Number of ni atoms in 3.4 mol of ni

1 mol of a substance = 6.022 x 10^23 of the substance 

an analogy would be: 

1 dozen of things = 12 things 

6.022 x 10^23 is thus the NUMBER of the things, not any sort of measurements. 

Thus for the first question, 
1 mol of nickel atoms = 6.022 x 10^23 nickle atoms 
2.5 mols of He atoms = (2.5)(6.022 x 10^23) He atoms.. and so on. 

And very helpfully, if you look at your periodic table, each element has the average atomic mass (which is measured in AMU, or atomic mass unit. Amu is like grams, pounds, or any other unit, but a special unit for such small mass of an atom, molecules, etc) 

For example, Oxygen (atomic number 8) has approximately 16 amu. The good thing about this is that one ATOM of oxygen will have the mass of 16 amu. One MOLE of oxygen will have 16 GRAMS. 

Thus, you can figure out how much the mass is by using conversion factors. 

1 mol of nickel atoms.. for example, would be 58.693 g. 
for 5 moles of water.. 
water = H2O = approximately atomic mass is 18 amu 
So 5 moles H2O = 5x 18 = 90 g 

Hope i helped