"her students to put two spoonfuls of seawater into their containers"
 We have then:
 (2) * (5) = 10 mililiter
 "he gives each of her 22 students a container"
 Thus,
 (22) * (10) = 220 mililiter
 Therefore, they will remain:
 (0.4 * 1000) - 220 = 400 - 220 = 180 milliliter
 Answer:
 There will be left 180 mililiter of seawater after all 22 students have filled their containers