NEET Previous Year Questions (2014-2024): Some Basic Concepts of Chemistry | Chemistry Class 11 PDF Download

Q.1. The highest number of helium atoms is in                   (2024)
(a) 4mol of helium
(b) 4u of helium
(c) 4g of helium
(d) 2.271098L of helium at STP
Ans: (a)
To determine which option contains the highest number of helium atoms, we need to analyze each option based on the amount of helium it represents and apply Avogadro's Law as required.
Option A: 4mol of helium
Using Avogadro's number, which is approximately 6.022 × 10²³ atoms per mole, the number of helium atoms in 4 moles can be calculated as: 4mol × 6.022 × 10²³ atoms/mol = 24.088 × 10²³ atoms
Option B: 4u of helium
The atomic mass of helium is approximately 4 u (atomic mass units). Therefore, 4u represents about 1 mole of helium atoms (since the molar mass of helium is approximately 4 g/mol, which equals 4 u). Thus, this option represents:
1mol × 6.022 × 10²³ atoms/mol = 6.022 × 10²³ atoms
Option C: 4g of helium
Similarly, as we've established that the molar mass of helium is 4 g/mol, 4g of helium equates exactly to:
1mol × 6.022 × 10²³ atoms/mol = 6.022 × 10²³ atoms
Option D: 2.271098L of helium at STP (Standard Temperature and Pressure)
At STP, one mole of any ideal gas occupies 22.4 L. Therefore, the amount of helium in moles for L
2.271098L  can be derived from:

Using this to find the number of atoms:
0.101mol × 6.022 × 10²³ atoms/mol ≈ 6.082 × 10²² atoms
Conclusion:
Comparing the numbers:
Option A: 24.088 × 10²³ atoms
Option B: 6.022 × 10²³  atoms
Option C: 6.022 × 10²³  atoms
Option D: 6.082 × 10²²  atoms
Option A clearly contains the highest number of helium atoms, which is 24.088 × 10²³ atoms

Q2:  1 gram of sodium hydroxide was treated with  25mL of  0.75M HCl  solution, the mass of sodium hydroxide left unreacted is equal to
(a) 750 mg
(b) 250 mg
(c) Zero mg
(d) 200 mg      (2024)
Ans: (b)
To find the mass of sodium hydroxide (NaOH) left unreacted, we first need to determine the moles of NaOH and HCl originally present and compare them to see which one is in excess.
The molar mass of NaOH is approximately  40 g/mol, so the number of moles of NaOH in 1 gram can be calculated as follows:
Next, we calculate the number of moles of HCl using its concentration and the volume of the solution. Recall that concentration (Molarity, M) is defined as moles of solute per liter of solution. Given that the concentration of HCl is  0.75 M and the volume of the solution is  25 mL or 0.025L, we can find the moles of HCl:
Moles of HCl = Concentration × Volume in liters = 0.75 M × 0.025 L = 0.01875 moles
Now, we compare the moles of NaOH and HCl. The stoichiometry of the reaction between NaOH and HCl is:
NaOH + HCl → NaCl + H₂O
Each mole of NaOH reacts with one mole of HCl. Given that there are more moles of NaOH (0.025 moles) than HCl (0.01875 moles), HCl is the limiting reactant and will be completely consumed. The excess NaOH can be calculated:
Excess moles of NaOH = Moles of NaOH − Moles of HCl = 0.025 moles − 0.01875 moles = 0.00625 moles
To find the mass of the unreacted NaOH:
Mass of unreacted NaOH = Moles of unreacted NaOH × Molar mass of NaOH = 0.00625 moles × 40 g/mol = 0.25 g = 250 mg
Therefore, the mass of sodium hydroxide left unreacted is 250 mg, which corresponds to Option B.

Q.3. A compound X contains 32% of A, 20% of B and remaining percentage of C. Then, the empirical formula of X is :
(Given atomic masses of A = 64 ; B=40 ; C = 32 u)
(a) A₂BC₂
(b) ABC₃
(c) AB₂C₂
(d) DABC₄           (2024)
Ans: (b)
So, empirical formula of

∴ The correct empirical formula of compound X  is ABC₃

Q.4. A compound with a molecular formula of C₆H₁₄ has two tertiary carbons. Its IUPAC name is :
(a) n-hexane
(b) 2-methylpentane
(c) 2,3-dimethylbutane
(d) 2,2-dimethylbutane       (2024)
Ans: (c)

Q.5. Select the correct statements from the following      (2023)
A. Atoms of all elements are composed of two fundamental particles.
B. The mass of the electron is 9.10939 × 10^–31 kg.
C. All the isotopes of a given element show same chemical properties.  
D. Protons and electrons are collectively known as nucleons.  
E. Dalton’s atomic theory, regarded the atom as an ultimate particle of matter.
Choose the correct answer from the options given below  
(a) A, B and C only
(b) C, D and E only
(c) A and E only
(d) B, C and E only
Ans: d

• Atoms consist of three fundamental particles:  Electrons, protons and neutrons
• The mass of the electron is 9.10939 × 10^–31 kg
• All the isotopes of a given element show same chemical properties.
• Protons and neutrons present in the nucleus are collectively called as nucleons.  
• Dalton’s atomic theory, regarded the atom as the ultimate particle of matter.
  So, the correct statements are B, C, E only 

Question for NEET Previous Year Questions (2014-2024): Some Basic Concepts of Chemistry

Try yourself:Q.2. The right option for the mass of CO₂ produced by heating 20 g of 20% pure limestone is (Atomic mass of Ca = 40)     (2023)

• A.

  1.12 g
• B.

  1.76 g
• C.

  2.64 g
• D.

  1.32 g

Explanation

 
From 100 g CaCO₃ → 44 g CO₂ produced  
As CaCO₃ is 20% pure

So, 100 g CaCO₃ → 44 g CO₂

= 1.76 g CO₂

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Q.6. What mass of 95% pure CaCO₃ will be required to neutralise 50 mL of 0.5 M HCl solution according to the following reactions?

[Calculate upto second place of decimal point]    (2022)
(a) 3.65 g
(b) 9.50 g
(c) 1.25 g
(d) 1.32 g
Ans: