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

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North Carolina K-12 Science Standard Course of Study

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Chemistry

Matter and its Interactions

PS.Chm.3.3

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Experimental Determination of Chemical Formula

عادة ما يتم وصف المركبات الكيميائية باستخدام صيغة تجريبية أو جزيئية. توفر هذه الصيغ معلومات،حول كمية الذرات المختلفة من العناصر المعنية. لكن كيف يتم إنشاء هذه الصيغ؟التحليل التجريبي،مثل تحلل المركبات،يستخدم لتقدير الكتل النسبية للعناصر المكونة في المركب.ثم يتم استخدام هذه الكتل النسبية لحساب عدد مولات كل عنصر لتحديد صيغة المركب الكيميائي. على سبيل المثال،عينة من مركب تحدد تجريبيًا احتواءه على 43.64 جرام فوسفور و 56.36 جرام أكسجين. باستخدام الكتل المولية كعوامل تحويل،فإنّالكتل النسبية من البيانات التجريبية يتم تحويلها إلى 1.41 مول للفوسفور و 3.52 مول للأكسجين.هذه القيم المولية،التي تم تعيينها كنصوص مؤقتة للعناصر،ينتج عنها صيغة زائفة للمركب. قسمة قيمة المولعلى أصغر قيمة للمول يزودنا بنسبة المول لحوالي 2.5 مول من الأكسجين إلى 1 مول من الفوسفور،والذي يتناسب مباشرة مع النسبة الفعلية للعناصر في المركب. إذا كان أحد حاصل القسمة لا يزال عددًا عشريًا،يتم ضرب جميع الأرقام في أصغر رقم عد يعطي أصغر نسبة عدد صحيح للرمز،و بذلك تنتج الصيغة التجريبية لـ P2O5.الصيغة الجزيئية للمركبات يمكن تحديدها من صيغتها التجريبية مع الكتلة المولية أو الوزن الجزيئي. على سبيل المثال،المركب الكيميائي باستخدام الصيغة التجريبية لـ P2O5 تم قياسه تجريبيا ليكون له كتلة مولية بقيمة 283.89 جرام لكل مول. صيغته الجزيئية عبارة عن مضاعف عدد صحيح من صيغته التجريبية،بينما كتلته المولية هو مضاعف عدد صحيح لكتلة صيغته التجريبية.نسبة الكتلة المولية وكتلة الصيغة التجريبية ينتج عدد وحدات الصيغة. ضرب الصيغة التجريبية في عدد وحدات الصيغة يعطي الصيغة الجزيئية. ومن ثم،من الصيغة الجزيئية P4O10،يتم التعرف على المركب على أنه رباعي أكسيد الفوسفور،أو الأكثر شيوعًا بصيغته التجريبية اسمه خامس أكسيد الفوسفور.

تحديد تجريبي للصيغة الكيميائية

تحدد التركيبة الأولية لمركب هويته الكيميائية، وتُشكل الصيغ الكيميائية الطريقة الأكثر إيجازاً لتمثيل هذه التركيبة الأولية. وعندما تكون صيغة المركب’s غير معروفة، فإن قياس كتلة العناصر المكونة له يكون غالباً الخطوة الأولى في تحديد الصيغة تجريبيا. <h4>تحديد الصيغ التجريبية </h4>إن النهج الأكثر شيوعاً في تحديد الصيغة الكيميائية للمركب’s يتلخص أولاً في قياس كتل العناصر المكونة له. ومع ذلك، تمثل الصيغ الكيميائية الأعداد النسبية وليس كتل الذرات في المادة. لذلك، يجب استخدام أي بيانات مشتقة من التجارب تتضمن الكتلة للحصول على الأعداد المتناظرة للذرات في المركب. ويتم ذلك باستخدام الكتل المولية لتحويل كتلة كل عنصر إلى عدد المولات الخاص به. وتُستخدم هذه الكميات المولية لحساب نسب الأعداد الكلية التي يمكن استخدامها لاستخلاص الصيغة التجريبية للمادة. لنعتبر أن عينة من مركب ما تم استنتاج أنها تحتوي 1.71 جرامًا من الكربون و0.287 جرامًا من الهيدروجين. هذا يقابله أعداد للذرات هي 0.142 مولات من الكربون و0.284 مولات من الهيدروجين. وبالتالي، يمكن تمثيل هذا المركب بالصيغة C0.142H0.284. بحسب الأصول، تحتوي الصيغ على أرقام سفلية كاملة، وهذا يمكن تحقيقه عن طريق تقسيم كل رقم سفلي على أصغر رقم سفلي والذي هو (0.142). وبالتالي فإن الصيغة التجريبية لهذا المركب هي CH2. فالأرقام السفلية الأحادية “1” لا تكتب بل يتم افتراضها عندما لا يكون هناك رقم آخر. وقد تكون هذه هي حقاً الصيغة الجزيئية للمركب’s أو قد لا تكون؛ ولهذا فإن معلومات إضافية لاتخاذ هذا القرار هي مطلوبة. وكمثال ثان، يتم تحديد عينة من المركب بحيث تحتوي على 5.31 جرامًا من الكلور و8.40 جرامًا من الأكسجين. اتخاذ نفس النهج ينتج عنه صيغة تجريبية مؤقتة من ClO3.5. في هذه الحالة، لا تزال القسمة على أصغر رقم سفلي تترك كسرًا عشريًا في الصيغة التجريبية. ولتحويل هذا الرقم إلى عدد صحيح، اضرب كل الأرقام السفلية في اثنين، مع الاحتفاظ بنفس نسبة الذرة وهذا يعطينا Cl2O7 كصيغة تجريبية نهائية. <h4>استخلاص الصيغ التجريبية من تركيب النسبة المئوية </h4>في الحالات التي يتوفر فيها التركيب المئوي لمركب، يتم استخدامه لحساب كتل العناصر الموجودة في المركب. بما أن مقياس النسب المئوية هو 100، فمن الملائم حساب كتلة العناصر الموجودة في عينة تزن 100 جرامًا. وتُستخدم الكُتل المحسوبة لاستخلاص الصيغة التجريبية. على سبيل المثال، افترض أن مركباً غازيّاً يحتوي على 27.29% C و72.71% O. وبالتالي، يتم التعبير عن النسب المئوية للكتلة ككسور: <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;" />>وتتطابق كتلة الكربون، 27.29 غ، مع 2.272 مول من الكربون، وتتطابق كتلة الأكسجين، 72.71 غ، مع 4.544 مول من الأكسجين. ولذلك فإن الصيغة التمثيلية هي C2.272O4.544. إن قسمة كل رقم سفلي على 2.272 تعطينا الصيغة التجريبية: CO2. 
 11257 <h4>اشتقاق الصيغ الجزيئية </h4>يتطلب تحديد الأعداد المطلقة للذرات التي تكوّن جزيء واحد من مركب تساهمي معرفة كل من صيغته التجريبية وكتلته الجزيئية أو كتلته المولية. وقد يتم تحديد هذه الكميات من خلال أساليب قياس مختلفة. فالكتلة الجزيئية، على سبيل المثال، غالباً ما تستمد من الطيف الكتلِ للمركب. ويتم اشتقاق الصيغ الجزيئية من خلال مقارنة الكتلة المولية للمركب أو الكتلة الجزيئية مع كتلة صيغته التجريبية. وكما يقترح الاسم، فإن كتلة الصيغة التجريبية هي مجموع متوسط الكتلة الذرية لجميع الذرات الممثلة في صيغة تجريبية. إذا كانت الكتلة المولية المعروفة للمادة مقسومة على كتلة الصيغة التجريبية، فإنها تنتج عدد وحدات الصيغة التجريبية لكل جزيء (n).  <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-03v2.png" style="height: 40px;" /> يتم بعدها الحصول على الصيغة الجزيئية عن طريق ضرب كل نص فرعي في الصيغة التجريبية في n، كما هو موضح في الصيغة التجريبية العامة Axby: <img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-04v2.png" style="height: 17px;" />على سبيل المثال، يتم تحديد الصيغة التجريبية لمركب تساهمي على أنها CH2O، وتبلغ كتلة صيغتها التجريبية 30 وحدة دولية تقريبًا. وإذا تم تحديد الكتلة الجزيئية للمركب بأنها 180 وحدة دولية فإن هذا يشير إلى أن جزيئات هذا المركب تحتوي على ستة أضعاف عدد الذرات الممثلة في الصيغة التجريبية.  <img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/1018/1018_Equations-05v2.png" style="height: 35px;" /> يتم بعدها تمثيل جزيئات هذا المركب بواسطة صيغة جزيئية تحتوي على أرقام سفلية هي ست أضعاف أكبر من تلك الموجودة في الصيغة التجريبية: (cH2O)6 = C6H12O6. تم اقتباس هذا النص من <a href="https://openstax.org/books/chemistry-2e/pages/3-2-determining-empirical-and-molecular-formulas">Openstax, Chemistry 2e, Section 3.2: Determining Empirical and Molecular Formulas.</a>

يحدد التركيب الأولي للمركب هويته الكيميائية ، والصيغ الكيميائية هي الطريقة الأكثر إيجازا لتمثيل هذا التركيب الأساسي. ...

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Molecules, Compounds, and Chemical Equations

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KEY TERMS AND CONCEPTS

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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

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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Molecular Model Types in Chemistry

Molecular models help chemists and students visualize the structures and shapes of atoms and molecules. They represent the molecular architecture of chemical compounds in a way that is easier to study than formulas alone. Skeletal models are simple two-dimensional drawings of compounds. They show the molecular framework, or the bonds, without drawing every atom. In this model, many carbon and hydrogen atoms are not shown directly. Their positions are implied by the ends and junctions of the...

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Organic Molecules in Everyday Chemistry

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

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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...

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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...

key terms and concepts

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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JoVE Core Chemistry brings general chemistry concepts to life using concise and easy-to-understand animated video lessons. Plus, in the scientist-in-action videos, we demonstrate classical and original research experiments performed in today's laboratories worldwide.

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Predicting Chemical Formulas

Scientific Laws and Theories

Scientific Laws

In science, a law is defined as a concise, verbal or mathematical, statement that summarizes a vast number of experimental observations. ...

The Scientific Method

Chemistry is an empirical science. Scientists often pose questions to understand the chemistry in everyday life and seek answers to these questions. To ...

Classifying Matter by State

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Classifying Matter by Composition

Matter: Pure Substances and Mixtures
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The Atomic Theory of Matter

The earliest recorded discussion of the basic structure of matter comes from ancient Greek philosophers. Leucippus and Democritus argued that all matter ...

Subatomic Particles

Dalton was only partially correct about the particles that make up matter. All matter is composed of atoms, and atoms are composed of three smaller ...

Elements: Chemical Symbols and Isotopes

A chemical symbol is an abbreviation used to indicate an element or an atom of an element. For example, the symbol for mercury is Hg. The same symbol is ...

Use models to explain how the scientific understanding of atomic structure has evolved.

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Radioactivity and Nuclear Equations

Nuclear chemistry is the study of reactions that involve changes in nuclear structure. The nucleus of an atom is composed of protons and, except for ...

Types of Radioactivity

The most common types of radioactivity are &#945; decay, &#946; decay, &#947; decay, neutron emission, and electron capture.
Alpha (&#945;) decay is the ...

Nuclear Stability

Protons and neutrons, collectively called nucleons, are packed together tightly in a nucleus. With a radius of about 10&#8722;15 meters, a nucleus is ...

Nuclear Binding Energy

The difference between the calculated and experimentally measured masses is known as the mass defect of the atom. In the case of helium-4, the mass defect ...

Radioactive Decay and Radiometric Dating

Radioactivity is a spontaneous disintegration of an unstable nuclide and is a random process, as all the nuclei in the sample do not decay simultaneously. ...

Nuclear Fission

Many heavier elements with smaller binding energies per nucleon can decompose into more stable elements that have intermediate mass numbers and larger ...

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Controlled nuclear fission reactions are used to generate electricity. Any nuclear reactor that produces power via the fission of uranium or plutonium by ...

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The process of converting very light nuclei into heavier nuclei is also accompanied by the conversion of mass into large amounts of energy, a process ...

Nuclear Transmutation

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Use models to compare nuclear reactions including alpha decay, beta decay and gamma decay; nuclear fusion and nuclear fission.

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Atomic Orbitals

An atomic orbital represents the three-dimensional regions in an atom where an electron has the highest probability to reside. The radial distribution ...

The Pauli Exclusion Principle

The arrangement of electrons in the orbitals of an atom is called its electron configuration. We describe an electron configuration with a symbol that ...

The Energies of Atomic Orbitals

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The Aufbau Principle and Hund's Rule

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The alkali metal sodium (atomic number 11) has one more electron than the neon atom. This electron must go into the lowest-energy subshell available, the ...

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Periodic Classification of the Elements

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The Periodic Table

As early chemists discovered more elements, they realized that various elements could be grouped by their similar chemical behaviors. One such grouping ...

Atomic Mass

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Molar Mass

The identity of a substance is defined not only by the types of atoms or ions it contains but by the quantity of each type of atom or ion. For example, ...

Use the Periodic Table as a model to predict the relative properties of elements based on the pattern of valence electrons in the outermost energy levels of atoms.

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Atomic Radii and Effective Nuclear Charge

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Ionic Radii

Ionic radius is the measure used to describe the size of an ion. A cation always has fewer electrons and the same number of protons as the parent atom; it ...

Ionization Energy

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Electron Affinity

The electron affinity (EA) is the energy change for adding an electron to a gaseous atom to form an anion (negative ion).



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Construct an explanation to infer the atomic size, reactivity, electronegativity, and ionization energy of an element based on its position in the Periodic Table.

PS.Chm.2.2

Types of Chemical Bonds

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Chemical bonds are complex interactions between two or more atoms or ions, which reduce the potential energy of the molecule. Gilbert N. Lewis developed a ...

Ionic Bonding and Electron Transfer

Ions are atoms or molecules bearing an electrical charge. A cation (a positive ion) forms when a neutral atom loses one or more electrons from its valence ...

The Born-Haber Cycle

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To draw Lewis structures for complicated molecules and molecular ions, it is helpful to follow a step-by-step procedure as outlined:


	Determine the ...

Resonance

The Lewis structure of a nitrite anion (NO2&minus;) may actually be drawn in two different ways, distinguished by the locations of the N-O and N=O ...

Formal Charges

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Exceptions to the Octet Rule

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	...

Bond Energies and Bond Lengths

Stable molecules exist because covalent bonds hold the atoms together. The strength of a covalent bond is measured by the energy required to break it, ...

Analyze and interpret data to explain the mechanisms and properties of the two main types of intramolecular (ionic and covalent) bonds.

PS.Chm.3.1

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Molecular Comparison of Gases, Liquids, and Solids

Particles in a solid are tightly packed together (fixed shape) and often arranged in a regular pattern; in a liquid, they are close together with no ...

Intermolecular vs Intramolecular Forces

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Intermolecular Forces

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Comparing Intermolecular Forces: Melting Point, Boiling Point, and Miscibility

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Surface Tension, Capillary Action, and Viscosity

Surface Tension
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Hydrogen Bonds

Hydrogen BondsHydrogen bonds are weak attractions between atoms that have formed other chemical bonds. One of these atoms is electronegative, like oxygen, ...

Van der Waals Interactions

Atoms and molecules interact with each other through intermolecular forces. These electrostatic forces arise from attractive or repulsive interactions ...

Construct an explanation to summarize the influences intermolecular forces have on the properties of chemical compounds.

PS.Chm.3.2

Analyze the structure of atoms and isotopes.

Finding Empirical and Molecular Formulas

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