Let $S_n$ denote the symmetric group on $n$ letters, and let $S_n(p)$ denote a Sylow $p$subgroup. Why is the image of $H_i(S_n(p))$ in $H_i(S_n)$ the $p$primary part of $H_i(S_n)$?
That is the content of the first part of theorem 10.1 in CartanEilenberg (up to the fact that their $\hat H^i$ is the same as $H_{i1}$ for $i<1$). In particular, this is true for all finite groups, not just the symmetric ones. 

