, MOPS is a middle strength acid (pKa = 7.2)
On the other hand, the required concentration of the buffer is 0.4M:
Now, we can calculate the required concentrations of MOPS and its Na-salt:
Now you take 2.40mol*209.3g/mol = 502g of MOPS and 1.92mol/10M = 0.192L of 10M solution of NaOH (react in a 1 to 1 molar relation):
C7H15NO4S + NaOH = C7H14NO4SNa + H2O
After the reaction is complete, you dissolve this mixture up to 6L in a special flask with distilled water.