The average human exhales about 900g of carbon dioxide per day. how much lithium hydroxide is needed to remove this amount each day? how much of each of the products is produced?

i believe you are right for all except the 3rd ( look at sqdancefan his answer is much better)

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.