The following solutions were prepared by dissolving 1 g of solute in 1 L of the solution. Arrange the following solutions in decreasing order of their molarity
(A) Glucose (molar mass = \( 180\, g\, mol^{-1} \))
(B) NaOH (molar mass = \( 40\, g\, mol^{-1} \))
(C) NaCl (molar mass = \( 58.5\, g\, mol^{-1} \))
(D) KCl (molar mass = \( 74.5\, g\, mol^{-1} \))
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → (B), (C), (D), (A)
Molarity is moles of solute per litre of solution. Here every solution has 1 g of solute in 1 L, so the molarity is \( 1/M \): the smaller the molar mass, the larger the molarity.
\( \displaystyle \text{(B) NaOH: } \frac{1}{40} = 0.025\ \mathrm{M} \)
\( \displaystyle \text{(C) NaCl: } \frac{1}{58.5} = 0.017\ \mathrm{M} \)
\( \displaystyle \text{(D) KCl: } \frac{1}{74.5} = 0.013\ \mathrm{M} \)
\( \displaystyle \text{(A) Glucose: } \frac{1}{180} = 0.0056\ \mathrm{M} \)
Shortcut: No calculation is needed: arrange the molar masses in increasing order, 40 < 58.5 < 74.5 < 180, and that is the decreasing order of molarity.