NEET 2015 Chemistry PYQs: Important Questions Every NEET Aspirant Must Master
Learn the Concept. Understand the Reaction. Master Chemistry.
Chemistry can become one of the highest-scoring subjects in NEET when students learn how to connect concepts, reactions, formulas and applications.
Previous Year Questions are one of the best ways to understand how concepts are tested in competitive examinations. NEET 2015 Chemistry questions covered important concepts from Organic Chemistry, Physical Chemistry and Inorganic Chemistry.
In this blog, Pentagon Institute explains important NEET 2015 Chemistry PYQ concepts in a simple and student-friendly format.
For every question, you will learn:
• Question• Correct Answer• Chapter• Step-by-Step Solution• Core Concept• Example• Quick NEET Revision Tip
Let's start.
1. Organic Chemistry – Addition of HBr to an Alkene
Question
An alkene reacts with HBr. How can we predict the major product of the reaction?
Correct Concept
The major product depends on the stability of the intermediate carbocation formed during the reaction.
Chapter
Hydrocarbons / Organic Chemistry – Some Basic Principles and Techniques
Step-by-Step Solution
When an alkene reacts with HBr, the reaction generally takes place through electrophilic addition.
Step 1: Protonation of the Double Bond
The double bond reacts with H⁺ from HBr.
This produces a carbocation intermediate.
Step 2: Formation of a Stable Carbocation
The reaction generally prefers the formation of a more stable carbocation.
Carbocation stability can be affected by:
• Resonance• Hyperconjugation• Alkyl substitution
Step 3: Attack by Bromide Ion
The Br⁻ ion attacks the carbocation.
This produces the final product.
Core Concept
The major product of an organic reaction can often be predicted by asking:
Which intermediate is more stable?
A general carbocation stability trend is:
Benzylic / Allylic > Tertiary > Secondary > Primary
Example
If one possible carbocation is stabilised by resonance with a benzene ring and another is not, the resonance-stabilised carbocation is generally more stable.
Therefore, the reaction pathway leading to the more stable intermediate is preferred.
NEET Quick Tip
Do not memorise only the final organic product.
Always ask:
What intermediate is formed?
Is it stable?
Can resonance occur?
2. Dumas Method – Estimation of Nitrogen
Question
In Dumas' method, nitrogen gas is collected over water.
How is the percentage of nitrogen calculated?
Chapter
Organic Chemistry – Purification and Characterisation of Organic Compounds
Core Concept
In Dumas' method, nitrogen present in an organic compound is ultimately converted into nitrogen gas.
The nitrogen gas is collected and its volume is measured.
If the gas is collected over water, the observed pressure contains the pressure of water vapour.
Therefore:
Pressure of Dry Nitrogen = Total Pressure – Aqueous Tension
Step-by-Step Method
Step 1: Find the Pressure of Dry Nitrogen
Pressure of N₂ = Total Pressure – Aqueous Tension
Step 2: Convert the Gas Volume
Use the combined gas equation:
P₁V₁/T₁ = P₂V₂/T₂
Convert the gas volume to standard conditions if required.
Step 3: Find the Mass of Nitrogen
At STP:
22.4 L of N₂ = 28 g
Use proportion to calculate the mass of nitrogen.
Step 4: Calculate Percentage of Nitrogen
Percentage of Nitrogen = (Mass of Nitrogen / Mass of Organic Compound) × 100
Example
Suppose:
Total Pressure = 725 mm Hg
Aqueous Tension = 25 mm Hg
Then:
Pressure of Dry Nitrogen = 725 – 25
Pressure of Dry Nitrogen = 700 mm Hg
The corrected pressure is used for further calculations.
NEET Quick Tip
Whenever a gas is collected over water:
Actual Gas Pressure = Total Pressure – Water Vapour Pressure
3. Why Are Metals Usually Not Found as Nitrates in Their Ores?
Question
Why are metals generally not found as nitrates in their ores?
Correct Answer
Because most metal nitrates are highly soluble in water.
Chapter
General Principles and Processes of Isolation of Elements
Detailed Solution
Minerals present in the Earth's crust are exposed to water over very long periods.
If a compound is highly soluble in water, it can dissolve and get carried away by water.
Most nitrates are highly soluble.
Therefore, metal nitrates generally do not remain concentrated in large deposits.
Hence, metals are usually not found as nitrate ores.
Core Concept
The occurrence of a mineral in nature depends on:
• Solubility• Stability• Reactivity• Environmental conditions
Highly soluble compounds are less likely to remain concentrated as mineral deposits.
Example
An insoluble compound can remain deposited in the Earth's crust for a long period.
A highly soluble compound can dissolve in water and move away.
NEET Quick Tip
Remember:
Nitrates → Highly Soluble → Usually Not Found as Ores
4. Molecular Orbital Theory – Bond Order of Oxygen Species
Question
How do we calculate the bond order of oxygen and its ionic species?
Chapter
Chemical Bonding and Molecular Structure
Core Formula
Bond Order = ½ (Number of Bonding Electrons – Number of Antibonding Electrons)
Important Oxygen Species
O₂
Bond Order = 2
O₂⁺
One electron is removed from an antibonding orbital.
Therefore:
Bond Order = 2.5
O₂⁻
One electron is added to an antibonding orbital.
Therefore:
Bond Order = 1.5
O₂²⁻
Two electrons are added to antibonding orbitals.
Therefore:
Bond Order = 1
Core Concept
Bond order helps us predict bond strength and bond length.
Higher Bond Order
Generally means:
• Stronger bond• Shorter bond length
Lower Bond Order
Generally means:
• Weaker bond• Longer bond length
Example
Compare:
O₂⁺ → Bond Order 2.5
O₂ → Bond Order 2
O₂⁻ → Bond Order 1.5
Therefore:
Bond Strength: O₂⁺ > O₂ > O₂⁻
NEET Quick Tip
Remember:
Removing an electron from an antibonding orbital increases bond order.
Adding an electron to an antibonding orbital decreases bond order.
5. Isoelectronic Species – Comparing Atomic and Ionic Radii
Question
Arrange the following isoelectronic species in increasing order of radius:
Ar, K⁺ and Ca²⁺
Correct Answer
Ca²⁺ < K⁺ < Ar
Chapter
Classification of Elements and Periodicity in Properties
Step-by-Step Solution
All three species contain:
18 electrons
Therefore, they are called isoelectronic species.
Now compare their number of protons.
Ar → 18 protons
K⁺ → 19 protons
Ca²⁺ → 20 protons
As nuclear charge increases, the attraction between the nucleus and electrons increases.
Therefore, the radius decreases.
Hence:
Ca²⁺ < K⁺ < Ar
Core Concept
For isoelectronic species:
More Protons → Stronger Nuclear Attraction → Smaller Radius
Example
Consider:
O²⁻, F⁻ and Na⁺
All contain 10 electrons.
As nuclear charge increases, the radius decreases.
NEET Quick Tip
For an isoelectronic series:
Count electrons.
Compare nuclear charge.
More protons mean a smaller radius.
6. Chemical Kinetics – Activation Energy and the Arrhenius Equation
Question
How can activation energy be determined using the Arrhenius equation?
Chapter
Chemical Kinetics
Core Formula
The Arrhenius equation is:
k = Ae⁻ᴱᵃ/ᴿᵀ
Taking logarithm:
ln k = ln A – Ea/RT
This can be written as:
ln k = ln A – (Ea/R)(1/T)
Important Graph
When we plot:
ln k versus 1/T
We get a straight line.
The slope is:
Slope = –Ea/R
Therefore:
Activation Energy = –R × Slope
Core Concept
The Arrhenius equation connects:
• Rate Constant• Temperature• Activation Energy
As temperature increases, the rate constant generally increases.
Example
If the slope of a graph between ln k and 1/T is known:
Slope = –Ea/R
Therefore:
Ea = –R × Slope
NEET Quick Tip
Memorise:
ln k vs 1/T → Slope = –Ea/R
7. Identifying an Organic Compound Using Chemical Tests
Question
An organic compound forms a phenylhydrazone but gives negative results with Tollens' test and the iodoform test.
How can the compound be identified?
Chapter
Aldehydes, Ketones and Carboxylic Acids
Step-by-Step Solution
Formation of a phenylhydrazone indicates the presence of a carbonyl group.
Therefore, the compound may contain:
• An aldehyde group• A ketone group
Now consider Tollens' test.
A negative Tollens' test suggests that the compound is not a typical aldehyde.
Therefore, the compound is more likely to be a:
Ketone
Now consider the iodoform test.
A negative iodoform test means that the compound does not contain the required methyl ketone arrangement.
Therefore, the final compound can be identified by combining all the information provided.
Core Concept
Do not solve compound identification questions by memorising structures alone.
Follow this process:
Molecular Formula → Functional Group Test → Positive Result → Negative Result → Eliminate Incorrect Options → Identify the Compound
Important Test Revision
Tollens' Test
Generally positive for:
Aldehydes
Iodoform Test
Generally positive for compounds containing:
CH₃CO–
or compounds that can be converted into this arrangement.
NEET Quick Tip
One Chemical Test = One Important Clue
Use all clues together before selecting the final answer.
What Should You Learn from NEET Chemistry PYQs?
Every Previous Year Question should teach you more than just the correct answer.
For example:
Question → Isoelectronic Species
Do not simply remember:
Ca²⁺ < K⁺ < Ar
Also understand:
Chapter → Periodicity
Concept → Isoelectronic Species
Rule → More Nuclear Charge = Smaller Radius
This is how one question can help you revise an entire topic.
The Smart PYQ Method for NEET Chemistry
Step 1: Identify the Chapter
Ask:
Which chapter does this question belong to?
Step 2: Identify the Branch of Chemistry
Ask whether the question belongs to:
• Physical Chemistry• Organic Chemistry• Inorganic Chemistry
Step 3: Identify the Core Concept
Ask:
What exactly is the question testing?
Step 4: Apply the Correct Method
Physical Chemistry → Formula + Calculation
Organic Chemistry → Reaction + Mechanism
Inorganic Chemistry → Concept + NCERT + Periodic Trends
Step 5: Analyse the Wrong Options
Understanding why other options are wrong helps prevent similar mistakes in future questions.
Pentagon Institute's Smart Chemistry Learning Formula
Read NCERT
↓
Understand the Concept
↓
Learn Important Reactions
↓
Practice Numerical Questions
↓
Solve MCQs
↓
Analyse PYQs
↓
Identify Mistakes
↓
Revise Again
Final Takeaway
The best NEET Chemistry students do not simply memorise reactions and formulas.
They understand:
• Which chapter the question belongs to• Which concept is being tested• Which formula or reaction should be used• Why the correct answer is correct• How the same concept can appear in another question
Do not study Previous Year Questions as:
Question → Answer
Study them as:
Question → Chapter → Concept → Formula or Reaction → Solution → Example → Revision
With regular NCERT revision, concept clarity, reaction practice and proper PYQ analysis, Chemistry can become one of the strongest scoring subjects in NEET.
Frequently Asked Questions
Are NEET 2015 Chemistry PYQs useful for current NEET preparation?
Yes. Previous Year Questions help students understand important concepts, common question patterns and different ways in which Chemistry concepts can be tested.
Should I memorise all Chemistry reactions?
No. First understand:
• Reactant• Reagent• Reaction Condition• Intermediate• Final Product
Understanding the reaction makes it easier to remember.
Which Chemistry topics should I focus on?
Build strong concepts in:
• Chemical Bonding• Periodic Table• Chemical Kinetics• Thermodynamics• Organic Reactions• Aldehydes and Ketones• Coordination Chemistry• Solutions• Electrochemistry
How Can I Improve My NEET Chemistry Score?
Follow this approach:
Read NCERT → Understand Concepts → Learn Formulas and Reactions → Practice MCQs → Analyse PYQs → Revise Mistakes
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