Molarity = moles / Litres of solution. Work out how many moles of each reagent you add to the initial solution. Then use the total volume of the initial solution to determine the molarity. Use this equation to solve. Solution Molarity = moles / Litres of solution. Work out how many moles of each reagent you add to the initial solution. Then use the total volume of the initial solution to determine the molarity. Use this equation to solve..