Please Help with 2 Gen Chem questions

[Holden21]

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I have 2 gen chem questions - I can't get the same answer that was given as correct answer. Can somebody please show me how to do these 2? I will be eternally in your debt.

1.When the following oxidation-reduction reaction is balanced, what is the correct stoichiometric coefficient for Sn+2?

Sn2+ + H+ +Cr2O72- > Sn4+ + Cr3+ + H2O

(the answer is supposed to be 3)

2. If a solution which is initially 1.00M in compound X undergoes a decomposition reaction for 20.0 sec. @ average rate of 0.020 mol/l.sec, the new concentration of X will be:
(the answer is supposed to be .60M)
Many thanks.

 

[justsusan]

split up the equation...

Sn+2 -> Sn+4

Cr2O72- > Cr3+

balance both..

Sn+2 -> Sn+4 + 2e-

(Cr2O7)2- +14H+ +6e- -> 2Cr3+ +7H20

to add the two equations, you need the number of electrons to be equal so you'll multiply the first equation by 3.

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if it decomposed for 20 s at a rate of 0.02 mol/l.sec, the decomposition is 20 x 0.02 mol/l

final concentration = 1.0 -0.4 = 0.6

 

[Holden21]

Thank you so much.