Molecular Formulas and Nomenclature (2023)

The content that follows is the substance of lecture 20. In this lecture we cover the Gas Laws: Partial Pressures, Kinetic Molecular Theory and Real Gases.

Partial Pressures:

Dalton’s Law of Partial Pressures: The total pressure exerted by a mixture of gases is the sum of their individual partial pressures.

Ptotal = P1 + P2 + P3 + …..

There are two ways to calculate partial pressures:

1)Use PV = nRT to calculate the individual pressure of each gas in a mixture.

2)Use the mole fraction of each gas to calculate the percentage of pressure from the total pressure assignable to each individual gas.

Example 1:

A 1.00 L sample of dry air at 25.0 oC contains 0.319 mol N2, 0.00856 mol O2, 0.000381 mol Ar, and 0.00002 mol CO2. Calculate the partial pressure of N2(g) in the mixture.

At first this looks really intimidating with all of the moles given for each gas but if you read the question carefully you realize that it just wants the pressure for nitrogen and you can calculate that very simply by use of the ideal gas law:

p = nRT/V = 0.319 mol(0.08206 L.atm/mol/K)(298.15K)/1.00L = 7.80 atm

Example 2:

A sample of intestinal gas was collected and found to consist of 44% CO2, 38% H2, 17% N2, 1.3% O2 and 0.0030% CH4, by volume. (The percentages do not total 100% because of rounding) What is the partial pressure of each gas if the total pressure in the intestine is 818 Torr?

For this question we need to remember that the total pressure is the total of each partial pressure and the partial pressure is equal to the mole fraction X of each gas times the total pressure.

PP = XPTotal

First we need to calculate the moles of each of the gases in the mixture. If we assume 100 grams, we can multiply the percentages given and use the resulting gram amounts to calculate each of the mole amounts:

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44% x 100 g = 44g CO2 x (1 mol CO2/44.0g CO2)= 1 mol CO2

38% H2 x 100 g = 38 g H2 x (1 mol H2/2.0g H2)= 19 mol H2

17% N2 x 100 g = 17 g N2 x (1 mol N2 /28 g N2)= 0.61 mol N2

1.3% O2 x 100 g = 1.3g O2 x (1 mol O2/32g O2)= 0.041 mol O2

0.003% CH4 x 100g = 0.003g CH4 x (1 mol CH4/16 g CH4)= 0.00019 mol CH4

Total Moles all gases = 1mol + 19 mol + 0.61 mol + 0.041 mol + 0.00019 mol = 20.6529 mol

Mole Fraction = mol gas/mol total

So calculating the mole fraction and then multiplying the fraction by the total pressure will give the partial pressure of each:

1 mol CO2/20.6529 mol x 818 Torr = 40. Torr

19 mol H2/20.6529 mol x 818 Torr = 750 Torr

0.61 mol N2 /20.6529 mol x 818 Torr = 24 Torr

0.041 mol O2/20.6529 mol x 818 Torr = 1.6 Torr

0.00019 mol CH4/20.6529 mol x 818 Torr = 0.0075 Torr

The pressures don't add up to 818 Torr exactly due to rounding but they should be close.

Partial Pressure Practice:

Kinetic Molecular Theory

Molecular Formulas and Nomenclature (1) The Kinetic Molecular Theory describes the relationship between the motion of gas particles and their properties. The theory makes several assumptions:

•Particles are point masses in constant, random, straight line motion.

•Particles are separated by great distances.

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•Collisions are rapid and elastic.

•No force between particles.

•Total energy remains constant.

Pressure – Assessing Collision Forces

The force of the impacts experienced by the walls of a container is what we use to determine the pressure of a gas.

•Translational kinetic energy

Molecular Formulas and Nomenclature (2)

•Frequency of collisions

Molecular Formulas and Nomenclature (3)

•Impulse or momentum transfer

Molecular Formulas and Nomenclature (4)

•Pressure proportional to impulse times frequency

Molecular Formulas and Nomenclature (5)

While these equations can look intimidating what they are really saying is that the pressure of a gas is simply proportional to the frequency and force with which the molecules are striking the container walls.

Pressure and Molecular Speed

Molecular Formulas and Nomenclature (6)

Assume 1 mole in 3 dimensions (E.G. 1/3)

PV = nRT so

NA * m = M (molar mass)

Then Rearrange to get the Root Mean Squared Speed of the gas

The derivation of the equation shown above also looks intimidating but the end result just says that the speed at which a gas travels is inversely proportional to the square root of its molar mass (M) or that bigger gases travel slower.

Molecular Formulas and Nomenclature (7)

Speed and Temperature:

Using the equations above and substituting the Ideal Gas Law and definition for kinetic energy (eK) into the equation for one mole:

Molecular Formulas and Nomenclature (8)

We can see from the final equation that the relationship between the kinetic energy of a gas and temperature is one of direct proportionality.

Diffusion and Effusion:

Two properties of a gas which derive directly from the relationships shown above are those of Diffusion and Effusion.

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Molecular Formulas and Nomenclature (9)

(a) Depicts the process of Diffusion = the mixing process of two or more gases (b) Depicts the process of Effusion = the process of a gas escaping from a container through a small hole.

In both cases, the net speed at which the process occurs is proportional to the molar mass of the gas(es) involved:

Molecular Formulas and Nomenclature (10)

The equation above is called Graham's Law of Effusion and is named after the scientist that derived it. The conditions under which the law can be applied are as follows:

•Only for gases at low pressure (natural escape, not a jet).

•Tiny orifice (no collisions)

•Does not apply to diffusion.

The law can be used to make a number of calculations:

–Rate of effusion (as above)

–Molecular speeds

–Effusion times

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–Distances traveled by molecules

–Amounts of gas effused.

Here are some practice problems for you to try: Questions and Answers

Real Gases

Up to this point we have been discussing the gases studied as Ideal. That is to say they obey the Ideal Gas Law. The reality is that very few gases obey this law except at very high temperatures or low pressures. Most gases act like Real Gases at room temperatures and pressures and this means they have a different set of properties that have to be considered when you try to calculate their pressures or volumes. Here are the differences that you should know:

Molecular Formulas and Nomenclature (11)

Because of the small but non-zero interactions and volumes of a real gas, the ideal gas law must be amended to consider these factors.

Volume Correction

In an Ideal gas we neglected the volume of the gas so no matter what the identity of the gas or how many molecules were present in a space we assumed the distance between them was the same. But in a real gas, the size of the molecule of gas DOES MATTER. The actual volume free to move in is less because of particle size and more molecules will have more effect.

The correction to the volume is there for the number of moles (n) of the gas times a constant (b) subtracted from the Ideal Gas volume where the value of b is individual to the identity of the gas.

Corrected volume V’ = V – nb

Pressure Correction

Because in a Real gas the molecules do have some interaction/attraction, the pressure observed will be less than that of an ideal gas. The correction must take into account both the concentration of the gas (n/V) as well as the attraction between the two gases (n/V)2 and again this factor must be individualized to the gas by a constant a. The entire correction is shown below:

Molecular Formulas and Nomenclature (12)

Substitution of these two corrections for pressure and volume into the normal Ideal Gas Law yields the Van der Waal's Equation:

Molecular Formulas and Nomenclature (13)

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Where the values of a and b are determined by experiment, and are different for each gas. The values of b will be larger for larger molecules of gas and since the interactions of the gas will depend on its attractive properties like polarity (unequal distribution of electrons throughout the molecule causes partial positive and negative areas that can be more attractive to molecules of opposite charges distributions), the values for a will increase with both size and polarity.

For our purposes, the calculation of this equation would be more of an exercise of mathematical skills rather than an understanding of the chemistry so I will not be presenting an example calculation here nor will put one on a test BUT I do expect you to know the differences between the properties of an Ideal versus Real Gas and how those differences are dealt with in the equation above.


How do you name and write formulas for molecular compounds? ›

write the names of the elements in the order listed in the formula. Use prefixes to indicate the number of each atom. End the name of the second element with -ide. To write the formula of a binary molecular compound, use the prefixes to determine the subscript of each element.

What are the formulas of molecules? ›

Molecular Formula=n(Empirical formula). It gives the type and number of atoms of every element present within the molecular compound. The formula will always be some integer multiple of the formula.

What are 10 common molecules? ›

Molecule Examples
  • Acetic acid - CH3COOH.
  • Benitoite - BaTiSi3O9
  • Caffeine - C8H10N4O2
  • Calcium hydroxide - Ca(OH)2
  • Chlorine - Cl2
  • Dieldrin - C12H8Cl6O.
  • Estradiol - C18H24O2
  • Fool's Gold (Iron Sulfide) - FeS2

What is nomenclature in chemistry? ›

Nomenclature is the process of naming chemical compounds with different names so that they can be easily identified as separate chemicals. Inorganic compounds are compounds that do not deal with the formation of carbohydrates, or simply all other compounds that do not fit into the description of an organic compound.

What are the 3 rules for writing a chemical formula? ›

The rule for writing chemical formula is as follow: Firstly, write the symbols with positive charge valency first. Secondly, write the valency of each atom on the top of its symbol. Thirdly, divide the valency number by their highest common factor ignore the positive or negative radicle.

How many types of molecular formulas are there? ›

There are three main types of chemical formulas: empirical, molecular and structural. Empirical formulas show the simplest whole-number ratio of atoms in a compound, molecular formulas show the number of each type of atom in a molecule, and structural formulas show how the atoms in a molecule are bonded to each other.

What are the 4 steps to calculating molecular formulas? ›

Find the empirical formula.
  1. Get the mass of each element by assuming a certain overall mass for the sample (100 g is a good mass to assume when working with percentages). ...
  2. Convert the mass of each element to moles. ...
  3. Find the ratio of the moles of each element. ...
  4. Use the mole ratio to write the empirical fomula.

How do you write the molecular formula explain with steps? ›

Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number or very close to a whole number. Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.

What are 20 examples of molecules? ›

Examples of Molecules
  • H2O (water)
  • N2 (nitrogen)
  • O3 (ozone)
  • CaO (calcium oxide)
  • C6H12O6 (glucose, a type of sugar)
  • NaCl (table salt)

What are the 4 types of molecules? ›

There are four major classes of large biological molecules—carbohydrates, lipids, proteins, and nucleic acids. Carbohydrates are made up of monomers called monosaccharides that contain carbon, oxygen, and hydrogen.

What are 20 examples of atoms? ›

Here are some examples of atoms:
  • Neon (Ne)
  • Hydrogen (H)
  • Argon (Ar)
  • Iron (Fe)
  • Calcium (Ca)
  • Deuterium, an isotope of hydrogen that has one proton and one neutron.
  • Plutonium (Pu)
  • F-, a fluorine anion.
1 Mar 2022

What is the most famous molecule? ›

Deoxyribonucleic acid, or DNA. It's probably your body's most famous molecule, and rightfully so. It's the one that tells yourselves what proteins to make, as well as how and where. Now, you usually hear people talk about DNA in the context of genetics.

What are the 6 main molecules in the human body? ›

The human body is approximately 99% comprised of just six elements: Oxygen, hydrogen, nitrogen, carbon, calcium, and phosphorus. Another five elements make up about 0.85% of the remaining mass: sulfur, potassium, sodium, chlorine, and magnesium.

What are the 4 rules of nomenclature? ›

Rules of Binomial Nomenclature
  • All the scientific names of organisms are usually Latin. ...
  • There exist two parts of a name. ...
  • When the names are handwritten, they are underlined or italicized if typed. ...
  • The name of the genus starts with a capital letter and the name of the species starts with a small letter.
13 Feb 2021

What are the 3 rules of nomenclature? ›

The full two-part name has to be written in italics (or emphasized when handwritten). The name of the genus is always read first. The name of the genus must be capitalized upon. Never capitalizing on the specific epithet.

What are the two types of nomenclature? ›

Systematic and trivial nomenclature are the two types of nomenclature recognised by nomenclatures.

What two methods are used in writing chemical formulas? ›

An empirical formula gives the simplest, whole-number ratio of atoms in a compound. A structural formula indicates the bonding arrangement of the atoms in the molecule.

What is Class 9 chemical formula? ›

A chemical formula shows the elements that make up the compound and the numbers of atoms of each element in the smallest unit of that compound, be it a molecule or a formula unit.

What are the 3 state symbols used in chemical equations? ›

Using state symbols in chemical equations
State symbolMeaning
(aq)aqueous (dissolved in water)

What are the 10 formulas? ›

Class 10 Maths Formulas for Algebra And Quadratic Equations
  • (a+b)2 = a2 + b2 + 2ab.
  • (a-b)2 = a2 + b2 – 2ab.
  • (a+b) (a-b) = a2 – b. ...
  • (x + a)(x + b) = x2 + (a + b)x + ab.
  • (x + a)(x – b) = x2 + (a – b)x – ab.
  • (a + b)3 = a3 + b3 + 3ab(a + b)
  • (a – b)3 = a3 – b3 – 3ab(a – b)
  • (x – a)(x + b) = x2 + (b – a)x – ab.
19 Jul 2022

What are the 7 types of compounds? ›

Types of Compounds
  • Metal + Nonmetal —> ionic compound (usually)
  • Metal + Polyatomic ion —> ionic compound (usually)
  • Nonmetal + Nonmetal —> covalent compound (usually)
  • Hydrogen + Nonmetal —> covalent compound (usually)

What are the types of formulas? ›

There are three basic types of chemical formula, the empirical formula, the molecular formula, and the structural formula.

What order do you write molecular formulas? ›

Writing a Chemical Formula Given a Chemical Structure

For organic compounds, the order is carbon, hydrogen, then all other elements in alphabetical order of their chemical symbols. If the structure does not contain carbon, write all elements, including hydrogen, in alphabetical order of their chemical symbols.

How do you memorize molecular compounds? ›

Molecular compounds are named with the first element first and then the second element by using the stem of the element name plus the suffix -ide. Numerical prefixes are used to specify the number of atoms in a molecule.

Which is the first step to write a molecular equation? ›

The first step in writing an accurate chemical equation is to write the skeleton equation, making sure that the formulas of all substances involved are written correctly. All reactants are written to the left of the yield arrow, separated from one another by a plus sign.

What is a molecular formula easy definition? ›

The molecular formula is an expression that defines the number of atoms of each element in one molecule of a compound. It shows the actual number of each atom in a molecule. For example, the molecular formula of propane is C4H10. In this formula, there are 4 carbon atoms and 10 hydrogen atoms in the given compound.

What is the concept of formula? ›

The formula is a fact or a rule written with mathematical symbols. It usually connects two or more quantities with an equal sign. When you know the value of one quantity, you can find the value of the other using the formula.

What are the 7 molecules in the human body? ›

Take a look at some of the most important molecules in the body.
  • Water. Water is an essential molecule for life. ...
  • Oxygen. About 20% of air consists of oxygen. ...
  • DNA. DNA codes for all the proteins in the body, not just for new cells. ...
  • Hemoglobin. ...
  • ATP. ...
  • Pepsin. ...
  • Cholesterol.
3 Jul 2019

What are 3 types of molecules? ›

Molecules are of three types: Molecule of an atom, Molecule of an element and Molecule of a compound.

Who discovered atom? ›

Thus, John Dalton known as father of atoms and atomic theory.

What is 5 carbon called? ›


What type of molecule is ATP? ›

ATP is a nucleotide consisting of an adenine base attached to a ribose sugar, which is attached to three phosphate groups. These three phosphate groups are linked to one another by two high-energy bonds called phosphoanhydride bonds.

What is the size of 1 molecule? ›

Molecular size

Molecules commonly used as building blocks for organic synthesis have a dimension of a few angstroms (Å) to several dozen Å, or around one billionth of a meter.

What are the 4 organic compounds? ›

Organic compounds essential to human functioning include carbohydrates, lipids, proteins, and nucleotides. These compounds are said to be organic because they contain both carbon and hydrogen.

What are the 4 main organic molecules? ›

There are 4 major kinds of organic molecules, carbohydrates, lipids, proteins and nucleic acids.
  • Each of these exists as a polymer, composed of the monomers shown in the table. ...
  • monosaccharide, disaccharides, and polysaccharides; quick energy for the cell. ...
  • and a little O. ...
  • sometimes S.

What are the 4 Micromolecules? ›

11.1 Introduction: The Four Major Macromolecules

These are the carbohydrates, lipids (or fats), proteins, and nucleic acids. All of the major macromolecule classes are similar, in that, they are large polymers that are assembled from small repeating monomer subunits.

What are the 118 atoms called? ›

Oganesson is a synthetic chemical element with the symbol Og and atomic number 118.

Are there 118 atoms? ›

There are only about 118 different kinds of atoms, but millions of compounds.

Is water an atom? ›

Atoms join together to form molecules. A water molecule has three atoms: two hydrogen (H) atoms and one oxygen (O) atom. That's why water is sometimes referred to as H2O. A single drop of water contains billions of water molecules.

What is the oldest molecule? ›

Thus, the first chemical bonds were formed. The new compound of helium and hydrogen was called helium hydride or helonium (HeH+), the very first molecule (of any sustained abundance) in the universe.

Which molecule is largest? ›

The aptly named titin weighs in at a molecular weight of 3 million and consists of a continuous chain of 27,000 amino acids, making it 20 to 50 times larger than the average-size protein.

What molecule is the strongest? ›

The strongest intermolecular force is hydrogen bonding, which is a particular subset of dipole-dipole interactions that occur when a hydrogen is in close proximity (bound to) a highly electronegative element (namely oxygen, nitrogen, or fluorine).

What are 4 molecules found in cells? ›

It is, however, the organic molecules that are the unique constituents of cells. Most of these organic compounds belong to one of four classes of molecules: carbohydrates, lipids, proteins, and nucleic acids.

What are the 4 molecules essential to life? ›

There are four major classes of biological macromolecules (carbohydrates, lipids, proteins, and nucleic acids); each is an important cell component and performs a wide array of functions. Combined, these molecules make up the majority of a cell's dry mass (recall that water makes up the majority of its complete mass).

How many atoms are in a human? ›

Suzanne Bell, an analytical chemist at West Virginia University, estimates that a 150-pound human body contains about 6.5 octillion (that's 6,500,000,000,000,000,000,000,000,000) atoms. The vast majority of them are hydrogen (humans are almost entirely water, which comprises two hydrogen atoms and an oxygen).

How do you write molecular formulas in Word? ›

Select Insert > Equation or press Alt + =. Select the equation you need. See the ribbon for more Structures and Convert options.

How do you make a molecular formula? ›

Divide the molar mass of the compound by the empirical formula mass. The result should be a whole number or very close to a whole number. Multiply all the subscripts in the empirical formula by the whole number found in step 2. The result is the molecular formula.

How do you do molecular formula? ›

STEP 1: Calculate the molar mass of the empirical formula. STEP 2: Divide the given molecular molar mass by the molar mass calculated for the empirical formula. STEP 3: Multiply each subscript by the whole number that resulted from step 2. This is now the molecular formula.

What is molecular formula explain with example? ›

The molecular formula is an expression that defines the number of atoms of each element in one molecule of a compound. It shows the actual number of each atom in a molecule. For example, the molecular formula of propane is C4H10. In this formula, there are 4 carbon atoms and 10 hydrogen atoms in the given compound.

How do you memorize chemical formulas? ›

Here are some of the best (and worst) ways to memorize chemistry.
  1. Memorizing Chemistry Using Repetition.
  2. Memorizing Chemistry Using Mnemonic Devices.
  3. Using Memory Palaces To Memorize Chemistry.
  4. Using a Memory Palace To Memorize Numbers.
15 Mar 2018

What are the two types of compounds? ›

Types of Compounds

Covalent or molecular compounds form when elements share electrons in a covalent bond to form molecules. Molecular compounds are electrically neutral. Ionic compounds are (usually) formed when a metal reacts with a nonmetal (or a polyatomic ion).

How do I calculate moles? ›

The unit is denoted by mol.
  1. The formula for the number of moles formula is expressed as.
  2. Given.
  3. Number of moles formula is.
  4. Number of moles = Mass of substance / Mass of one mole.
  5. Number of moles = 95 / 86.94.

How do I calculate molarity? ›

To find the molarity of a solution, we divide the number of moles of solute by the total volume of liters of solution.

How do you read a molecular formula? ›

Each element is represented by its atomic symbol in the Periodic Table – e.g. H for hydrogen, Ca for calcium. If more than one atom of a particular element is present, then it's indicated by a number in subscript after the atomic symbol — for example, H2O means there are 2 atoms of hydrogen and one of oxygen.


1. Writing Chemical Formulas For Covalent Molecular Compounds
(The Organic Chemistry Tutor)
2. How To Name Covalent Molecular Compounds - The Easy Way!
(The Organic Chemistry Tutor)
3. Writing Ionic Formulas: Introduction
(Tyler DeWitt)
4. Chemistry: Naming of Molecules: How to memorize in 30 min
5. Empirical Formula & Molecular Formula Determination From Percent Composition
(The Organic Chemistry Tutor)
6. Naming Ionic Compounds
(Professor Dave Explains)
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