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[Claire_95]

Okay, so there are 1.49 moles of Mg(NO3)2. There are 1.229*10E19 atoms of N in an unknown sample of Mg(NO3)2. How many miligrams will the sample weigh? I've tried everything I can think of, and I'm still stuck!

[Arkcon]

OK, you have 3 different units in this question:  moles, number of atoms, and grams.  Do you know the relationship between the 3 so you can convert?

[Claire_95]

Yes

[Borek]

Numbers given (1.49 moles and 1.229x10¹⁹) are unrelated - they don't describe the same sample. Ignore 1.49, it is there just to confuse you.

[vmelkon]

Sounds like you have 2 samples of Mg(NO3)2, but they are asking for the one with 1.229*10E19 atoms.

Use Avogadro's number to convert # of atoms to moles (because we all like working with small numbers instead of insanely large numbers).

1.229*10E19 atoms / Avogadro = number of moles of Mg(NO3)2

Then convert the # of moles to grams. Find the molar mass of Mg(NO3)2 which a simple search will provide or just calculate it yourself.