Chemistry molarity question

What's the method behind converting 0.4% of CuSO4 to molarity?

Assuming 0.4% v/v then there are 0.4 mL CuSO4 per 100 mL.
Convert 4 mL to mass using mass = density * volume = 3.603 g/mL * 4 mL
= 14.416 g. 100 mL solution contains 14.416 g CuSO4 so 1 liter contains 144.16 g.
Moles = mass / molar mass = 144.6 g / 159.82 g/mol = 0.9048 moles.
Molarity = moles/liter = 0.9048 M.

Assuming 0.4% m/v then there are 0.4 g CuSO4 per 100 mL = 4 g/L
Molarity = 4 g/L / 159.82 g/mol = 0.0250 M.

Since Molarity is defined as number of moles per liter of solution??? . You could do a bit of lab work and get mass and density and calculate a volume I suppose, but this sounds like a problem out of a book or from a teacher??????????. The percentage can be converted to mass and then to moles, but...

If this is not a lab exercise, you do not appear to have the information you need.

Find out the GMW of CuSO4 MW of Cu is 63.54, S 32.06 O 16 GMW CuSO4 = 63.54 + 32.06 + (16)4 (why did I multiply 16 by 4?) = 159.6
in a 0.4% solution of anything that means there is 4g/1000ml 4 is 0.4% of 1000
so to figure moles do this x/4g = 1/159.6 for every mole of CuSO4 there is 159.6 g of CuSO4
solve for x 159.6x = 4 x = 4/159.6 x = 0.025
It does not matter how much liquid is in the stock solution just as long it is at a given percent.
now to figure how many grams of CuSO4 is a known volumn (let's say 5 ml) do the following
4g/1000ml = x/5mls solve for x

Work it by assuming that if you have a liter of water, then 0.4% by mass is going to be CuSO4. that's the way % usually works. So convert that to moles.

0.4% = .4 / 100 * 1000 = 4 grams.

1 mole of CuSO4 = 63.5 + 32 + 4*16 = 159.5 grams

4 grams // 159.5 grams / mol = 0.0251 mols

Therefore the molarity is 0.0251 moles / 1 Liter.