February 1, 2015

Homework Help: chem

Posted by hannah on Wednesday, November 10, 2010 at 4:18pm.

so i posted this question yesturday and got some help. which is the stuff below. can you look at the last thing i said which was the answer i got by doing what DrBob222 said to do and tell me if its correct.(the question is the first section) thanks

chem - hannah,

A salt crystal has a mass of 0.40 mg. How many NaCl formula units does it contain?

chem - DrBob222,

moles = grams/molar mass
Substitute "number formula units" for moles. Substitute "mass formula unit" for molar mass. Solve for moles.
0.04/about 58.5 or so. You do it more precisely.

chem - hannah,

so do i have to convert .4 mg to grams first? that would be .0004 right?

so then i just divide .004/58.44 and that will be my answer?

chem - DrBob222,

Yes, and I notice I didn't see the decimal point.
0.4 mg = 0.0004 grams and that divided by 58.44 = ?? formula units.

chem - hannah,

so it would be .000006845 formula units?

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