Grahams law of diffusion: Worked example and importance

len Alfred Ajibola - Mon, 29th April, 2019 @ 4:19 PM

Topics in Chemistry

Gas laws in chemistry Charles's law explained Calculation questions on Charles's law Examples of Charles's law in real life Boyle's law explained Calculation questions on Boyle's law Examples of Boyle's law in real life Summary on the kinetic molecular theory of gases Postulates of kinetic theory of gases Avogadro's number explained with worked examples Mole and Avogadro's Number explained Le Chatelier's Principle: Changes in concentration and pressure in dynamic equilibrium Chemical Equilibrium: Dynamic Equilibrium in Chemistry Static and Dynamic Equilibrium explained with their differences Chemistry Scheme of Work, SS1, First Term Chemistry Scheme of Work, SS1, Second Term Chemistry Scheme of Work, SS1, Third Term Compounds in Chemistry: Characteristics of Compounds Types of Mixture: Homogenous and Heterogeneous Mixtures What are mixtures? Characteristics of mixtures

Academic Questions in Chemistry

Please click here to see all Questions and Answers

With regards to redox reactions, which of the following statement is wrong?

  • A. Redox reaction are examples of chemical change

  • B. Reduction is the gain of electron

  • C. Substances that donates election during chemical reaction are termed reductants

  • D. Oxidizing agents are always reduced in chemical reactions

  • E. Oxidation occurs at the cathode in electrolysis

  • F. Hydrogen is a reducing agent

In chemistry, physical change is associated only with the rearrangement of molecules while the internal composition of the substance remains the same.

  • A. True

  • B. False

_____ electron(s) is a term that describes the number of electron(s) in the outermost shell of an atom.

  • A. Outer

  • B. Excess

  • C. Valence

  • D. Positive

  • E. Negative

  • F. Last

_____ is the negative electrode in electrolysis.

  • A. Anode

  • B. Anion

  • C. Cathode

  • D. Cation

  • E. Ion

  • F. Electrolyte

Electrovalent Bond - Len Academy

The above diagram shows the _____ type of bond.

  • A. Covalent

  • B. Polar covalent

  • C. Coordinate covalent

  • D. Metallic

  • E. Van dear walls

  • F. Ionic

 Metals are referred to as _____ in their impure state.

  • A. Diluted

  • B. Consecrated

  • C. Coloured

  • D. Ores

  • E. Stained

  • F. Strained

Metals generally have the quality to shine, glow, sparkle, glitter, reflect light and be polished. This characteristic of metals is termed _____.

  • A. State

  • B. Ductility

  • C. Luster

  • D. Malleability

  • E. Hardness

  • F. Inflorescence

The _____ spectrometry experiment conducted on isotopic elements gave a confirmation for the existence of isotopes.

  • A. Mole

  • B. Volume

  • C. Weight

  • D. Number of moles

  • E. Mass

  • F. Amount of substance

Graham’s Law of Diffusion:

Diffusion involves the movement of gaseous, liquid and solid particles from a region of higher concentration to that of lower concentration. Consider the analogy below:

When a perfume is sprayed in a room, the particles of the perfume will move from a region of higher concentration (region where the perfume was sprayed) to a region of lower concentration (other regions in the room where the perfume wasn’t sprayed). As a result, anyone present in this region of lower concentration will also perceive the scent of the perfume.

This movement of particles from a region of higher concentration to a region of lower concentration is termed diffusion.

The particles undergoing diffusion can be gaseous, liquid or solid. Importantly, the movement of such particles must be from a region of higher concentration to a region of lower concentration.

Please read on physical and chemical change here.

Diffusion is fastest in gaseous particles. This is due to the fact that gaseous particles are able to move freely since the cohesive or binding forces between them are extremely small or negligible.

Please read more on the kinetic theory of gases here.

It is noteworthy to state that diffusion is slowest in solid particles.

Diffusion is technically not the same thing as effusion Diffusion always accompanies effusion; with both happening almost simultaneously.

Effusion is the movement of gaseous particles through a very small opening; and is always accompained by diffusion.

For instance, when an inflated balloon is pierced with a needle, both effusion and diffusion will occur.

When diffusion occurs, it continues until equilibrium is attained. With reference to our previous instance, the scent of the perfume will continue to move from the region of higher concentration to the region of lower concentration until every part of the room becomes equally filled, assuming all external openings have been shut.

When a gas has a high rate of diffusion, it will take a shorter time for diffusion to occur in such gas. This implies that the gas will diffuse rapidly.

Conversely, diffusion will take a longer time for gases with lower rate of diffusion: that is, such gases will diffuse slowly.

Please read on the characteristics of solids, liquids and gases here.

The Scottish chemist Thomas Graham (1805-1869) studied the rate at which gases diffuse. He established a law of diffusion which was named after him. This is the Graham’s law of diffusion.

Graham’s law of diffusion states that the rate (r) of diffusion in a gas at a given temperature is inversely proportional to the square root of its density or molecular mass (m).

Please read on molecular mass and molar mass here.

Below is an equation on Graham's law of diffusion.

Diffusion Equation - Len Academy

Note that the vapour density (v.d) of a gas is equal to half its relative molecular mass(r.m.m).

Therefore: r.m.m. = 2 x v.d

Please read on density and specific gravity here.

Worked example of Graham's law

30cm3 of a gas with an empirical formula of CH3 diffuses through a porous partition in 45.2s. If 30 cm3 of hydrogen diffused in 11.7s under the same conditions. Calculate:

  1. The vapour density of the CH3 gas

  2. The molecular formula of the gas CH3

(Mass of H2 = 2)


(i) Vapour density of the CH3 gas.

Using the formula derived from the above diagram:

  • tx/tH= √mx /√mH

  • t = time, m = molar mass, x = CH3 and H = hydrogen

  • Time taken for CH3 to diffuse = 45.2s

  • Time taken for H2 to diffuse = 11.7s

  • Mass of H2 = 2

  • Mass of CH3 = ?

Let us substitute these values into the above formula:

  • 45.2/11.7 = √(mx/2)

  • 2(45.2/11.7)2 = mx

  • mx = 2 x 14.92

  • mx = 29.84g

  • Mass of CH3 = 29.84g

  • r.m.m = 2 x v.d

Where r.m.m = Relative Molecular Mass and v.d = Vapour Density

  • v.d = r.m.m./2

  • v.d = 29.84/2

  • v.d = 14.92

(ii) Molecular formula of the gas CH3

  • xCH3 = 30

  • 15x = 30

  • x = 2

Therefore, molecular formula = C2H6. This is ethane

Please read on emperical and molecular formula here.


Importance of Graham's law:

  1. It is applied in the separation of gases with different densities.
    Please read on separation of mixtures here.
  2. It is applied in the separation of elements with the same atomic number but different mass number (isotopes).
    Please read on Dalton's atomic theory and it's modifications here.
  3. To determine the densities and molecular masses of unknown gases by comparing their rates of diffusion with known gases.

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Amazing facts in Chemistry

Plastic and Glass can decompose, but not in our lifetimes. It takes an average time of 450 years for plastics to decompose. As for the decomposition of glasses, it takes about 4,000 years

The only letters that failed to appear on the periodic table are letters:

J     &     Q

Gold and copper are the only two non-silvery colored metals.

Copper is the only metal that is naturally antibacterial. For this reason, some children utilize 'copper water bottles' in schools

Water freezes faster when it’s warm than when it’s cold

Most element in their pure state exists physically in different forms. For instance, pure carbon can exist as both diamond and graphite. This phenomenon is called allotropy

If you pour a handful of salt into a full glass of water, the water level will go down rather than overflowing the glass.

Similarly, if you mix half liter of water and half litre of alcohol, the total volume of the liquid will be les than one litre

Notable points in Chemistry

Below are the physical properties of metals:

  1. They exist in solid state.

  2. The have high densities.

  3. They are good conductors of heat and electricity.

  4. The have the ability to be polished, to glow, sparkle and reflect light.

  5. They can be bent, flattened and made into sheets called foils.

  6. They can be drawn into wires.

  7. Iron undergoes magnetism while most metals are poorly magnetized.

  8. They typical have high melting point.

  9. They generally have high boiling point.

  10. With the exception of lithium, sodium and potassium, most metals are generally hard.

  11. They have the ability to make sound when in contact with other objects or metals.

  12. Please read details on the physical properties of metals here

John Dalton is an English chemist who brought clarity into the composition of matter and the basis for their chemical reactions.

Below are Dalton's Atomic Theory:

  1. All matter consists of tiny indivisible particles called atoms.

  2. Atoms of the same element are identical to each other in every aspect because they have the same shape and mass, while atoms of different elements are different in all respect.

  3. Atoms are indestructible and can neither be created nor destroyed.

  4. Atoms of different elements can combine with each other in simple whole number ratios to form compounds.

  5. Atoms of the same element share similar physical and chemical properties. They can also combine in more than one ratio to form two or more compounds.

Meanwhile, understand that the above theories of John Dalton had been modified.

Please read on Dalton's atomic theory and its modifications here

Periodic Table - Len Academy

The periodic table, also called periodic table of elements or Mendeleev's table, is a table that shows an organized arrangement of the 118 chemical elements according to their atomic number.

Out of the 118 elements; elements 1 - 94 are present in nature while elements 95 - 118 are synthesized artificially.

The manner at which elements are arranged on this table reveals some similarities in their electronic configurations and chemical properties.

Please read on the periodic table of elements here.

A compound composed of iron (Fe) and oxygen (O) was analyzed and found to contain 69.94% iron and 30.06% oxygen. Find the empirical formula of the compound. (Molar mass of Fe=55.85, O=16)

  • Solution:

Step 1: Identify the given parameter from the question.

  • Fe = 69.94%,   O = 30.06%.

  • Empirical formula = Fe?O?


Step 2: Convert the percentages to gram. (just attribute grams to the %).

  • 69.94% = 69.94g while 30.06% = 30.06g


Step 3: To get the mole ratio of each element, convert the gram to moles using the formula (mole = mass/molarmass). Please merorize this formula because we always work with moles in emperical formula.

  • Mole of Fe: 69.94/55.85 = 1.252mol

  • Mole of O: 30.06/16 = 1.879mol


Step 4: Divide both sides by the smallest mole ratio.

  • Iron has the smallest mole ratio in our case, therefore: 1.252/1.252 = 1,    1.879/1.252 = 1.5

  • We now have the formula = Fe1O1.5


Step 5: Multiply each of the moles by the smallest whole number that will convert each into a whole number. (In our case, the number '2' is the smallest whole number that will make '1.5' and '1' whole numbers when multiplied by it.

  • For iron (Fe), we will have 1 x 2 = 2

  • For oxygen (O), we will have 1.5 x 2 = 3


Step 6: Write the empirical formula.

  • The empirical formula= Fe2O3

  • Iron(III)tetraoxosulphate(VI)

In chemistry, hydrocarbons can be classified as either aliphatic or aromatic. Recently, both classifications of hydrocarbon were based on their structure rather than their origin.

Aliphatic hydrocarbons are put into three main groups according to the types of bonds they possess. These are:

  1. Alkanes

  2. Alkenes

  3. Alkynes

They are shown in the image below:

Alkanes, Alkenes and Alkynes - Len Academy

It's important to note the followings:

  • Alkanes have single bonds (only) in their structures.

  • Alkenes always have a carbon-carbon double bond present in their structure.

  • Alkynes always have a carbon-carbon triple bond present in their structure.


Aromatic hydrocarbons are classified into:

  • Arenes: They contain benzene ring as a structural unit. Below is the structure of a benzene ring.

Benzene Ring - Len Academy


  • Nonbenzenoid aromatic hydrocarbons: They possess special stability but lack a benzene ring as a structural unit.