Thursday

April 24, 2014

April 24, 2014

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

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?

- chem -
**DrBob222**, Wednesday, November 10, 2010 at 4:48pmYes, 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 6.845E-6 to 7E-6 formula units for the answer.

- chem -
**hannah**, Wednesday, November 10, 2010 at 4:53pmI 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?

- chem -
**DrBob222**, Wednesday, November 10, 2010 at 9:12pmYes, 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) = ??

**Related Questions**

Advice...about chemistry degree Dr.Bob - Based on the question topic posted by ...

history - can some one check my work? i can where is it It's the one with the ...

history - what are the two towns near boston that in 1775 the first battles of ...

CHEM (DrBob222) - I had posted the question to that previous chem question, ...

To drwls - I posted some Physics questions yesterday, but no one got the chance ...

French - I have been on for 3 hours and no one is posting or anything. Can ...

chem-repost - I ddin't know if anybody would look far down to page to help me ...

English - 1. Some said yes and others said no. 2. Some said yes and the others ...

psychology - I posted this question before and you told me to go to answers to ...

chemistry - thanks DrB I got AgCl also Could you check this problem as well? If ...