Atomic weights: -
Pb = 207,2
Sb = 121,76
O = 16
Let the mass of Sb be x grams
Then mass of Pb is (1,34 - x ) grams.
In Sb2O4 the ratio of O to Sb is the same as in SbO2
Therefore 121,76 g Sb combines with 32 g O2 and
207,2 grams of Pb combines with 32 grams of O2
So oxygen that reacts with Sb = 32x / 121,76
and O2 that combines with Pb = 32/207,2 X (1,34 - x)
Total combined O2 = 1,558 -1,340 = 0,218 g
Therefore: -
32x/121,76 + (1,34 -x) X 32/207,2 = 0,218
Simplify 32X207,2x + 5221,0688 - 3896,32x = 5499,85
6630,4x + 5221,0688 - 3896,32x = 5499,85
2734,08 x = 278,781
x = 0,10197 grams
Sb = 0,102 g
Pb = 1,238 g
Therefore Sb = 7,61%