chem
posted by hannah .
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?

Yes, although if your prof is a stickler for significant figures, you can't get away with that. 0.4 has only one s.f.; therefore, you are allowed only one in the answer so you round that 6.845E6 to 7E6 formula units for the answer.

I used that answer and it was wrong.
we have a practice version : the question was
A salt crystal has a mass of 0.18 mg. How many NaCl formula units does it contain?
and the answer was 1.9e+18 formula units. so wouldn't the answer for my question be in the positive exponents? 
Yes, it should and I goofed. I guess I just don't know what a formula unit is. I work in moles and molecules. Apparently this problem is asking for the number of molecules. Here is how you do the practice problem, followed by the other problem.
0.18 mg = 0.00018 grams but in one step we get, 0.18 mg x (1 g/1000 mg) x (1 mole/58.44 g) x (6.02 x 10^23 molecules/mole) = 1.85 x 10^18 molecules (formula units) which I would round to 1.8E18 molecules but your prof rounded to 1.9E18. My rule on rounding is to round to the even number but follow your prof's rules.
The other problem is done exactly the same way.
0.4mg*(1 g/1000 mg) x (1 mole/58.44 g)x (6.02 x 10^23 molecules/mole) = ??