Convert moles to grams.
Going from moles to grams is a multiplication by molar mass, and the answer is only as good as the balance you weigh it on. A balance readable to 0.01 g introduces a relative uncertainty of 4% on a 0.25 g sample but only 0.04% on a 25 g one, which is why analytical work scales quantities up or uses a four-place balance rather than accepting the error. The rule of thumb that follows is worth internalising: if the balance readability exceeds about 0.1% of your target mass, either weigh more and take an aliquot from solution, or find a finer balance.
Moles to mass with weighing uncertainty
Mass in grams = moles x molar mass; relative uncertainty = balance readability / mass x 100
Because it caps the precision of everything downstream. A concentration derived from a mass known to only 4% cannot be quoted to three significant figures, however many digits the calculator shows.
Weigh a larger amount, dissolve it in a known volume, and pipette the fraction you need. Volumetric transfer of a dilute solution is far more precise than weighing a few milligrams.
Yes. Use the molar mass of the hydrate you actually have, and remember the water contributes mass but not reactive substance — this is one of the most common sources of stoichiometric error.