What is ionizable hydrogen and why we say it replaceable hydrogen

There are 6.022 x 10^23 atoms of hg in 1 mole of hg. the number of atoms in 4.5 moles of hg can be found by multiplying 4.5 by 6.022 x 10^23 a. 2.7 x 10^24 b. 27 x 10^23 c. 2.71 x10^24 d. 27.099 x 10^23

The best solution for preventing harm to people and pets from severe hurricanes involves determining and warning residents about what

Can someone me with 3 and 4 plz. this is for masteries test.