If the solute is properly distributed in the given volume, there are 2.642 g of (NH4)2SO4 per 10 mL. For the new solution, divide the 2.642 g by the molar mass of the compound. The answer is 0.02 moles. Then, divide this by the new volume, 50 mL or 0.05 L. The concentration of the new solution is 0.4 M.