Q1 / 97 · NTA Abhyas (JEE Main) 2020 · Chemistry · Solutions

The freezing point of a 3% aqueous solution of 'A' is equal to the freezing point of 9% aqueous solution 'B'. If the molecular weight of 'A' is 60, then the molecular weight of 'B' will be

  • Correct answer
    A
    191.9
  • B
    90
  • C
    45
  • D
    20

Solution

(T f ) A  =  (T f ) B
⇒ (ΔT f ) A  =  (ΔT f ) B
⇒i ×  K f  ×  m A  = i ×  K f  ×  m B
⇒ m A =  m B
⇒ w A M A  ×  1000 w solvent (g)  =  w B M B  ×  1000 w solvent (g)
⇒ 3 60  ×  1000 97  =  9 M B  ×  1000 91
⇒ M B = 9×60×97 3×91
∴ M B =191.9 g/mol

Hence, the correct answer is option: 'A'.

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