Arrange the following solutions in increasing order of their concentration.
(A) 7.5 g KCl (74.5 g \( mol^{-1} \)) in 100 mL solution
(B) 3 g Urea (60 g \( mol^{-1} \)) in 1 L solution
(C) 3.6 g glucose (180 g \( mol^{-1} \)) in 100 mL solution
(D) 4 g NaOH (40 g \( mol^{-1} \)) in 1 L solution
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B), (D), (C), (A)
The concentration of each solution is compared as its molarity, the number of moles of solute per litre of solution:
\( \displaystyle M = \frac{\text{mass of solute}/\text{molar mass}}{\text{volume of solution in L}} \)
(A) KCl: \( \displaystyle \frac{7.5/74.5}{0.1} = \frac{0.1007}{0.1} = 1.01\ \mathrm{mol\,L^{-1}} \)
(B) Urea: \( \displaystyle \frac{3/60}{1} = 0.05\ \mathrm{mol\,L^{-1}} \)
(C) Glucose: \( \displaystyle \frac{3.6/180}{0.1} = \frac{0.02}{0.1} = 0.2\ \mathrm{mol\,L^{-1}} \)
(D) NaOH: \( \displaystyle \frac{4/40}{1} = 0.1\ \mathrm{mol\,L^{-1}} \)
In increasing order of concentration:
\( \displaystyle 0.05\,(\text{B}) \lt 0.1\,(\text{D}) \lt 0.2\,(\text{C}) \lt 1.01\,(\text{A}) \)
The 100 mL volumes in (A) and (C) multiply their moles by 10, which is why they come out on top.
Shortcut: Urea gives the smallest number, 0.05 mol in a whole litre, and only Option (3) starts with (B).
NCERT (2026-27 edition):
Expressing Concentration of Solutions – “… (vi) Molarity: Molarity (M) is defined as number of moles of solute dissolved in one litre (or one cubic decimetre) of solution,”