The needed volume is 0.119 L
To calculate the volume we use the following formula:
We know that the final concentration should be 0.156 molar (or mol/L) and we need to prepare 12.0 L of that solution.
We have a solution of 15.7 mol/L in stock, so how much of that solution must we use?
To do this, we first calculate the amount of mol that we should have in our final solution using the formula above. We obtain:
We need to make a solution with 1.872 mol from the stock solution. We again use the above formula:
This means that if you take 0.119 L from your 15.7m