0.777 g/L K+ solution means 0.777g of K+ present in
1L of solution
                                  Â
1L of solution contains 0.777g of K+
  1000ml of solution contains 0.777g of
K+
510ml of solution contains = 0.777*510/1000Â Â =0.39627
g of K+
                                Â
2*39g of K+ obtain from 138g of k2CO3
                              Â
0.39627G OF k+ obtain from= 138*0.39627/2*39Â Â
= 0.701g of K2CO3