Under certain conditions the rate of this reaction is zero order in ammonia with a rate constant of 0.0021Ms1, 2NH3(g)N2(g)+3H2(g) Suppose a 3.0 L. fask is charged under these conditions with 500 . mmol of ammonia. After how much time is there only 250. mmol left? You may assume no other reaction is important. Be sure your answer has a unit symbol, if necessary, and round it to the correct number of significant digits