How many Liters of a 2M solution can be made with 12 moles of NaCl?

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

when [oh-] is 10-5  m, and [h3o+] is 10-9  m, the poh is 5 and ph is 9. again in both cases the sum of ph and poh is 14. however, the hydrogen ion concentration is not always going to be equal to exactly 1 x 10 raised to a negative number. for example, we skipped over the value of 2.0 x 10-7.

yes the answer is total mass in unchanged   : )