1.

A 42.12% (w/v) solution of NaCl causes precipitation of a certain sol in 10 hours. The coagulating value of NaCl for the sol is[Given : Molar mass : Na = 23.0 g mol–1 ; Cl = 35.5 g mol–1 ](A) 36 mmol L–1 (B) 36 mol L–1 (C) 1440 mol L–1 (D) 1440 mmol L–1

Answer»

Correct option is D) 1440 mmol L–1

Data insufficient.

The minimum concentration of an electrolyte in mmol/L to cause coagulation of a sol in 2 HOURS is coagulating value

Here 42.12% (w/v) 

⇒ 42.12 gram salt in 100ml water
⇒ 421.2 gram salt (NaCl) in 1000ml or 1 lit water
⇒ \(\frac{421.2}{58.5}\)= 7.2 moles of salt
7.2 moles → 10 hours 
x moles → 2 hours

⇒  x=\(\frac{7.2\times \:2}{10}\)=1.44 mol/L 
⇒  x= 1440mmol/L


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