The answer is
I've done a little research and discovered that, for mannitol (a sugar alcohol derived from the sugar manose by reduction) solutions, the concentration is usually w/v.
A w/v solution is used when a substance is mixed as a dry mass (grams) per mL (volume):
So, a 20% w/v solution would have 20.0 g of mannitol for every 100 mL of solution -> 20g/100mL . Therefore, the volume needed for 5.0 g of mannitol is