Răspuns:
Na2S sulfura de sodiu
r.a. Na: S=2:1
r.m Na:S=46:32=23:16
MNa2S=2×23+32=78g/mol
78g Na2S......46gNa.....32gS
100gNa2S......x%Na........y%S
x=100×46/78=calculează
y=32×100/78=calculează
78g Na2S.........32gS
780gNa2S........xgS
x=780×32/78=320gS
MS=32g/mol
n=m/M320/32=10moli S.
