MHT-CET Chemistry · Some Basic Concepts of Chemistry

SI Units, Physical Properties and Atomic Abundance

The measurement toolkit of chemistry: the seven SI base units and the derived units built from them, how a property behaves when you change the sample size (intensive vs extensive), and how an element's average atomic mass falls out of its isotopes' masses and abundances.

All Some Basic Concepts of Chemistry notes MHT-CET Chemistry strategy

Why this matters

Six PYQs here, all EASY and all pure recall or one-line computation — the reliable free marks of this chapter. They cluster three ways: name-the-unit (SI unit of viscosity, of rate of diffusion; the quantity measured in candela), classify-the-property (which pair is intensive), and one abundance calculation (find isotopic percentages from the average atomic mass of chlorine). So the work is mostly memorising two short reference tables plus one mixing formula — the kind of subtopic where a student should never drop a mark.

Concept 1 of 5

The seven SI base units

Intuition

Every measurement in chemistry is ultimately built from seven agreed-upon base units. The bank tests these as a straight name-the-unit or name-the-quantity recall, so learn the pairing both ways: quantity to unit and unit to quantity.

Definition

The SI (International System) defines seven base quantities, each with one base unit and symbol:

Mass in kilogram $\text{kg}$ ; length in metre $\text{m}$ ; time in second $\text{s}$ .
Temperature in kelvin $\text{K}$ ; amount of substance in mole $\text{mol}$ .
Electric current in ampere $\text{A}$ ; luminous intensity in candela $\text{cd}$ .

Everything else (volume, force, pressure, viscosity, ...) is a derived unit assembled from these.

Base quantity	Unit	Symbol
Mass	kilogram	kg
Length	metre	m
Time	second	s
Temperature	kelvin	K Note kelvin has no degree sign: write $300\ \text{K}$ , not $300\ ^\circ\text{K}$ .
Amount of substance	mole	mol
Electric current	ampere	A
Luminous intensity	candela	cdQ The odd one out that PYQs love — candela measures luminous intensity, not energy, force or work.

Learn the pairing in both directions — quantity to unit and unit to quantity.

Practice this conceptself-check · 4 quick reps

Try it yourself

Which physical quantity has the SI base unit mole?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
SI base unit of amount of substance?
2.
Candela is the SI unit of which quantity?
3.
SI base unit of temperature?
4.
Which quantity has the SI base unit ampere?

From the bank · past-year question

Example 1Some Basic Concepts of ChemistryEASY

Identify the physical quantity that is measured in Candela.

[Q60 · 10th May Shift 1 · 2024]

Candela measures luminous intensity, not energy

A common distractor set offers energy, work or force for candela. Candela

\text{cd}

is strictly the base unit of luminous intensity; energy and work are measured in joules and force in newtons.

Concept 2 of 5

Common SI derived units

Intuition

A derived unit is just base units multiplied and divided together, dictated by the defining formula of the quantity. If you can write the quantity as a formula, you can build its unit — but the bank usually just wants you to recognise the finished unit.

Definition

A derived unit is assembled from base units through the quantity's own defining relation:

Volume $= (\text{length})^3$ , so its unit is $\text{m}^3$ (litre and $\text{dm}^3$ are common non-SI equivalents).
Rate of diffusion $= \dfrac{\text{volume}}{\text{time}}$ , giving $\text{dm}^3\,\text{s}^{-1}$ .
Coefficient of viscosity has unit $\text{N s m}^{-2} = \text{Pa s}$ (pascal-second).
Density, force and pressure follow the same build-from-the-formula rule.

Quantity	Defining relation	SI derived unit
Volume	length cubed	$\text{m}^3$
Density	mass / volume	$\text{kg m}^{-3}$
Force	mass $\times$ acceleration	newton $\text{N} = \text{kg m s}^{-2}$
Pressure	force / area	pascal $\text{Pa} = \text{N m}^{-2}$
Rate of diffusion	volume / time	$\text{dm}^3\,\text{s}^{-1}$ Q
Coefficient of viscosity	stress / velocity gradient	$\text{N s m}^{-2} = \text{Pa s}$ Q Watch the exponents on $\text{s}$ and $\text{m}$ : the correct form is $\text{N s m}^{-2}$ , not $\text{N s}^{-1}\text{m}^{-2}$ .

Every derived unit is the base units of its defining formula, combined.

Practice this conceptself-check · 4 quick reps

Try it yourself

Rate of diffusion of a gas is measured as the volume diffused per unit time. Volume is in dm3 and time in seconds. What is its SI-style unit?

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
SI unit of coefficient of viscosity?
2.
SI unit of density?
3.
SI unit of pressure?
4.
Unit of rate of diffusion (volume in $\text{dm}^3$ )?

From the bank · past-year question

Example 2Some Basic Concepts of ChemistryEASY

Which of following is SI Unit of coefficient of viscosity?

[Q57 · 9th May Shift 2 · 2024]

The exponents on the viscosity unit matter

Viscosity is

\text{N s m}^{-2}

— one power of

\text{s}

in the numerator and

\text{m}^{-2}

. Distractors flip these to

\text{N s}^{-1}\text{m}^{-2}

\text{N s m}^{2}

. Read the exponents carefully before choosing.

Drill 1 more on common si derived units

Concept 3 of 5

Intensive vs extensive properties

Intuition

Ask one question: if I take a bigger sample, does the value change? If it does, the property is extensive; if it stays put, it is intensive. The bank tests this by asking you to pick the pair that are both intensive.

Definition

Physical properties split by their dependence on the amount of substance:

Extensive properties depend on the amount — mass, volume, internal energy, heat capacity all double if you double the sample.
Intensive properties are independent of the amount — temperature, density, boiling point, surface tension, viscosity and specific heat are the same for a drop or a bucketful.
A tell-tale: any ratio of two extensive properties is intensive (density = mass/volume; specific heat = heat capacity/mass).

Property	Type	Why
Mass	Extensive	Doubles when the sample doubles.
Volume	Extensive	Scales directly with amount.
Internal energy	Extensive	Total energy grows with amount.
Heat capacity	Extensive	Whole-sample quantity; scales with mass.
Temperature	Intensive	A drop and a bucket of the same liquid share it.
Density	Intensive	Ratio mass/volume — the amounts cancel.
Boiling point	Intensive	Fixed for a pure substance, any amount.
Surface tension	Intensive	A material property, independent of quantity.Q
Viscosity	Intensive	Same for a drop or a barrel of the liquid. Surface tension and viscosity are the intensive pair the bank tests — both material properties, unchanged by sample size.
Specific heat	Intensive	Heat capacity per unit mass — a ratio, so amounts cancel.

Change the sample size in your head: if the value moves, it is extensive.

Practice this conceptself-check · 4 quick reps

Try it yourself

Classify each as intensive or extensive: density, mass, boiling point, volume.

Practice — Level 1 (4 reps)

Quick reps to lock in the method. Try each, then check.

1.
Is temperature intensive or extensive?
2.
Is volume intensive or extensive?
3.
Density is a ratio of which two extensive properties?
4.
Name the intensive pair among: surface tension, mass, viscosity, volume.

From the bank · past-year question

Example 3Some Basic Concepts of ChemistryEASY

Which among the following pair of properties are intensive?

[Q52 · 11th May Shift 1 · 2023]

Heat capacity is extensive; specific heat is intensive

Heat capacity is a whole-sample quantity — it scales with mass, so it is extensive. Divide it by mass and you get specific heat, which is intensive. The 'heat capacity and specific heat' option is a mixed pair, not an intensive one.

Concept 4 of 5

Average atomic mass from isotopic abundance

Intuition

A natural sample of an element is a mixture of isotopes, so its atomic mass is a weighted average — each isotope's mass counted according to how common it is. The same formula runs backwards: given the average and the isotope masses, you can solve for the abundances.

Definition

The average (relative) atomic mass is the abundance-weighted mean of the isotope masses:

Multiply each isotope mass by its fractional abundance and add.
Fractional abundance = percentage abundance $\div 100$ ; the fractions must sum to 1.
For a two-isotope element with abundances $x$ and $1-x$ , set up $m_1 x + m_2(1-x) = \bar{m}$ and solve for $x$ .

Average atomic mass

\bar{m} = \sum_i m_i\,f_i \quad\text{where}\quad \sum_i f_i = 1

\bar{m}average atomic mass
m_imass of isotope i
f_ifractional abundance of isotope i (percentage / 100)

Worked example

Boron has two isotopes of masses 10 and 11. Their percentage abundances are 20% and 80%. Find the average atomic mass of boron.

Convert to fractions: $f_{10} = 0.20$ , $f_{11} = 0.80$ .
Weighted sum: $\bar{m} = 10(0.20) + 11(0.80)$ .
$\bar{m} = 2.0 + 8.8 = 10.8$ .

Answer:

10.8

Practice this conceptself-check · 3 quick reps

Try it yourself

Silver has two isotopes of masses 107 and 109. Its average atomic mass is 107.9. Find the percentage abundance of each isotope.

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Two isotopes, masses 20 and 22, abundances 90% and 10%. Average mass?
2.
If fractional abundances are 0.75 and 0.25, what do they sum to?
3.
Convert 25% abundance to a fraction.

From the bank · past-year question

Example 4Some Basic Concepts of ChemistryEASY

Chlorine exists in two isotopic forms

^{35}\text{Cl}

^{37}\text{Cl}

. If average atomic mass of chlorine is 35.5, what is the percentage abundance of these isotopes respectively?

[Q94 · 10th May Shift 1 · 2023]

Weight by abundance, not a plain average

For chlorine's masses 35 and 37, a plain average would give 36. The true value 35.5 is lower because the lighter isotope is far more abundant (75%). Always multiply each mass by its fraction before adding — never just average the isotope masses.

Concept 5 of 5

Abundance of elements on Earth

Intuition

The bank asks which element is most abundant, and the answer depends on where you look: the Earth's crust versus the whole Earth versus the universe give different winners. For the crust — the default 'on Earth' answer — oxygen leads.

Definition

Element abundance is context-dependent, and PYQs default to the crust:

In the Earth's crust (by mass), oxygen is most abundant (~46%), then silicon, then aluminium.
For the Earth as a whole, iron dominates (the core is iron-rich).
In the universe, hydrogen is by far the most abundant.

Read the question's frame, but 'most abundant on Earth' unqualified means the crust — oxygen.

Domain	Most abundant element	Approx. share
Earth's crust (by mass)	Oxygen	about 46%Q This is the default 'most abundant element on Earth' answer the bank wants — oxygen.
Earth's crust (2nd)	Silicon	about 28%
Earth's crust (3rd)	Aluminium	about 8%
Whole Earth (by mass)	Iron	about 32%
Universe (by mass)	Hydrogen	about 74%

The winner changes with the domain — match the answer to what the question asks.

Practice this conceptself-check · 3 quick reps

Try it yourself

Which element is the most abundant in the Earth's crust by mass?

Practice — Level 1 (3 reps)

Quick reps to lock in the method. Try each, then check.

1.
Most abundant element in the Earth's crust?
2.
Second most abundant element in the Earth's crust?
3.
Most abundant element in the universe?

From the bank · past-year question

Example 5Some Basic Concepts of ChemistryEASY

Which from following elements is most abundant on earth?

[Q71 · 10th May Shift 2 · 2024]

Crust versus universe versus whole Earth

Oxygen tops the crust, but hydrogen tops the universe and iron tops the whole Earth. An unqualified 'most abundant on Earth' means the crust, so choose oxygen — not hydrogen.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (1)

Average atomic mass from isotopic abundance
Average atomic mass
$\bar{m} = \sum_i m_i\,f_i \quad\text{where}\quad \sum_i f_i = 1$

Reference tables (4)

The seven SI base units7 rows

Base quantity	Unit	Symbol
Mass	kilogram	kg
Length	metre	m
Time	second	s
Temperature	kelvin	K Note kelvin has no degree sign: write $300\ \text{K}$ , not $300\ ^\circ\text{K}$ .
Amount of substance	mole	mol
Electric current	ampere	A
Luminous intensity	candela	cdQ The odd one out that PYQs love — candela measures luminous intensity, not energy, force or work.

Learn the pairing in both directions — quantity to unit and unit to quantity.

Common SI derived units6 rows

Quantity	Defining relation	SI derived unit
Volume	length cubed	$\text{m}^3$
Density	mass / volume	$\text{kg m}^{-3}$
Force	mass $\times$ acceleration	newton $\text{N} = \text{kg m s}^{-2}$
Pressure	force / area	pascal $\text{Pa} = \text{N m}^{-2}$
Rate of diffusion	volume / time	$\text{dm}^3\,\text{s}^{-1}$ Q
Coefficient of viscosity	stress / velocity gradient	$\text{N s m}^{-2} = \text{Pa s}$ Q Watch the exponents on $\text{s}$ and $\text{m}$ : the correct form is $\text{N s m}^{-2}$ , not $\text{N s}^{-1}\text{m}^{-2}$ .

Every derived unit is the base units of its defining formula, combined.

Intensive vs extensive properties10 rows

Property	Type	Why
Mass	Extensive	Doubles when the sample doubles.
Volume	Extensive	Scales directly with amount.
Internal energy	Extensive	Total energy grows with amount.
Heat capacity	Extensive	Whole-sample quantity; scales with mass.
Temperature	Intensive	A drop and a bucket of the same liquid share it.
Density	Intensive	Ratio mass/volume — the amounts cancel.
Boiling point	Intensive	Fixed for a pure substance, any amount.
Surface tension	Intensive	A material property, independent of quantity.Q
Viscosity	Intensive	Same for a drop or a barrel of the liquid. Surface tension and viscosity are the intensive pair the bank tests — both material properties, unchanged by sample size.
Specific heat	Intensive	Heat capacity per unit mass — a ratio, so amounts cancel.

Change the sample size in your head: if the value moves, it is extensive.

Abundance of elements on Earth5 rows

Domain	Most abundant element	Approx. share
Earth's crust (by mass)	Oxygen	about 46%Q This is the default 'most abundant element on Earth' answer the bank wants — oxygen.
Earth's crust (2nd)	Silicon	about 28%
Earth's crust (3rd)	Aluminium	about 8%
Whole Earth (by mass)	Iron	about 32%
Universe (by mass)	Hydrogen	about 74%

The winner changes with the domain — match the answer to what the question asks.

Watch out for (5)

Candela measures luminous intensity, not energy
→ The seven SI base units
The exponents on the viscosity unit matter
→ Common SI derived units
Heat capacity is extensive; specific heat is intensive
→ Intensive vs extensive properties
Weight by abundance, not a plain average
→ Average atomic mass from isotopic abundance
Crust versus universe versus whole Earth
→ Abundance of elements on Earth

Mastery check — 1 interleaved questions

Try each one before clicking. Questions are interleaved across the concepts above, not grouped — interleaving sharpens transfer.

Example 1Some Basic Concepts of ChemistryEASY

What is the SI unit of rate of diffusion for a gas?

[Q77 · 21 April Shift I · 2025]

Drill every past-year question on this subtopic

6 questions from the bank — paginated, with cart and Word-export support.

Drill the 6 questions

Related notes

Mole Concept and Stoichiometry (NDA Chemistry) →