FeBr₃ ⇒ limiting reactant
mol NaBr = 1.428
Reaction
2FeBr₃ + 3Na₂S → Fe₂S₃ + 6NaBr
Limiting reactant⇒ smaller ratio (mol divide by coefficient reaction)
211 g of Iron (III) bromide(MW=295,56 g/mol), so mol FeBr₃ :
[tex]\tt n=\dfrac{mass}{MW}\\\\n=\dfrac{211}{295,56}\\\\n=0.714[/tex]
186 g of Sodium sulfide(MW=78,0452 g/mol), so mol Na₂S :
[tex]\tt n=\dfrac{186}{78.0452}=2.38[/tex]
Coefficient ratio from the equation FeBr₃ : Na₂S = 2 : 3, so mol ratio :
[tex]\tt FeBr_3\div Na_2S=\dfrac{0.714}{2}\div \dfrac{2.38}{3}=0.357\div 0.793[/tex]
So FeBr₃ as a limiting reactant(smaller ratio)
mol NaBr based on limiting reactant (FeBr₃) :
[tex]\tt \dfrac{6}{3}\times 0.714=1.428[/tex]