Concentration and Molarity Worked Example Problem

Concentration and Molarity Worked Example Problem Question a) Explain how to prepare 25 liters of a 0.10 M BaCl2 solution, starting with solid BaCl2.b) Specify the volume of the solution in (a) needed to get 0.020 mol of BaCl2. Solution Part a): Molarity is an expression of the moles of solute per liter of solution, which can be written: molarity (M) moles solute / liters solution Solve this equation for moles solute: moles solute molarity Ãâ€" liters solution Enter the values for this problem: moles BaCl2 0.10 mol/liter times 25 litermoles BaCl2 2.5 mol To determine how many grams of BaCl2 are needed, calculate the weight per mole. Look up the atomic masses for the elements in BaCl2 from the Periodic Table. The atomic masses are found to be: Ba 137Cl 35.5 Using these values: 1 mol BaCl2 weighs 137 g 2(35.5 g) 208 g So the mass of BaCl2 in 2.5 mol is: mass of 2.5 moles of BaCl2 2.5 mol Ãâ€" 208 g / 1 molmass of 2.5 moles of BaCl2 520 g To make the solution, weigh out 520 g of BaCl2 and add water to get 25 liters. Part b): Rearrange the equation for molarity to get: liters of solution moles solute / molarity In this case: liters solution moles BaCl2 / molarity BaCl2liters solution 0.020 mol / 0.10 mol/literliters solution 0.20 liter or 200 cm3 Answer Part a). Weigh out 520 g of BaCl2. Stir in sufficient water to give a final volume of 25 liters. Part b). 0.20 liter or 200 cm3