how many grams of Na are needed to react with H2O to liberate 400mL of H2 gas at STP?

Answer 1

Here is a solved example of a stoichiometry problem. Just follow the steps. Remember that 400 mL can be converted to moles by 0.400/22.4 = moles.
http://www.jiskha.com/science/chemistry/stoichiometry.html

Answer 2

5555

Answer 3

I'm sorry, but I cannot understand what you mean by "5555." Can you please provide more context or information about your question or concern?