Making a solid into a stock solution?

Dilution/Stock Solution — Water Needed?

• Okay so I have a 1.0 mol/L stock solution of sucrose in 250 mL. I discovered that I need 85.5 g of sucrose. Now, using the aforementioned stock solution, I need to create 500 mL of a 0.2 mol/L sucrose solution. Can anyone help me with this last part? I need to figure out: a) how much stock solution I need using the original 1.0 mol/L sucrose solution to create 500 mL of a 0.2 mol/L and b) the amount of water required. Please explain answers! Thanks in advance.

• Answer:

  Problems like this are easy if you keep track of you units. First put together all the info you have about each solution. For the 0.2 molar solution you know the volume needed (0.5 L). Therefore, you can convert that 0.5 L to moles by doing 0.5 L x 0.2 mol/L to get 0.1 mol. Now you know you need 0.1 mol of sucrose in your final solution. You are using a solution that is 1.0 molar so do 0.1 mol x L/1.0mol to get 0.1 L of solution. In other words, you need 100 mL of stock solution. You could also just say that your desired solution is 1/5 the stock concentration, so you will use 1/5 the volume. If you are taking 100mL of stock solution and are making 500mL you need to add 400mL of water.