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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Claire_95 on December 09, 2011, 08:57:11 PM

Title: Help with finding weight of an unknown sample....
Post by: Claire_95 on December 09, 2011, 08:57:11 PM

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!

Title: Re: Help with finding weight of an unknown sample....
Post by: Arkcon on December 09, 2011, 09:08:18 PM

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?

Title: Re: Help with finding weight of an unknown sample....
Post by: Claire_95 on December 09, 2011, 09:10:55 PM

Yes

Title: Re: Help with finding weight of an unknown sample....
Post by: Borek on December 10, 2011, 02:43:31 AM

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.

Title: Re: Help with finding weight of an unknown sample....
Post by: vmelkon on December 10, 2011, 10:18:20 AM

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.