[my first name is jack]

On a test if you are only allowed to use periodic table and calculator how are you able to answer a question like this and what are the steps to answering a question like this...

Lanthanum is composed of two isotopes: La-138 and La-139. What is the percentage abundance of both isotopes?




THANKS TO ALL WHO HELP =D

[ARGOS++]

Dear my first name is jack;

This is a typically problem of two equations with two unknown.

I give you the first equation:      x%   +  y%  =   100 %
Are you able to give the second equation using the periodic table?

Good Luck!
                    ARGOS⁺⁺

[my first name is jack]

how would i be able to give the second equation using the periodic table???
sorry i actually dont understand this at all

[ARGOS++]

Dear my first name is jack;

Think about that:   x% ¹³⁸La   +   y% ¹³⁹La  must give 100%( = 1.0)
With the periodic table you can do similar:
         x%  * ra(_¹³⁸La)    +     y% * ra(_¹³⁹La)    =   138.9055 amu

And the relative atom masses (ra) are:
      ¹³⁸La  =  137.9071 amu        ¹³⁹La  =  138.9063 amu

Can you now solve the equation system?

Good Luck!
                    ARGOS⁺⁺

[my first name is jack]

On my periodic table it only says La 138.91 so I don't understand how you got...


And the relative atom masses (ra) are:
      138La  =  137.9071 amu        139La  =  138.9063 amu



thanks ARGOS i appreciate your help a lot.

[ARGOS++]

Dear my first name is jack;

You may check on:
              http://physics.nist.gov/cgi-bin/Compositions/stand_alone.pl

Good Luck!
                    ARGOS⁺⁺

[my first name is jack]

oh okay but on the test we wont be able to use that information so is there any other way of solving this problem?

[ARGOS++]

Dear my first name is jack;

Then you have to do it less precise with only the assumption from the question:
The relative atom masses (ra):
      ¹³⁸La  =  138.0 amu        ¹³⁹La  =  139.0 amu


Good Luck!
                    ARGOS⁺⁺