Name: mole, mass, and avogadro’s number
Uploaded: 2020-09-03T13:38:25.000Z
Duration: 1 min 54 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.2.1

Molar Mass

הטבלה המחזורית מציגה את הממוצע של המסה האטומית של כל אטום. יחד עם זאת, דוגמיות מקרוסקופיות מכילות מספרים גדולים מאוד של אטומים. לכן חיוני להשתמש ביחידת ספירה הנקראת מספר אבוגדרו.מספר אבוגדרו, שסמלו הוא NA, מעוגל בדרך כלל ל-6.022 X 23^10 ערכו שווה למספר האטומים ב-12 גרמים של איזוטופ פחמן-12 טהור. המושג מול, המסומל באותיות mol, משמש לתאר מספר אבוגדרו, באותו האופן שהמונח המוכר, תריסר, משמש לתאר את המספר 12. יחד עם זאת, משקלם של תריסר פריטים שונים יכול להיות שונה מאוד, למרות שמספר הפריטים בכל תריסר הוא זהה.באופן דומה, מולים של יסודות שונים הם בעלי מסות שונות אבל מכילים תמיד את אותו מספר אטומים. המסה בגרמים של מול אחד של יסוד נקראת מסה מולרית ומבוטאת בגרמים מול. הערך המספרי של מסה מולרית שווה למסה האטומית של היסוד.לכן הגדרת המול מאפשרת לנו לספור חלקיקים על ידי שקילתם. ניתן להשתמש במסה המולרית ובמספר אבוגדרו כגורמי המרה. הם מאפשרים לחשב את מספר האטומים במטיל זהב במשקל חמישה גרמים.המסה המולרית הממוצעת של זהב היא 196.96 גרמים למול. אז כדי להמיר את המסה למולים, מחלקים את המסה של המטיל במסה המולרית הממוצעת. לאחר מכן מכפילים את מספר המולים במספר אבוגדרו כדי לקבל את מספר האטומים הכולל בדגימה.מכאן שמטיל הזהב הזה מכיל 1‪529 X 10‪22 אטומים.

מסה מולרית

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 זהותו של חומר מוגדרת לא רק לפי סוגי האטומים או היונים שהוא מכיל אלא לפי הכמות של כל סוג של אטום או יון. לדוגמה, מים, H2O ומי חמצן, H2O2, דומים בכך שהמולקולות שלהם מורכבות מאטומי מימן וחמצן. עם זאת, מכיוון שמולקולת מי חמצן מכילה שני אטומי חמצן, בניגוד למולקולת המים, שיש לה רק אחד, שני החומרים מציגים תכונות שונות מאוד. 

 אטומים ומולקולות הם קטנים ביותר. לכן, למדידת הכמויות המקרוסקופיות שלהם, יש צורך ביחידה מדעית סטנדרטית. מול היא יחידת כמות הדומה ליחידות מוכרות כמו זוג, תריסר, ברוטו וכו'. היא מספקת מדד ספציפי למספר האטומים או המולקולות בדגימה של החומר. הקונוטציה הלטינית למילה "חפרפרת" היא "מסה גדולה" או "תפזורת", מה שעולה בקנה אחד עם השימוש בה כשם ליחידה זו. המול מספקת קישור בין תכונה מקרוסקופית הנמדדת בקלות, מסה בתפזורת, לבין תכונה בסיסית חשובה ביותר, מספר אטומים, מולקולות וכו'.

 מול של חומר היא הכמות שבה יש 6.02214076 × 1023 ישויות נפרדות (אטומים או מולקולות). מספר גדול זה מעוגל בצורה נוחה ל-6.022 × 1023, הוא קבוע בסיסי המכונה המספר של אבוגדרו (NA) או קבוע אבוגדרו לכבודו של המדען האיטלקי אמדאו אבוגדרו. קבוע זה מדווח כהלכה עם יחידה מפורשת של "מול".

 בהתאם להגדרתו כיחידת כמות, 1 מול של כל יסוד מכיל את אותו מספר של אטומים כמו מול 1 של כל יסוד אחר. המסות של מול 1 של יסודות שונים, לעומת זאת, שונות, מכיוון שהמסות של האטומים הבודדים שונות באופן דרסטי. המסה המולרית של יסוד (או תרכובת) היא המסה בגרמים של מול 1 של אותו חומר, תכונה המתבטאת ביחידות של גרם למול (גרם/מול). 

 המסה המולרית של כל חומר שווה בערך מספרי למשקל האטומי או הנוסחה שלו בamu. לפי הגדרת הamu, אטום פחמן בודד שוקל 12 amu (המסה האטומית שלו היא 12 amu). מול של פחמן שוקל 12 גרם (12 גרם C = 1 מול אטומי C = 6.022 × 1023 אטומי C), והמסה המולרית של פחמן היא 12 גרם/מול. קשר זה מתקיים עבור כל היסודות שכן המסה האטומית שלהם נמדדת ביחס לזו של חומר המיוחס לamu, פחמן-12. בהרחבת עיקרון זה, המסה המולרית של תרכובת בגרמים שווה באופן מספרי גם למסת הנוסחה שלה בamu. לדוגמה, להליום יש מסה אטומית של 4.002 amu ומסה מולרית של 4.002 גרם/מול. 

 בעוד שמסה אטומית ומסה מולרית שוות ערך מספרי, זכור כי הם שונים בתכלית מבחינת קנה המידה. כדי להעריך את גודל המול, שקול טיפה קטנה של מים במשקל של כ-0.03 גרם. למרות שזה מייצג רק חלק זעיר של מול 1 מים (~18 גרם), הוא מכיל יותר מולקולות מים ממה שניתן לדמיין. אם המולקולות היו מפוזרות באופן שווה בין שבעה מיליארד בני אדם על פני כדור הארץ, כל אדם היה מקבל יותר מ-100 מיליארד מולקולות מים. 

 המול מגדיר את הקשר בין המסה למספר האטומים. זה מאפשר לחשב את מספר האטומים על בסיס צורות מתאימות של מקדם ההמרה: 1 מול של אטומים = 6.022 × 1023 אטומים. כדי להמיר בין המסה של יסוד (בגרמים) למספר מולים, המסה המולרית של היסוד (גרם/מול) משמשת כמקדם המרה. 

 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></div>

זהותו של חומר מוגדרת לא רק על ידי סוגי האטומים או היונים שהוא מכיל, אלא גם על ידי הכמות של כל סוג אטום או יון. לדוגמה...

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mole, mass, and avogadro’s number

The mole links mass and the number of particles in a substance. It gives chemists a standard way to count atoms and molecules in samples that are far too small to see. It also helps show why substances with the same kinds of atoms can still have very different properties, like water, H2O, and hydrogen peroxide, H2O2.

The mole is an amount unit, like a pair or a dozen. One mole contains 6.02214076 × 10^23 discrete entities, such as atoms or molecules. This number is usually rounded to 6.022 × 10^23 and is called Avogadro’s number, or the Avogadro constant, named for Amedeo Avogadro. The constant is written with the unit per mole.

One mole of any element contains the same number of atoms as one mole of any other element. The masses are different, though, because atoms have different individual masses. That is why one mole of one element does not weigh the same as one mole of another element.

Molar mass is the mass in grams of one mole of a substance. It is reported in grams per mole, or g/mol. The molar mass of an element or compound is numerically equal to its atomic mass or formula mass in amu. For example, carbon has an atomic mass of 12 amu, so 1 mole of carbon has a mass of 12 g and a molar mass of 12 g/mol.

This same relationship works for other elements and for compounds. Helium, for example, has an atomic mass of 4.002 amu and a molar mass of 4.002 g/mol. Although the numbers match, atomic mass and molar mass are very different in scale.

A tiny drop of water can still contain an enormous number of molecules. A drop that weighs about 0.03 g is only a small part of 1 mole of water, which is about 18 g. Even so, it holds more water molecules than can easily be imagined. If those molecules were shared equally among the roughly seven billion people on Earth, each person would still get more than 100 billion molecules of water.

The mole also lets scientists convert between mass and amount. Using the relationship 1 mole of atoms = 6.022 × 10^23 atoms, the number of atoms can be calculated from a sample. The molar mass of an element is also used as a conversion factor to move between grams and moles.

Mole, Mass, and Avogadro’s Number

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molar mass

The mole links mass and the number of particles in a substance. It gives chemists a standard way to count atoms and molecules in samples that are far too small to see. It also helps show why substances with the same kinds of atoms can still have very different properties, like water, H2O, and hydrogen peroxide, H2O2. The mole is an amount unit, like a pair or a dozen. One mole contains 6.02214076 × 10^23 discrete entities, such as atoms or molecules. This number is usually rounded to 6.022 ×...

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Learn how the mole connects mass, atoms, and molecules through Avogadro’s number and molar mass, with clear examples for high school chemistry.

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Mole, Mass, and Avogadro’s Number

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