In order to make 100mls a 6.5mm solution of magnesium sulfate what do you need to know?

The question is incomplete. Complete question is:
In order to make 100mls a 6.5mM solution of magnesium sulfate what do you need to know? 1)the molarity of water 2) the solvent which you are making it in 3) the molecular weight of magnesium sulfate
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Option 3: the molecular weight of magnesium sulfate

Reason:
Molarity of solution is mathematically expressed as,
Molarity =

From the above equation, it ca be seen that, if molecular weight of MgSO4 is known, weight of solute required to prepared 100ml of 6.5mM solution can be easily estimated. 

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