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    lisanoce's Avatar
    lisanoce Posts: 30, Reputation: 1
    Junior Member
    		  
    #1

    Apr 24, 2009, 10:22 AM
    stoichiometry of redox reactions
    I just need someone to check my work. The question was:

    Solder is an alloy containing the metals tin and lead. A particular sample of this alloy weighing 1.50 g was dissolved in acid. All the tin was converted to the +2 oxidation state. Next, it was found that 0.368 g of Na2Cr2O7 was required to oxidize the Sn^2+ to Sn^4+ in an acidic solution. In the reaction the chromium was reduced to Cr^3+ ion.

    a) Write the balanced net ionic equation for the reaction b/w the Sn^2+ and Cr2O7^2- in an acidic solution.

    My answer:

    9(Sn^2+ --> Sn^4+ + 2e-)
    2(9e- + 14H+ + Cr2O7^2- --> 2Cr^3+ + 7H2O )
    ----------------------------------------------------------
    9Sn^2+ + 18e- + 28H+ + 2Cr2O7^2- --> 9Sn^4+ + 18e- + 4Cr^3+ + 14H2O

    cross out the electrons: 9Sn2+ + 28H+ + 2Cr2O7^2- --> 9Sn^4+ + 4Cr^3+ + 14H2O

    b) Calculate the number of grams of tin that were in the sample of solder.

    Do I have to use the 0.368g of Na2Cr2O7 in order to find the grams of tin? ( using stiochiometry )

    Thanks
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
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    #2

    Apr 24, 2009, 07:24 PM

    Yup, the only way. You cannot use the 1.50g of solder since that includes both the lead and tin. Your only information which hints about tin is the amount of used.

    The 1.50g of solder is perhaps used to find the percentage of tin or lead by mass in the solder. :)
    lisanoce's Avatar
    lisanoce Posts: 30, Reputation: 1
    Junior Member
    		  
    #3

    Apr 25, 2009, 02:30 PM
    Quote Originally Posted by Unknown008 View Post
    Yup, the only way. You cannot use the 1.50g of solder since that includes both the lead and tin. Your only information which hints about tin is the amount of used.

    The 1.50g of solder is perhaps used to find the percentage of tin or lead by mass in the solder. :)

    Thanks a lot for helping : )
    Unknown008's Avatar
    Unknown008 Posts: 8,076, Reputation: 723
    Uber Member
    		  
    #4

    Apr 25, 2009, 07:40 PM

    Glad to help! Ask any time! :)
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