KVPY Sample Paper KVPY Stream-SX Model Paper-27

A student finds a crystal of NaCl having mass 58.5 mg. He uses some of it to prepare 0.05 M, 10 mL NaCl solution. How many unit cells are remaing in the crystal? \[({{N}_{A}}=6\times {{10}^{23}})\]

A) \[1.5\times {{10}^{20}}\]

B) \[7.5\times {{10}^{19}}\]

C) \[3.5\times {{10}^{20}}\]

D) None of these

Correct Answer: B

Solution :

NaCl left \[=\text{1}{{\text{0}}^{-\,3}}-5\times {{10}^{-}}^{\,4}=5\times {{10}^{-\,4}}\text{mol}\]

No. of unit cells \[=\frac{5}{4}\times {{10}^{-\,4}}\times 6\times {{10}^{23}}=7.5\times {{10}^{19}}\]