What is the freezing point of a solution containing $$8.1$$ g HBr in $$100$$ g of water. Assuming the acid to be $$90$$% ionised? (Kf$$ for water $$=$$ $$1.86$$ K kg $$mol-1)

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Infinity Learn · Accepted Answer

∆$$Tf=i$$×Kf×m;         $$HBr1-$$α→ $$H+$$α$$+Br-$$αTotal ions $$=1-$$α$$+$$α$$+$$α$$=1+$$α   ∴                    $$i=1+$$αGiven,          $$Kf=1.86$$ K kg $$mol-1$$ Mass of $$HBr=8.1$$ g  Mass of $$H2O=100$$ g $$($$α$$)=degree$$ of ionisation $$=90$$%$$molality($$ $$m)$$ $$=mass$$ of $$solute/molecular$$ weight of solutemass of $$solvent(in$$ $$kg)m(molality)=8.1/81100/1000$$                    $$i=1+$$α$$=1+90100=1.9$$              ∆$$Tf=i$$×Kf×m                      $$=1.9$$×$$1.86$$×$$8.1$$×$$81100$$×$$1000=3.534oC$$  ∆$$Tf(depression$$ in freezing $$point)$$    $$=$$ freezing point of water $$-$$ freezing point of solution    $$3.534$$ $$=$$ $$0$$ $$-$$ freezing point of solution ∴Freezing point of solution $$=$$ $$-3.534o$$ C.

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