Answer :
Hey there!
Given the reaction:
P4 + 10 Cl2 ------------------ 4 PCl5
Molar mass P4 = 124 g/mol
Number of moles P4:
n = mass of solute / molar mass
n = 24.0 / 124
n = 0.1935 moles of P4
Therefore:
1 mole P4 --------------- 4 moles PCl5
0.1935 moles P4 ------- moles PCl5
moles PCl5 = 0.1935 * 4
= 0.774 moles of PCl5
Hope that helps!