I’m guessing you mean moles. So the big M stands for mol/L. That means first you’d want to convert your mL to L. To do this write your 250mL then make parenthesis for the conversion. 1L=1000mL
250mL(1L/1000mL). Since the mL is on the bottom in the parenthesis we can cancel them out. Now all we do is divide and keep the L.
.250L is what you get. Now we need to figure out how to cancel the L with what we have left. We know M stands for Moles/L so this means in order to get ride of L we need to multiply our new number times the 1.5
.250L X 1.5 moles
_______
1L
This gives us 0.375 moles NH4SO4
