A 0.250 L solution of NH4Cl contains (0.250 L)(0.35 mol/L) = 0.0875 moles of dissolved NH4Cl. Converting this quantity to mass, we get (0.0875 mol NH4Cl)(53.491 g/mol) = 4.7 g NH4Cl (two significant figures).
So, approximately 4.7 g of NH4Cl dissolved in a 0.250 L solution will have a concentration of 0.35 L.