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

K12 Standards

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NC K-12 Science Standard Course of Study

9-12

Chemistry

Matter and its Interactions

PS.Chm.3.3

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Formula Mass and Mole Concepts of Compounds

أصغر وحدة تمثيلية للمركب هي الجزيء الذي يصور بالصيغة الكيميائية. الكتل الذرية لجميع الذرات في الصيغة الكيميائية،إذا أضيفت،تعطي الكتلة الجزيئية للمركب،و تقاس بوحدة الكتلة الذرية. على سبيل المثال،لإيجاد الكتلة الجزيئية للسكروز أو سكر المائدة،الكتلة الذرية لكل عنصر مضروبا في عدد الذرات من العنصر المقابل.يعطي المجموع الكلي الكتلة الجزيئية للسكروز. كتلة كل الجزيئات الموجودة في مول واحد من أي مركب كيميائي تسمى الكتلة المولية،وتقاس بالجرام لكل مول. الكتلة المولية للمركب هي عدداًمساوي لكتلته الجزيئية على سبيل المثال،السكروز،له الكتلة الجزيئية 342.3 وحدة كتلة ذرية،سيكون له الكتلة المولية 342.3 جرام لكل مول.إنّمعرفة العدد الفعلي للجزيئات أو الأجزاء الموجودة في مادة كيميائية،أساسي لفهم تكوينها وتفاعلاتها وتركيباتها. ومع ذلك،في عينة معينة من المركب،فإنّحساب العدد الفعلي للجزيئات بالوسائل العادية غير عملي. من خلال توظيف العلاقة بين الكتلة المولية،المول،وعدد أفوجادرو،فإنّمجموع الجزيئات الموجودة يمكن حسابه.وفقًا لهذا المفهوم فإنّالكتلة المولية للمركب،وهي ما يعادل مول واحد من المركب،تشكل 6.022 10 23 جسيمًا. لذلك،يحتوي 342.3 جرامًا من السكروز على 6.022 10 23 من جزيئات السكروز. ومع ذلك،فإذا كانت عينة السكروز تزن 150 جرامًا على سبيل المثال،يمكن تحديد عدد الجزيئات عن طريق الحساب باستخدام طريقة الضرب التبادلي.تظهر النتيجة أن 150 جرام من السكروز تشكل 2.63 10 23 جزيء سكروز. النسبة المئوية لكل عنصر في المركب يمكن التعبير عنها باستخدام قيمة نسبة-الكتلة. ولحسابها،فإنّكتلة العنصر في واحد مول من المركب تقسم على الكتلة الكلية لمول واحد من المركب وتحويلها إلى نسبة مئوية.في الممارسة العملية،لحساب نسبة كتلة الهيدروجين في السكروز،فإنّالكتلة الكلية لذرات الهيدروجين والكتلة الكلية لجزيئات السكروز مدرجين في الصيغة. توضح القيمة المحسوبة أن السكروز يحتوي على 6.4 كتلة في المئة من الهيدروجين.

مفاهيم كتلة الصيغة والمول للمركبات

<h4> كتلة الصيغة للمركبات التساهمية </h4> تمثل الصيغ الكيميائية التركيب العنصري للمواد. بالنسبة للمركبات التساهمية ، تمثل الصيغة عدد وأنواع الذرات المكونة لجزيء واحد من المادة ؛ لذلك ، يمكن الإشارة بشكل صحيح إلى كتلة الصيغة على أنها كتلة جزيئية. تشير الصيغة الجزيئية للكلوروفورم (CHCl3)، وهو مركب تساهمي، إلى أن الجزيء الفردي يحتوي على ذرة كربون واحدة، وذرة هيدروجين واحدة، وثلاث ذرات كلور. متوسط ​​الكتلة الجزيئية لجزيء الكلوروفورم يساوي مجموع متوسط ​​الكتل الذرية لهذه الذرات: <img alt="Eq1" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;" /><h4> كتلة الصيغة المركبات الأيونية </h4> تتكون المركبات الأيونية من الكاتيونات والأنيونات المنفصلة التي يتم دمجها بنسب لإنتاج مادة لها شحنة إجمالية متعادلة كهربائيًا. يتم حساب كتلة الصيغة للمركب الأيوني بنفس طريقة حساب كتلة صيغة المركبات التساهمية: عن طريق جمع متوسط ​​الكتل الذرية لجميع الذرات في صيغة المركب’s. ومع ذلك، فإن صيغة المركب الأيوني لا تمثل تكوين جزيء منفصل، لذلك قد لا تتم الإشارة إليه بشكل صحيح باسم ‘molecular mass’. على سبيل المثال، ملح الطعام الشائع أو كلوريد الصوديوم (NaCl) مركب أيوني يتكون من كاتيونات الصوديوم (Na+) وأنيونات الكلوريد (Cl−) معًا بنسبة 1: 1. يتم حساب كتلة الصيغة لهذا المركب عن طريق إضافة متوسط ​​الكتل الذرية للعناصر المكونة له: <img alt="Eq2" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;" /> عند حساب كتلة صيغة المركب الأيوني، تم استخدام متوسط ​​كتل ذرات الصوديوم والكلور المتعادلة، بدلاً من كتل كاتيونات الصوديوم وأنيونات الكلوريد. على الرغم من أن كاتيون الصوديوم يحتوي على كتلة أصغر قليلاً من ذرة الصوديوم (نظرًا لأنه ينقصه إلكترون)، فإن هذا الاختلاف سيعوضه حقيقة أن أنيون كلوريد أكبر قليلاً من ذرة الكلور (بسبب الإلكترون الإضافي). علاوة على ذلك، فإن كتلة الإلكترون صغيرة بشكل مهم بالنسبة إلى كتلة الذرة النموذجية. <h4> تكوين نسبة الكتلة </h4> يحدد التركيب العنصري للمركب هويته الكيميائية ، والصيغ الكيميائية هي الطريقة الأكثر إيجازًا لتمثيل هذا التركيب العنصري. تسمى النسبة المئوية بالكتلة لكل عنصر في المركب نسبة الكتلة لهذا العنصر المعين. يمكن حساب النسبة المئوية للتركيب بقسمة كتلة كل عنصر على الكتلة الكلية للمركب ثم تحويلها إلى نسبة مئوية. يعد التركيب بالنسبة المئوية مفيدًا في تقييم الوفرة النسبية لعنصر معين في مركبات مختلفة من الصيغ المعروفة. طالما أن الصيغة الجزيئية أو التجريبية للمركب المعني معروفة، يمكن اشتقاق التركيب المئوي من الكتل الذرية أو المولية لعناصر المركب. على سبيل المثال ، يحتوي جزيء واحد من حمض النيتريك (HNO3) على ذرة  نيتروجين واحدة تزن 14.01 وحدة دولية ، ذرة  هيدروجين تزن 1.008 وحدة دولية ، وثلاث ذرات  أكسجين تزن (3 مرات ؛ 16.00) amu) = 48.00 وحدة دولية. وبالتالي ، فإن كتلة الصيغة لحمض النيتريك هي (14.01 amu + 1.008 amu + 48.00 amu) = 63.02 amu ، وتركيبتها المئوية هي: <img alt="Eq3" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-03.png" style="height: 40px;" /><img alt="Eq4" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-04.png" style="height: 40px;" /><img alt="Eq5" src="https://www.jovecore.com/CDNSource/lessons/1017/1017_Equations-05.png" style="height: 40px;" />هذا النص مقتبس من <a href="https://openstax.org/books/chemistry-2e/pages/3-1-formula-mass-and-the-mole-concept">Openstax, Chemistry 2e, Section 3.1: Formula Mass and the Mole Concept</a> and <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>

كتلة الصيغة للمركبات التساهمية

تمثل الصيغ الكيميائية التركيب الأولي للمواد. بالنسبة للمركبات التساهمية ، تمثل الصيغة ...

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Molecules, Compounds, and Chemical Equations

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KEY TERMS AND CONCEPTS

المصطلحات والمفاهيم الأساسية

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

chemical-formula-mass-and-percent-composition

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

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

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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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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 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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Chemical Formula Mass and Percent Composition

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