If you have 250 mL of 0.75 M LINO2.
How many grams of this substance
would you have?

Mass of this substance = 9.928 g

Further explanation

Molarity is a way to express the concentration of the solution

Molarity shows the number of moles of solute in every 1 liter of solute or mmol in each ml of solution

Where

M = Molarity

n = Number of moles of solute

V = Volume of solution

So to find the number of moles can be expressed as

mol of substance -Lithium nitrite - LiNO₂ :

V = 250 ml = 0.25 L

M = 0.75 M

So mass of this substance -  LiNO₂ (MW=52,947 g/mol) :

I think the ratio of copper to iron will still be the same even if you add more nails in the beaker. This is because each nail placed is assumed to have the same composition with the other nails and is assumed to be homogeneous.