Mole Concept, Avogadro’s Number, and Molecular Weight – MCAT General Chemistry | MedSchoolCoach

## Mole Concept, Avogadro’s Number, and Molecular Weight

##### MCAT General Chemistry Chapter 1 - Section 1.3 - Stoichiometry
1. Home
2. »
3. MCAT Masterclass
4. »
5. Chemical and Physical Foundations of Biological Systems
6. »
7. General Chemistry
8. »
9. Stoichiometry
10. »
11. Chemical Formula
12. »
13. Mole Concept, Avogadro’s Number, and Molecular Weight – MCAT General Chemistry

## Sample MCAT Question - Mole Concept, Avogadro’s Number, and Molecular Weight​

Approximately how many hydrogen atoms exist in 9 g of H2O?

a) 18

b) 3.0 x 1023

c) 6.0 x 1023

d) 1.2 x 1024

C is correct. 6.0 x 1023.

For every mole of H2O, there are two moles of hydrogen atoms. We can find the number of moles of H2O using the given mass in the question stem and the molar mass. The molar mass of H2O is 2 g/mol (2 H) + 16 g/mol (O) = 18 g/mol. Therefore, the number of moles of H2O is 9 g / 18 g/mol = 0.50 mol. Using our previous reasoning, if there are 0.50 moles of H2O, there is 1 mole of hydrogen atoms. Thus, the number of hydrogen atoms is simply equal to Avogadro's number: 6.0 x 1023.

## Get 1-on-1 MCAT Tutoring From a Specialist

With MCAT tutoring from MedSchoolCoach, we are committed to help you prepare, excel, and optimize your ideal score on the MCAT exam.

For each student we work with, we learn about their learning style, content knowledge, and goals. We match them with the most suitable tutor and conduct online sessions that make them feel as if they are in the classroom. Each session is recorded, plus with access to whiteboard notes. We focus on high-yield topics if you’re pressed for time. If you have more time or high-score goals, we meticulously cover the entire MCAT syllabus.

## Molecular Weight versus Molar Mass

As we will discuss in future lessons, the relationship between different reactants and products in a reaction has less to do with their mass or weight and is instead a function of their quantity. However, while the mass of a solution or substance can be easily measured in a laboratory setting, determining the quantity of molecules in a substance is less immediately clear. This brings us to our next major concepts: molecular weight, molar mass, the mole, and Avogadro’s number.

The molecular weight of a molecule is simply the summation of the individual weights of each atom in a molecule. These can be determined by looking at the periodic table. In the example of sodium hydroxide, we would add the atomic weight of sodium (23 Daltons, or Da), oxygen (16 Da), and hydrogen (1 Da), giving us a molecular weight of approximately 40 Da for sodium hydroxide.

The number of molecules in a sample can be determined by dividing the mass of the sample by 40 Da. However, due to the extremely high number of molecules in even a minuscule sample of a chemical substance, this conversion quickly becomes impractical. Therefore, we will instead typically use the more accessible value of molar mass, which is defined as the mass of a compound (in grams) present in one mole (the SI unit of quantity) of that substance. The molar mass of a molecule typically has the same absolute value as its molecular weight. So the molar mass of sodium hydroxide, which had a molecular weight of 40 Da, would be approximately 40 grams per mole.

## The Mole Concept and Avogadro's Number

A mole, which we mentioned above, is a value of quantity equal to 6.02 x 1023 particles of whatever is being measured. This number is referred to as Avogadro’s number, and was determined experimentally by a 19th century Italian scientist named Amedo Avogadro, by dividing the mass of 12 grams of carbon-12 by the mass of a single atom of carbon-12. This conversion allows us to work with the extremely high values of molecular quantity more intuitively: instead of viewing 12 grams of carbon-12 as consisting of an almost unfathomably large number of atoms, we can simply describe it as consisting of one mole of carbon atoms.

## Explore More MCAT Masterclass Chapters

Take a closer look at our entire MCAT Masterclass or explore our Biochemistry lessons below.

Start Lesson »

Start Lesson »

Start Lesson »

Start Lesson »

Start Lesson »

Start Lesson »

Start Lesson »

### One-on-One Tutoring

Are you ready to take your MCAT performance to a whole new level? Work with our 99th-percentile MCAT tutors to boost your score by 12 points or more!

See if MCAT Tutoring can help me

### MCAT Go Audio Course

Engaging audio learning to take your MCAT learning on the go, any time, any where. You'll be on the way to a higher MCAT score no matter where you are. Listen to over 200+ lessons.

### MCAT Practice Exams

Practice makes perfect! Our mock exams coupled with thorough explanations and in-depth analytics help students understand exactly where they stand.

### MCAT Prep App

Access hundreds of MCAT videos to help you study and raise your exam score. Augment your learning with expert-created flashcards and a question banks.