A commercial sample (2.0189 gram) of NaOH containing Na2CO3 as impurity was dissolved in water to make 250ml solution . A 10 ml portion of this solution required 20 ml of 0.1 N H2SO4 for complete neutralization. Calculate the percentage by weight of Na2CO3 in the sample.

it’s previous models   because when i picked that son of a gun’s answer it was wrong look for previous models because they might make it a different letter  

it is the minimum amount of material needed to sustain a fission reaction.

critical mass is the minimum amount of fissionable material that must be assembled for a sustained fission reaction to take place. although two to three neutrons are produced for every fission, not all of these neutrons are available for continuing the fission reaction. if the conditions are such that the neutrons are lost at a faster rate than they are formed by fission, the chain reaction will not be self-sustaining.   at the point where the chain reaction can become self-sustaining, this is referred to as critical mass.