A solution of 4.00 ml of #sf(Sn^(2+))# of unknown concentration is titrated against an acidified solution of 0.495M #sf(Cr_2O_7^(2-)#. The mean titre is 4.94 ml. What is the concentration of the #sf(Sn^(2+))# solution ?

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Answer 1 · 2016-12-17T18:07:17

#sf(1.83color(white)(x)"mol/l")#

Start with the two 1/2 equations:

#sf(Sn^(2+)rarrSn^(4+)+2e" "color(red)((1)))#

#sf(Cr_2O_7^(2-)+14H^(+)+6erarr2Cr^(3+)+7H_2O" "color(red)((2)))#

To get the electrons to balance we multiply #sf(color(red)((1))# by 3 and add to #sf(color(red)((2))rArr)#

#sf(Cr_2O_7^(2-)+14H^(+)+cancel(6e)+3Sn^(2+)rarr2Cr^(3+)+7H_2O+3Sn^(4+)+cancel(6e))#

This tells us that 1 mole of Cr(VI) reacts with 3 moles of Sn(II).

#sf(n_(Cr(VI))=cxxv=0.495xx4.94/1000=2.4453xx10^(-3))#

#:.##sf(n_(Sn(II))=2.4453xx10^(-3)xx3=7.3359xx10^(-3))#

#sf(c=n/v)#

#:.##sf([Sn^(2+)]=(7.3359xxcancel(10^(-3)))/(4.00/cancel(1000))=1.83color(white)(x)"mol/l")#