Unless there is more to this than meets the eye, this appears to be regular stoichiometry,  a little surprised to see it here. For 40 wt% you'd need to add ~ 350mL water.

For 40%wt%, you need = 40g of salt X (100/98)  to 100 ml of dist. water

Aashranth and Samir, Thank you for helping me.  By using Samia's suggestion, I need to add about 583 ml of water to 238g of powder . That is quite different from 350 ml. 

That's because wt% is 100 x wt. of solute/wt. of solution

(not 100 x wt.of solute/wt. of solvent...wt. of solution = wt. of solute + wt. of solvent)

(Ref. :http://www.iun.edu/~cpanhd/C101webnotes/aqueoussolns/wwpcnt.html)

Effective weight of 238g of 98% powder =238 * 0.98 =  233.24g

40 wt % of solution = 100*(233.24g of salt (solute))/(233.24 + wt. of solvent))

So wt. of solvent ~ 350g ~350mL in case of water.

Nearly all measures of solutions have mass/volume of solution in the denominator, except for molality, which has mass of solvent.

40% means ( weight ratio , not more ) ; 40 gm of solute : 100 gm of solvent

thereby , i believed that the mass of solvent must not included the mass of solute, only the mass of solvent 

So 40% wt of solution =(238* 0.98) * (100/40) = 583.1 gm (ml) of water

Dear Samir,

Your calculation is perfect as per your definition of wt%, however this is the first time I have come across this formulation. So far, in chemistry as well as metallurgy, the definition I have found (as linked in previous answer) is wt. of solute/wt. of solution. This is also what the IUPAC definition says:

https://en.wikipedia.org/wiki/Mass_fraction_(chemistry)#cite_note-1

Perhaps you could confirm and add a reference if you find a different formula? It would be a new learning for me as well.

  Dear Aashranth

Your link related to mass fraction , this is differ completely from concentration

its well known that ;

quantity of substances expressed as mg , g , mole , mole fraction which may be as wt% or .....

while concentration expressed as w/w% , w/v%, v/v% , Normality N or Molarity M

Dear Samir,

I believe Miki is looking for  wt%, which would correspond to the weight fraction (converted to % of course). There is a difference between mg/g/mole and mole fraction/wt% - the first units simply deal with quantity while the second units inherently deal with concentration.

Anyhow, the concentration units you mentioned also, eg. Normality, Molarity all are basically quantity (wt/vol/moles) of solute/quantity of solution. Nowhere does quantity of solvent appear, except for Molality, which is not what this question deals with.

The difference is essentially a matter of convention; it is possible that there is some convention where your formula is used and I have not come across it. So it would be nice if you can confirm and put a reference. 

Wt % or molar or ....

This is Wt%.