Дано
m((HCOО)2Mg = 200 g
W(примес)=15%
-------------------------
m(HCOOH)-?
m чистого(HCOO)2Mg) = 200 - ( 200*15%/100%) = 170 g
170 X
(HCOO)2Mg+H2O-->2HCOOH+MgO
114 2*46
M((HCOO)2Mg= 114 g/mol , M(HCOOH) = 46 g/mol
X = 170*92 / 114 = 137.2 g
ответ 137.2 г