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

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Chemistry

Matter and its Interactions

PS.Chm.3.3

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Formula Mass and Mole Concepts of Compounds

יחידה הייצוג הקטbv ביותר של תרכובת היא מולקולה המתוארת כנוסחה כימית. המסה האטומית של כל האטומים בנוסחה הכימית, נזכמת לתת את המסה המולקולרית של התרכובת ונמדדת ביחידת מסה אטומית. לדוגמה, על מנת לאתר את המסה המולקולרית של סוכרוז המוכר כסוכר, המסה האטומית של כל יסוד מוכפלת במספר האטומים של אותו יסוד.הסכום של כל המכפלות הוא המסה המולקולרית של סוכרוז. המסה של כל המולקולות המהוות מול אחד של תרכובת כימית נקראת המסה המולרית ונמדדת בגרמים למול. המסה המולרית של תרכובת שווה במספרה למסה המולקולרית לדוגמה, סוכרוז, שהמסה המולקולרית שלו היא 342.3 יחידות מסה אטומית, יהיה בעל מסה מולרית של 342.3 גרם למול.ידיעת המספר הממשי של מולקולות או חלקיקים שמכיל כל חומר כימי הכרחית להבנה של ההרכב שלו, האינטראקציות שלו והקומבינציות שלו. עם זאת, בדוגמית נתונה של התרכובת, ספירת המספר הממשי של מולקולות בדרכים מקובלות אינה מעשית. על ידי שימוש בקשר שבין המסה המולרית, מול, ומספר אבוגדרו, סך המולקולות הנוכחות ניתן לחישוב.לפי התאוריה, המסה המולרית של תרכובת שווה למול אחד בתרכובת, הכולל 6.022 x 10^23 חלקיקים. לכן, 342.3 גרמים של סוכרוז 6.022 x 10^23 מכילים מולקולות של סוכרוז. יחד עם זאת, אם דוגמית של סוכרוז שוקלת 150 גרם, לדוגמה, מספר המולקולות יכול להימצא על ידי חישוב המשתמש בשיטת הערך המשולש התוצאה מראה ש-150 גרמים של סוכרוז מכילים 2.63X 10^23 מולקולות סוכרוז.האחוז של כל יסוד בתרכובת יכול להיות מבוטא על ידי שימוש בערך אחוז המסה. כדי לחשב אותו, המסה של היסוד במול אחד של התרכובת מחולקת בסך המסה של מול אחד של התרכובת ומומרת לאחוזים. בפועל, כדי לחשב את אחוז המסה של המימן בסוכרוז, סך המסה של אטומי מימן וסך המסה של מולקולות הסוכרוז נכללת בנוסחה.הערך המחושב מראה שסוכרוז מכיל 6.4 אחוזי מסה של מימן.

מסת הנוסחה והמושגים של המול בתרכובות

<div style="direction:rtl"><h4>מסת הנוסחה של תרכובות קוולנטיות</h4> נוסחאות כימיות מייצגות את ההרכב היסודי של חומרים. עבור תרכובות קוולנטיות, הנוסחה מייצגת את המספרים והסוגים של האטומים המרכיבים מולקולה אחת של החומר; לכן, ניתן להתייחס בצורה נכונה למסת הנוסחה כמסה מולקולרית. הנוסחה המולקולרית של כלורופורם (CHCl3), תרכובת קוולנטית, מצביעה על כך שמולקולה אחת מכילה אטום פחמן אחד, אטום מימן אחד ושלושה אטומי כלור. המסה המולקולרית הממוצעת של מולקולת כלורופורם שווה לסכום המסה האטומית הממוצעת של אטומים אלה: <img alt="Eq1" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-01.png" style="height: 40px;"> <h4>מסת הנוסחה של תרכובות יוניות</h4> תרכובות יוניות מורכבות מקטיונים נפרדים ואניונים המשולבים ביחסים כדי להניב חומר בתפזורת נייטרלית חשמלית. מסת הנוסחה עבור תרכובת יונית מחושבת באותו אופן כמו מסת הנוסחה עבור תרכובות קוולנטיות: על ידי סיכום המסות האטומיות הממוצעות של כל האטומים בנוסחה של התרכובת. עם זאת, הנוסחה של תרכובת יונית אינה מייצגת את ההרכב של מולקולה בדידה, כך שייתכן שלא ניתן להתייחס אליה בצורה נכונה כ"מסה מולקולרית". לדוגמה, מלח שולחן רגיל או נתרן כלורי (NaCl) הוא תרכובת יונית המורכבת מקטיוני נתרן (Na+) ואניונים כלוריים (Cl−) המשולבים ביחס של 1:1. מסת הנוסחה של תרכובת זו מחושבת על ידי הוספת המסה האטומית הממוצעת של היסודות המרכיבים אותה: <img alt="Eq2" src="https://www.jove.com/files/core/CDNSource/lessons/1017/1017_Equations-02.png" style="height: 40px;"> בעת חישוב מסת הנוסחה של תרכובת יונית, נעשה שימוש במסה הממוצעת של אטומי נתרן וכלור ניטרליים, במקום המסות של קטיוני נתרן וכלוריד. למרות שלקטיון נתרן יש מסה מעט קטנה יותר מאטום נתרן (מאחר שחסר לו אלקטרון), הבדל זה יקוזז על ידי העובדה כי אניון כלורי הוא מעט יותר מאסיבי מאטום כלור (בשל האלקטרון הנוסף) . יתרה מכך, המסה של אלקטרון קטנה באופן זניח ביחס למסה של אטום טיפוסי. <h4>הרכב אחוזי המסה</h4> ההרכב היסודי של התרכובת מגדיר את זהותה הכימית, ונוסחאות כימיות הן הדרך התמציתית ביותר לייצג את המרכיב היסודי הזה. אחוז במסה של כל יסוד בתרכובת נקרא אחוז המסה של אותו יסוד מסוים. ניתן לחשב אחוז הרכב על ידי חלוקת המסה של כל יסוד במסה הכוללת של התרכובת ולאחר מכן להמיר לאחוז. אחוז הרכב שימושי להערכת השפע היחסי של יסוד נתון בתרכובות שונות של נוסחאות ידועות. כל עוד ידועה הנוסחה המולקולרית או האמפירית של התרכובת המדוברת, הרכב האחוז עשוי להיגזר מהמסה האטומית או המולרית של יסודות התרכובת. לדוגמה, מולקולה אחת של חומצה חנקתית (HNO3) מכילה אטום חנקן אחד במשקל 14.01 amu, אטום מימן אחד במשקל 1.008 amu ושלושה אטומי חמצן במשקל (3 × 16.00 amu) = 48.00 amu. מסת הנוסחה של חומצה חנקתית היא אפוא (14.01 אמיו + 1.008 אמיו + 48.00 אמיו)= 63.02 אמיו, והרכב האחוזים שלה הוא: <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;"> טקסט זה מותאם מ <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></div>

נוסחה מסת תרכובות קוולנטיות;

נוסחאות כימיות מייצגות את ההרכב היסודי של חומרים. עבור תרכובות קוולנטיות, הנוסחה מייצגת את ...

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KEY TERMS AND CONCEPTS

מונחים ומושגים מרכזיים

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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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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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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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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Chemical Formula Mass and Percent Composition

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