🧪AP Chemistry
7 min read•Last Updated on June 18, 2024
Dalia Savy
Jeremy Kiggundu
Dalia Savy
Jeremy Kiggundu
Moles and molar mass are essential key concepts to understand when studying chemistry since they will allow you to accurately calculate and interpret chemical quantities and conversions. Think about it this way: how do scientists perform laboratory work when it's nearly impossible to count the atoms they are working with🤔?
First, imagine an atom, the basic unit of matter that makes up all elements. Well, most likely you can't even begin to grasp how small an atom even is⚛️. An atom is made up of three types of subatomic particles: protons, neutrons, and electrons.
Protons and neutrons are located in the nucleus at the center of the atom, while electrons orbit the nucleus. The nucleus is a small, dense core at the center of the atom. Since protons have a + charge and neutrons are neutral, the nucleus is very overall very positive. Electrons, orbiting the nucleus, have a negative charge and counteract the positive center of the atom.
We'll discuss the atom in more depth later in this unit, but it is important to understand how small it is. This is where the concept of a mole emerged.
Since scientists cannot count the tiny particles and atoms they are experimenting with, there must be a correlation between the mass of substances involved in a chemical reaction and the number of particles undergoing change. This is exactly what the mole is!
A mole relates the mass of an element to the number of particles there are. The analogy between a mole and a dozen of eggs can be helpful in understanding the concept of a mole in chemistry. Just as a dozen is a unit of measurement for eggs, a mole is a unit of measurement for particles in a substance.
The molar mass of a substance is the number of grams there are in a mole. Hence, the units for molar mass are grams/mole (or g/mol). Molar mass is important because it allows us to convert between mass, moles, and the number of particles.
Finding the molar mass of an element or compound is not as hard as it might seem: the only things that you need to know are which elements are involved and how many of them are present. This is also where the periodic table of elements comes in.
On the periodic table, each element is represented by a one-to-two-letter abbreviation. You can also see a number above and below each chemical symbol. The number above, going chronologically across the periodic table, is the atomic number. The atomic number represents the number of protons in the nucleus of an atom of that element. The number below each symbol is the element's atomic mass. This is the mass of one atom of the element in atomic mass units (amu).
👉 Cool Interactive Periodic Table
Let's first calculate the molar mass of water (H2O). First, break down the compound of interest. In one molecule of water, we have 2 atoms of hydrogen and 1 atom of oxygen.
From here, we have to take a look at the periodic table and find out how much each atom of hydrogen and oxygen weighs. This is where we have to take a look at the atomic mass of an element. The atomic mass of hydrogen is 1.008 g/mol and the atomic mass of oxygen is 16.00 g/mol. Since there are two atoms of hydrogen and one atom of oxygen in water, we must multiply 1.008 by 2, and then add that product to 16.00. This is how you can calculate the molar mass of water: 18.02 g/mol! 🥳
Let's say we now have the compound CO2 or carbon dioxide. Remember, to calculate the molar mass, you simply have to multiply the atomic mass of each specific element by its subscript, and then add it all together.
Carbon has a subscript of 1 and an atomic mass of 12.01 grams according to the periodic table. Oxygen has a subscript of 2 in this compound and has an atomic mass of 15.99 grams. Always multiply the subscript by the atomic mass of the element:
Carbon: 1 x 12.01 = 12.01 g
Oxygen: 2 x 16.00 = 32.00 g
Finally, we add 32.00 grams to 12.01 grams to get 44.01 grams. Therefore, CO2 has a molar mass of 44.01 grams per mole.
Remember the analogy between a mole and a dozen? Just as a dozen eggs contains 12 eggs, a mole of a substance contains 6.022 x 10^23 particles. This very large number is Avogadro's number. To put this into perspective, a mole of hockey pucks would be equal to the mass of the Moon. In chemistry, this number is relevant when converting from grams to moles to atoms.
One of the most fundamental takeaways from this unit is dimensional analysis, and you'll be using it throughout the rest of this course! This is a technique used to convert between different units of measurement, and you've probably implicitly done it before.
For example, you can use dimensional analysis to convert from miles per hour to meters per second, or from inches to centimeters.
When doing dimensional analysis, you start by identifying the units you are trying to convert from and the units you want to convert to. Then, you write down the conversion factor that will allow you to make the conversion. Finally, you multiply the value you are trying to convert by the conversion factor to get the final result.
A conversion factor is a ratio of equivalent units that can be used to convert one set of physical units to another. These are usually known facts, such as 1 foot = 12 inches and 1 meter = 3.28 feet.
Now that we've discussed the fundamental concepts of moles and molar mass, let's try converting a sample of 50.0 grams of CO2 between units.
Since we know we have to convert from grams to moles, we have to figure out what conversion factor can help us do this. First, put the number that is given to you in the problem, which in this case, is 50.0 grams of CO2.
Then, you want to multiply 50.0 by the molar mass in order to convert it to the moles of CO2. The unit that you have (grams of CO2) should always be on the bottom of the next ratio in order for the units to cancel out. Here, the grams of CO2 cancel out and you are left with a measurement in moles.
Tip: It is good to memorize that moles = grams/molar mass. The conversion factor in this problem is actually using this concept since you are ultimately dividing the number of grams you have by the molar mass to get the number of moles.
Now let's convert 1.14 moles of CO2 into atoms using Avogadro's number.
Here, you are once again taking the number that you have and putting it first. Then, you are putting the unit of measurement that you want over the unit of measurement that you have, making that step the conversion factor. This enables the moles of CO2 to cancel out, leaving you with just 6.84x10^23 atoms of CO2.
Keep in mind that the concept of Avogadro's number serves as the conversion factor when going from moles to atoms.
Since the subscript on Carbon is 1, the number of atoms of CO2 is equivalent to the number of carbon atoms in CO2. There is nothing to multiply by because of this 1-to-1 ratio; therefore the number of carbon atoms in this 50.0g sample of CO2 is 6.84x10^23.
Unlike carbon, oxygen has a subscript of 2. This makes the ratio of CO2 atoms to oxygen atoms 1:2. Therefore, we have to use dimensional analysis again:
Since there are two atoms of O in one atom of CO2, we had to multiply by 2 to get the number of atoms of O.
You got this! Once you practice multiple problems involving dimensional analysis, it'll seem like a piece of cake. Sadly, these problems become more difficult as the course progresses but as always, practice makes perfect.
An atom is the smallest unit of a chemical element that retains the properties of that element. It consists of a nucleus (containing protons and neutrons) surrounded by electrons.
Term 1 of 25
An atom is the smallest unit of a chemical element that retains the properties of that element. It consists of a nucleus (containing protons and neutrons) surrounded by electrons.
Term 1 of 25
An atom is the smallest unit of a chemical element that retains the properties of that element. It consists of a nucleus (containing protons and neutrons) surrounded by electrons.
Term 1 of 25
An atom is the smallest unit of a chemical element that retains the properties of that element. It consists of a nucleus (containing protons and neutrons) surrounded by electrons.
Nucleus: The central part of an atom, containing protons and neutrons.
Electron: A subatomic particle with a negative charge that orbits the nucleus.
Element: A substance consisting entirely from one type of atom.
Subatomic particles are particles smaller than an atom. They include protons, neutrons, and electrons which make up atoms.
Quarks: Fundamental particles that combine to form protons and neutrons.
Leptons: A group of subatomic particles that includes electrons.
Bosons: Particles responsible for all physical forces except gravity.
Protons are positively charged subatomic particles found within atomic nuclei.
Neutrons: Neutral subatomic particles found within atomic nuclei alongside protons.
Atomic Number: The number of protons in an atomic nucleus which determines its chemical properties and place in the periodic table.
Ion: An atom or molecule with a net electric charge due to loss or gain of one or more electrons.
Neutrons are subatomic particles found in the nucleus of an atom. They have no electric charge and a mass slightly larger than that of a proton.
Proton: A positively charged particle located within the atomic nucleus.
Atomic Mass Unit (AMU): A unit of mass used to express atomic and molecular weights, roughly equivalent to the mass of one nucleon (either a proton or neutron).
Strong Nuclear Force: The force that acts between protons and neutrons in an atomic nucleus, overcoming electromagnetic repulsion and holding them together.
Electrons are subatomic particles with a negative electric charge. They orbit around the nucleus of an atom in specific energy levels or shells.
Orbitals: Regions around the nucleus where electrons are most likely to be found.
Valence Electrons: The electrons present in the outermost shell of an atom that participate in chemical reactions.
Ionization Energy: The amount of energy required to remove an electron from its orbital.
The nucleus is at the center of an atom consisting mainly of protons and neutrons. It carries most of an atom's mass but occupies only a tiny space compared to the total size of the atom.
Atomic Number: The number of protons in the nucleus of an atom, which determines the chemical properties of an element and its place in the periodic table.
Nucleons: Protons and neutrons collectively are known as nucleons.
Radioactive Decay: A process by which an unstable atomic nucleus loses energy by radiation. It can involve the release of alpha or beta particles from the nucleus.
This refers to the process where atoms or molecules (particles) change their structure, arrangement, or energy state. This can occur through chemical reactions, phase changes, or nuclear reactions.
Chemical Reaction: A process that leads to the transformation of one set of chemical substances to another.
Phase Change: A transition of matter from one state (solid, liquid, gas) to another.
Nuclear Reaction: A process in which two nuclei or nuclear particles collide to produce products different from the initial particles.
A standard quantity used to express and compare measurements. In chemistry, common units include meters for length, grams for mass, and seconds for time.
Metric System: An international decimal-based system of measurement.
SI Units: The International System of Units; it's an updated version of the metric system and is now widely used worldwide for scientific measurements.
Conversion Factor: A ratio derived from the equality between two different units that can be used to convert from one unit to another.
The atomic number is equal to the number protons found in an atom's nucleus. It defines what element an atom is and its place on the periodic table.
Protons: Positively charged particles found in the nucleus of an atom.
Electrons: Negatively charged particles that orbit around the nucleus of an atom.
Neutrons: Neutral particles (no charge) found in the nucleus of an atom.
Atomic mass units, or amu, is a standard unit of mass that quantifies mass on an atomic or molecular scale. One amu is approximately equal to the mass of one proton or neutron.
Proton: A subatomic particle carrying a positive electric charge, found in the nucleus of an atom. Its mass is roughly equivalent to 1 amu.
Neutron: A subatomic particle found in the nucleus of an atom. It carries no electrical charge and its mass is also roughly equivalent to 1 amu.
Electron: A subatomic particle carrying a negative electric charge. It's much lighter than protons and neutrons with a relative mass close to 0 amu.
The atomic mass of oxygen is the weighted average mass of an atom of oxygen, based on the relative abundance of different isotopes. It's approximately 16 atomic mass units (amu).
Isotope: An isotope is a variant form of an element with a different number of neutrons in its nucleus, leading to a different atomic mass.
Atomic Mass Unit (amu): This is a unit used to express atomic and molecular weights. It's one-twelfth (1/12) the weight of a carbon-12 atom.
Relative Abundance: This refers to how common each isotope of an element is in nature.
Avogadro's number, also known as Avogadro's constant, represents the number of atoms or molecules in one mole of any substance. It’s approximately 6.022 x 10^23 particles per mole.
Mole: A unit in chemistry representing Avogadro's number worth of particles (atoms, molecules, ions etc.)
Atoms/Molecules/Ions: These are basic units that make up matter; atoms form molecules and ions are charged atoms or groups of atoms.
Stoichiometry: The part of chemistry dealing with quantitative relationships between reactants and products in chemical reactions; often involves calculations using moles and Avogadro’s number.
Dimensional analysis is a mathematical technique used in chemistry and physics to convert from one unit to another using conversion factors.
Conversion Factor: A ratio expressing how many of one unit are equal to another unit.
Units of Measurement: Standard quantities used to express physical quantities, such as meters, grams, or seconds.
Significant Figures: The digits in a number that carry meaningful information about its precision; significant figures are often important when performing dimensional analysis.
A conversion factor is a ratio (or fraction) which represents the relationship between two different units. It's used to convert one unit of measurement into another.
Ratio: A comparison of two quantities that both have the same unit of measure.
Unit Conversion: The process of changing from one unit to another, often using a conversion factor.
Dimensional Analysis: A method used in science to convert from one system of units to another, or to check if an equation makes sense dimensionally.
Physical units are standard quantities used to express and measure physical quantities such as length, mass, time, etc.
SI Units (International System): An international system of physical units based on metric measurements including meter for length, kilogram for mass, second for time etc.
Metric System: A decimal-based system of measurement that is universally used in science due its ease of use and conversions.
Measurement Error: The difference between a measured value and its true value.
The atoms in CO2, or carbon dioxide, are the basic units that make up this molecule. It consists of one carbon atom and two oxygen atoms.
Carbon Atom: This is an element with atomic number 6. It's known for its ability to form stable covalent bonds with other elements, including itself, making it a key component in many molecules necessary for life.
Oxygen Atom: An element with atomic number 8. Oxygen readily forms compounds by reaction with almost any other element.
Molecule: A group of two or more atoms held together by chemical bonds. In this case, CO2 is a molecule composed of one carbon atom and two oxygen atoms.