer te geven. Wanneer de formule van een verbinding onbekend is, is het meten van de massa van de samenstellende elementen vaak de eerste stap bij het experimenteel bepalen van de formule. <h4>Bepaling van empirische formules</h4> De meest gebruikelijke benadering voor het bepalen van de chemische formule van een verbinding is eerst het meten van de massa van de samenstellende elementen. Chemische formules vertegenwoordigen echter de relatieve aantallen en niet de massa van atomen in de stof. Daarom moeten alle experimenteel verkregen gegevens over massa worden gebruikt om het overeenkomstige aantal atomen in de verbinding te verkrijgen. Dit wordt bereikt met behulp van molaire massa's om de massa van elk element om te zetten in het aantal mol. Deze molaire hoeveelheden worden gebruikt om gehele getalsverhoudingen te berekenen die kunnen worden gebruikt om de empirische formule van de stof af te leiden. Beschouw een monster van een verbinding waarvan is vastgesteld dat deze 1,71 gram koolstof en 0,287 gram waterstof bevat. De overeenkomstige aantallen atomen zijn 0,142 mol koolstof en 0,284 mol waterstof. Deze verbinding kan dus worden weergegeven door de formule C0.142H0.284. Volgens afspraak bevatten formules subscripts van hele getallen, wat kan worden bereikt door elk subscript te delen door het kleinste subscript (0,142). De empirische formule voor deze verbinding is dus CH2. Subscripts van “1” worden niet geschreven, maar aangenomen als er geen ander nummer aanwezig is. Dit kan wel of niet de molecuulformule van de verbinding zijn; er is echter aanvullende informatie nodig om die beslissing te nemen. Als tweede voorbeeld wordt bepaald dat een monster van een verbinding 5,31 gram chloor en 8,40 gram zuurstof bevat. Dezelfde aanpak levert een voorlopige empirische formule voor Cl3,5O op. In dit geval blijft er bij het delen door het kleinste subscript nog steeds een decimaal over in de empirische formule. Om dit in een geheel getal om te zetten, vermenigvuldigt u elk van de subscripts met twee, waarbij u dezelfde atoomverhouding behoudt en Cl2O7 oplevert als de uiteindelijke empirische formule. <h4>Empirische formules afleiden uit procentuele samenstelling</h4> In gevallen waarin de procentuele samenstelling van een verbinding beschikbaar is, wordt deze gebruikt om de massa van de elementen in de verbinding te berekenen. Omdat de schaal voor percentages 100 is, is het handig om de massa van de elementen te berekenen die aanwezig zijn in een monster van 100 gram. De verkregen massa's worden gebruikt om de empirische formule af te leiden. Stel bijvoorbeeld dat een gasvormige verbinding 27,29% C en 72,71% O bevat. De massapercentages worden daarom uitgedrukt als fracties: <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;"> De massa koolstof, 27,29 g, komt overeen met 2,272 mol koolstof, en de massa zuurstof, 72,71 g, komt overeen met 4,544 mol zuurstof. De representatieve formule is daarom C2.272O4.544. Door elk subscript te delen door 2,272 ontstaat de empirische formule: CO2. <h4>Afleiding van molecuulformules</h4> Het bepalen van het absolute aantal atomen waaruit een enkel molecuul van een covalente verbinding bestaat, vereist kennis van zowel de empirische formule als de molecuulmassa of molaire massa ervan. Deze grootheden kunnen experimenteel worden bepaald door middel van verschillende meettechnieken. De molecuulmassa wordt bijvoorbeeld vaak afgeleid van het massaspectrum van de verbinding. Molecuulformules worden afgeleid door de molmassa of molecuulmassa van de verbinding te vergelijken met de empirische formulemassa. Zoals de naam al doet vermoeden, is de massa van een empirische formule de som van de gemiddelde atoommassa's van alle atomen die in een empirische formule worden weergegeven. Als de bekende molaire massa van een stof wordt gedeeld door de empirische formulemassa, levert dit het aantal empirische formule-eenheden per molecuul (n) op. <img alt="Eq3" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-03v2.png" style="height: 40px;"> De molecuulformule wordt vervolgens verkregen door elk subscript in de empirische formule te vermenigvuldigen met n, zoals blijkt uit de generieke empirische formule AxBy: <img alt="Eq4" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-04v2.png" style="height: 17px;"> Er wordt bijvoorbeeld bepaald dat de empirische formule van een covalente verbinding CH2O is, en de massa van de empirische formule bedraagt ongeveer 30 amu. Als wordt vastgesteld dat de molecuulmassa van de verbinding 180 amu is, geeft dit aan dat de moleculen van deze verbinding zes keer het aantal atomen bevatten dat in de empirische formule wordt weergegeven. <img alt="Eq5" src="https://www.jove.com/files/core/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;"> Moleculen van deze verbinding worden vervolgens weergegeven door een molecuulformule met subscripts die zes keer groter zijn dan die in de empirische formule: (CH2O)6 = C6H12O6.

Experimentele bepaling van chemische formule

The elemental makeup of a compound defines its chemical identity, and chemical formulas are the most concise way of representing this elemental makeup. ...

Education

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Scheikunde

Eén

Molecules, Compounds, and Chemical Equations

Moleculen, verbindingen en chemische vergelijkingen

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KEY TERMS AND CONCEPTS

BELANGRIJKE TERMEN EN CONCEPTEN

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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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...

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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 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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Structure And Properties Of Matter

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CH.3

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CHEM.7

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Finding Empirical and Molecular Formulas

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