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Chemistry Question on Solutions

A 5.25%5 .25\% solution of a substance is isotonic with a 1.5%1.5\% solution of urea (molar mass = 60 g mol−1)60 \,g \,mol^{-1})in the same solvent. If the densities of both the solution are assumed to be equal to 1.0 g cm−31.0 \,g \,cm^{-3} molar mass of the substance will be

A

210.0 g mol−1210.0 \,g \,mol^{-1}

B

90.0 g mol−190.0 \,g \,mol^{-1}

C

115.0 g mol−1115.0\, g \,mol^{-1}

D

105.0 g mol−1105.0\,g\,mol^{-1}

Answer

210.0 g mol−1210.0 \,g \,mol^{-1}

Explanation

Solution

Isotonic solutions have the same molar concentration. Hence, Molar cone. of the substance=Molar cone of urea 52.5g L−1M=15 g L−160g mol−1\frac{52.5 g \, L^{-1}}{M} = \frac{15 \, g \, L^{-1}}{60 g\, mol^{-1}} (100 g solution = 100 mL as d = 1 g mol−1mol^{-1}) M = 52.5×6015\frac{52.5 \times 60}{15} = 210 g mol−1^{-1}