JEE Main 2022 · 29 June · evening · Single correct · +4 / −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 in 2 hours is :
[Given : Molar mass : Na = 23.0 g mol⁻¹ ; Cl = 35.5 g mol⁻¹]
- (a) 36 m mol L⁻¹
- (b) 36 mol L⁻¹
- (c) 1440 mol L⁻¹
- (d) 1440 m mol L⁻¹
Answer and solution
Answer (d) 1440 m mol L⁻¹
Solution
Chemistry ma’am · 4 steps
Turn the percentage into millimoles per litre first, then scale the 10-hour amount down to 2 hours.
- What is asked. The coagulating value is the amount of electrolyte, in millimoles per litre, needed to coagulate the sol in 2 hours. So the answer has to be in mmol L⁻¹ and has to refer to 2 hours.
- Convert to molarity. 42.12% (w/v) means 42.12 g of NaCl in 100 mL, which is 421.2 g in 1 L. The molar mass of NaCl is 23.0 + 35.5 = 58.5 g mol⁻¹.
(421.2 g L⁻¹)/(58.5 g mol⁻¹) = 7.2 mol L⁻¹ = 7200 mmol L⁻¹
- Scale to 2 hours. Take the electrolyte as acting at a steady rate, so the millimoles used grow in proportion to time. 7200 mmol per litre over 10 hours is 720 mmol per litre each hour, so 2 hours needs 720 × 2 = 1440 mmol L⁻¹.
- So. The coagulating value of NaCl for this sol is 1440 mmol L⁻¹, option (d).
The idea to keep. Coagulating value always means millimoles per litre for coagulation in 2 hours. Convert w/v to mol per litre, multiply by 1000, then scale the amount to 2 hours.
Answer from the NTA final answer key. Solution by PrepWiser.