Please make sure to write your final answers in your own words.
49. Polarity is based off the electronegativity differences between the two atoms involved in the bond. Then trend for electronegativity is that it increases across a period and down a group. So since Cl is more electronegative than S, when they are bonded to the same atom (P in this case), the P - Cl bond will have a greater difference in the electronegativity then the P - S bond. This means that PCl5 will be more polar than P2S5 because of the greater differences in electronegativity.
50. The first thing you are going to need to solve this problem is the molar mass of CH3Br. This is found by adding together the atomic masses of each atom.
Molar mass of CH3Br = 12.0107 + 3*1.00794 + 79.904 = 94.93852 g / mol
Now divide the mass by the molar mass of CH3Br to find the number of moles.
19 g CH3Br / (94.93852 g / mol) = 0.200 mol CH3Br
I hope that this makes sense to you. If you have additional questions about these problems please let me know.