Using "goro" would mean "approximately" and "ni" would confirm that it's on five. Go ji goro benkyoushimasu （５時頃勉強します。）＝I will study around five. Go ji ni benkyoushimasu (５時に勉強します。）= I will study at five. But in this sense, it would be more natural to say "Go ji kara benkyoushimasu （５時から勉強します。）" = I will study from five.