To prepare 250 cm^3 of 2mol/dm^3, you need to find the mass of the substance being used. This is calculated by using the formula for concentration = mass of the substance/ volume of solution. Therefore, 2mol/dm^3 = mass / 250 cm^3. Change the volume to dm^3 to get 1 centimeter^3 = 0.001 decimeters^3, 250 * 0.001 = 0.25dm^3. Mass = 0.25 * 2 = 0.5. Find out more from http://www.ehow.com/how_5700768_prepare-potassium-chloride.html.