Concentration in terms of M (molarity) = moles solute/volume of solution.
M is usually given in units of mol/L.
To calculate volume, divide the number of moles by the concentration.
M = n/V so V = n/M
Ex. What is the volume of 2 M NaOH needed to react with 3 moles of H+ ions?
V = n/M = 3 moles / 2 mol/L = 1.5 L
To convert L to mL, multiply by 1000.
Ex. 1.5 L = 1500 mL
Don't forget to use the stoichiometric coefficients (mole ratio) of the balanced reaction when converted between chemical species!
(the ratio above is 1:1 since 1 OH- reacts with 1 H+)
Hope this helps.
Finola