A) 60
B) 30
C) 120
D) 240
Correct Answer: C
Solution :
Key Idea: Calculate\[\Delta {{T}_{f}}\]for both the solutions (containing urea and A) separately divide the two equations to get the molecular mass of A. Given mass of urea = 0.75g, mass of = 1.5 g, mass of water = 100 g, mol. wt. of urea = 60.\[\Delta {{T}_{f}}\]for urea \[=\frac{1000\times {{K}_{f}}\times mass\text{ }of\text{ }urea}{mass\text{ }of\text{ }water\times molecular\text{ }mass\text{ }of\text{ }urea}\] \[=\frac{1000\times {{K}_{f}}\times 0.75}{60\times 100}\] ?.. (i) \[\Delta {{T}_{f}}\]for 'A' \[=\frac{1000\times {{K}_{f}}\times mass\text{ }of\text{ }A}{mass\text{ }of\text{ }water\times molecular\text{ }mass\text{ }of\text{ }A}\] \[=\frac{1000\times {{K}_{f}}\times 1.5}{100\times molecular\text{ }mass\text{ }of\text{ }A}\] ? (ii) Dividing Eq. (i) by (ii), we get \[=\frac{1000\times {{K}_{f}}\times 0.75\times 100\times molar\text{ }mass\text{ }of\text{ }A}{1000\times {{K}_{f}}\times 1.5\times 60\times 100}\] \[=\frac{0.75\text{ }x\text{ }molecular\text{ }mass\text{ }of\text{ }A}{1.5\times 60}\] Molecular mass of A = 120
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