Molarity = moles of solute/ volume of solution. Moles= weight of substance / molecular weight of that substance. Therefore, 0.051= moles of K2SO4 / volume of solution. 0.051= weight of K2SO4 /molecular weight of K2SO4 x volume of solution. Therefore, weight of K2SO4 =0.051 x 174 / 4 So weight = 2.2185 gm