Solved Examples: Some Basic Concepts in Chemistry | NCERT Exemplar & Revision Notes for NEET PDF Download

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:
C_xH_y + (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,
M₁V₁ = 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.