This process involves the dilution of the 12 molar HCl. To reduce the concentration, we need to set up an equality so that we know how much of the 12M we need to make the 3.5M.
12 moles HCl 3.5 moles HCl
——————— = ———————
1 Liter of Soln ‘x’ Liters of Soln
Notice that the 12 moles over the 1 liter is equal to 12 molar; in doing this, we’re maintaining the concentration of the initial HCl. By setting it equal to the 3.5 over ‘x’, we’re still maintaining the concentration.
After solving, we find that ‘x’ equals 0.292. This value means that in 0.292 liters of our 12 M HCl solution, there are 3.5 moles of HCl. But, we’re not done yet.
0.292 liters of 12 M HCl can make 1 liter of 3.5 M HCl, but the question asks for 1.5 liters. To get this, multiply 0.292 liters by 1.5, and the new result, 0.4375, represents the amount of 12 M HCl required to prepare a 1500 mL 3.5 M HCl solution.
