consider the reaction P4 + F2 gives PF3 . what mass of F2 is needed to produce 176 gram of PF3 if the reaction has 80% yield

Dear student,

  As per the reaction : P₄  +6F₂ $\stackrel{}{\to }$4 PF₃
   As per the stochiometry of reaction ; 6 moles of F₂ gives 4 moles of PF₃
    Now the molar mass  of PF₃ = 88 gm /mol and the molar mass of F₂ = 38 gm /mol
    Again 4 x 88 gm of PF₃ is formed by 6 x 38 gm of F₂
 Then 176 gm of PF₃ is formed by $\frac{6\times 38}{4\times 88} \times 176$  =  114 gm 
The above amount is for the 100 % yield 
Now for the 80 % yield the amount is  = 114 x 0.8 = 91.2 gm
  Regards