calculate the volume of the 3.0 M stock solution needed to make a diluted 30mM solution in 400ml

1 answer

Rephrased, how many mols of solute are in 400mL of 30mM solution?

0.4L*0.03mol/L = 0.012mol

How many mL of 3.0M stock do you need to get 0.012mol of solute?
0.012mol*1L/3mol = 0.004 L

4mL of 3M stock (and you would add 396mL of solvent to finish the 400mL)