Some Basic Concepts of Chemistry — Class 11 Chemistry NCERT Solutions (Free)

Free step-by-step NCERT solutions for Class 11 Chemistry chapter "Some Basic Concepts of Chemistry" — 8 important questions with detailed answers for CBSE board exam preparation.

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TL;DR: Free step-by-step NCERT solutions for Class 11 Chemistry chapter "Some Basic Concepts of Chemistry" — 8 important questions with detailed answers for…

Written & reviewed by the Syllab.in Academic Team (CBSE/NCERT subject experts) · Updated Jul 23, 2026

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Key Questions Covered:

Calculate the number of moles in 27 g of Al (Atomic mass of Al = 27 u).
Calculate the mass of 3.2 mol of oxygen atoms. (Atomic mass of O = 16 u)
Calculate the number of molecules in 11.2 L of CO2 at STP (Molar mass of CO2 …
What is the empirical formula of a compound containing 75% C and 25% H by mas…
Define atomic mass unit (amu). What is the relationship between amu and gram?
A compound has the molecular formula C₂H₆O. What are the possible structures?…
+ 2 more questions in the full chapter

Solutions Summary:

Question	Status
Calculate the number of moles in 27 g of Al (Atomic mass …	✓ Solved
Calculate the mass of 3.2 mol of oxygen atoms. (Atomic ma…	✓ Solved
Calculate the number of molecules in 11.2 L of CO2 at STP…	✓ Solved
What is the empirical formula of a compound containing 75…	✓ Solved
Define atomic mass unit (amu). What is the relationship b…	✓ Solved
A compound has the molecular formula C₂H₆O. What are the …	✓ Solved

Showing 6 of 8 questions

Q1: Calculate the number of moles in 27 g of Al (Atomic mass of Al = 27 u).

Number of moles = mass (g) / molar mass (g/mol) Given: mass = 27 g Molar mass of Al = 27 g/mol Number of moles = 27 g / 27 g/mol Number of moles = 1 mol Answer: 1 mole of Al is present in 27 g.

Q2: Calculate the mass of 3.2 mol of oxygen atoms. (Atomic mass of O = 16 u)

mass (g) = number of moles × molar mass (g/mol) Given: Number of moles = 3.2 mol Molar mass of O = 16 g/mol mass = 3.2 mol × 16 g/mol mass = 51.2 g Answer: The mass of 3.2 mol of oxygen atoms is 51.2 g.

Q3: Calculate the number of molecules in 11.2 L of CO2 at STP (Molar mass of CO2 = 44 g/mol).

Step 1: Find number of moles using molar volume at STP. At STP, 1 mole of gas = 22.4 L Number of moles = Volume (L) / Molar volume (L/mol) Number of moles = 11.2 L / 22.4 L/mol Number of moles = 0.5 mol Step 2: Find number of molecules. Number of molecules = number of moles × Avogadro's number Number of molecules = 0.5 mol × 6.022 × 10²³ /mol Number of molecules = 3.011 × 10²³ Answer: Number of molecules in 11.2 L of CO2 at STP is 3.011 × 10²³.

Q4: What is the empirical formula of a compound containing 75% C and 25% H by mass? (Atomic mass: C = 12, H = 1)

Step 1: Convert percentage to grams (assume 100 g sample). Mass of C = 75 g Mass of H = 25 g Step 2: Convert to moles. Moles of C = 75 g / 12 g/mol = 6.25 mol Moles of H = 25 g / 1 g/mol = 25 mol Step 3: Find mole ratio (divide by smallest). Mole ratio C : H = 6.25 : 25 Dividing by 6.25: C : H = 1 : 4 Answer: The empirical formula is CH₄.

Q5: Define atomic mass unit (amu). What is the relationship between amu and gram?

Atomic Mass Unit (amu) definition: Atomic mass unit is defined as 1/12th of the mass of a carbon-12 atom. Relationship with gram: 1 amu = 1.66054 × 10⁻²⁴ g Or, in reverse: 1 g = 6.02214 × 10²³ amu Note: This relationship shows why Avogadro's number (6.02214 × 10²³) is defined as it is — it converts between atomic mass units and grams.

Q6: A compound has the molecular formula C₂H₆O. What are the possible structures? (This relates to isomerism.)

Molecular formula: C₂H₆O Molecular mass = (2 × 12) + (6 × 1) + (1 × 16) = 24 + 6 + 16 = 46 u Possible structures (isomers): 1. Ethanol (CH₃CH₂OH) Structure: H₃C-CH₂-OH Functional group: Alcohol 2. Dimethyl ether (CH₃OCH₃) Structure: H₃C-O-CH₃ Functional group: Ether Note: Both have the same molecular formula but different structures, properties, and functional groups. This phenomenon is called structural isomerism.

Showing 6 of 8 questions. Visit the full page for complete solutions.