how to make 400 ml of 5% w/w h2so4

Answer 1

5% w/w H2SO4 means 5 g H2SO4/100 g solution.
So you want 5g H2SO4 and 95 g H2O BUT be sure you add the H2SO4 to the H2O and not the other way around. That gives you 100 g solution. To make 400 mL just multiply everything by 4.