Question

6.02 × 10²⁰ molecules of urea are present in 100 ml of its
solution. The concentration of urea solution is
(a) 0.02 M (b) 0.01 M (c) 0.001 M (d) 0.1 M
(Avogadro constant, NA = 6.02 × 10²³ mol⁻¹)

Answer 1

The concentration of urea solution is 0.01 M

Explanation:

Molarity of a solution is defined as the number of moles of solute present per one liter of the solution.

• $Molarity =\frac {Number\, of \,moles}{Volume \,of\, the\, solution(in\,ltr)}$
• From the given values,
• Number of urea molecules = $6.02\times 10^{20}$
• Number of moles = $\frac{Number \,of\,Molecules }{Avogadro \, number }$
• $=\frac{6.02\times 10^{20}}{6.02\times 10^{23}} = 10^{-3} moles$
• Volume of solution = 100 ml = 100/1000 L = 0.1 L
• $\therefore Molarity =\frac{Number\,of\,moles}{Volume\,of\,the\,solution}$  
• $=\frac{10^{-3}}{0.1} =\frac{0.001}{0.1} =0.01 M$