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Question 1: A gaseous hydrocarbon gives upon combustion 0.72 g of water and 3.08 g of CO2. The empirical formula of the hydrocarbon is:    (IIT JEE -2013)
(a) C3H4
(b) C6H5
(c) C7H8
(d) C2H4
Answer: c
Solution:
General equation for combustion of hydrocarbon:
CxHy + (x+ y/4)O2 → xCO2 + (y/2)H2O
Number of moles of CO2 produced = 3.08/44 = 0.07
Number of moles of H2O produced = 0.72/18 = 0.04
SO, x / (y/2) = 0.07/0.04 =  7/4
The formula of hydrocarbon is C7H8
Hence, the correct option is c.

Question 2: 29.2 % (w/w) HCl stock solution has density of 1.25 g mL1. The molecular weight of HCl is 36.5 g mol-1. The volume (mL) of stock solution required to prepare a 200 mL solution of 0.4 M HCl is    (IIT JEE 2012)
Solution:
Let mass of the stock solution = 100g
Mass of HCl in 100 g of 29.2 % (w/w) HCl stock solution = 29.2 g
Volume of the stock solution = 100g/1.25 g mL1 = 80 g
Number of moles of HCl in stock solution = 29.2/36.5 = 0.8
Molarity of the stock solution = (0.82/80)×1000 = 10 M
Using,
M1V= M2V2
10× V1 =0.4×200
or
V1 = 8mL
Hence, the volume required is 8 mL.

Question 3: Dissolving 120 g of urea (mol. wt. 60) in 1000 g of water gave a solution of density 1.15 g/mL. The molarity of the solution is    (IIT JEE-2011)
(a) 1.78 M
(b) 2.00 M
(c) 2.05 M
(d) 2.22 M
Answer: c
Solution:
For calculating molarity of the solution we require to know two things, (i) number of moles of urea & (ii) total volume of the solution.
Number of moles of urea dissolved in the solution = 120 g/60g = 2.
Total mass of the solution = 1000g + 120 g = 1120 g
Volume of the total solution = 1120g/1.15 gmL-1 = 974 mL = 0.974 L
Molarity = 2/0.974 = 2.05 M
Hence, the correct option is c.

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FAQs on Solved Examples: Some Basic Concepts in Chemistry - NCERT Exemplar & Revision Notes for NEET

1. What are some basic concepts in chemistry?
Ans. Some basic concepts in chemistry include understanding the structure of atoms, elements, compounds, chemical reactions, stoichiometry, and balancing equations.
2. How can I calculate the molar mass of a compound?
Ans. To calculate the molar mass of a compound, you need to determine the atomic mass of each element present in the compound and multiply it by the number of atoms of that element. Finally, sum up the masses of all the elements to obtain the molar mass.
3. What is the difference between an element and a compound?
Ans. An element is a pure substance composed of only one type of atom, such as oxygen or gold. On the other hand, a compound is a substance composed of two or more different elements chemically bonded together, such as water (H2O) or carbon dioxide (CO2).
4. How do chemical reactions occur?
Ans. Chemical reactions occur when bonds between atoms in reactant molecules break and new bonds form to create product molecules. This process involves the rearrangement of atoms to form different substances.
5. How do I balance a chemical equation?
Ans. To balance a chemical equation, you need to ensure that the number of atoms of each element is the same on both sides of the equation. You can achieve this by adjusting the coefficients in front of each compound or molecule in the equation.
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