Question

show with calculations that average atomic mass of chlorine is 35.5 u

Answer 1

It's due to isotopes of chlorine
calculation:
There are two principal Chlorine isotopes. About 75% of Chlorine atoms have a relative atomic mass of 35 and 25% have a relative atomic mass of 37.

Hence in a sample of Chlorine, the average relative atomic mass would be:
(75%×35)+(25%×37)=35.5