Find concentration (units M) by dividing:
Number of moles / volume
Volume must be in liters.
Looking at your first example:
0.00200 mol of sodium phosphate in 10.0 mL of solution
You are given the number of moles = 0.00200.
You are given the volume = 10.0 ml. Convert ml to liters by dividing by 1000.
Volume = 0.0100 l.
Concentration = moles / volume
= 0.002 / 0.01
= 0.200 M
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