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?

chem 
DrBob222
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.

chem 
hannah
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? 
chem 
DrBob222
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) = ??
Respond to this Question
Similar Questions

CHEM (DrBob222)
I had posted the question to that previous chem question, regarding Mo & V...they said that it should have been correct. What I am thinking that I was missing...must have been that I did not include 4s & then just not fill it. I had … 
Algebra
I have a few problems I need you to take a look at and tell me if I arrived at the right answer. 1. I am to use the intercepts to graph the equation x+3y=6 I used the formula y=mx+b using 3 and 6 for x I got these as my ordered pairs … 
Algebra/math
I have a few problems I need you to take a look at and tell me if I arrived at the right answer. 1. I am to use the intercepts to graph the equation x+3y=6 I used the formula y=mx+b using 3 and 6 for x I got these as my ordered pairs … 
algebra
I have a few problems I need you to take a look at and tell me if I arrived at the right answer. 1. I am to use the intercepts to graph the equation x+3y=6 I used the formula y=mx+b using 3 and 6 for x I got these as my ordered pairs … 
Math/Check
I have a few problems I need you to take a look at and tell me if I arrived at the right answer. 1. I am to use the intercepts to graph the equation x+3y=6 I used the formula y=mx+b using 3 and 6 for x I got these as my ordered pairs … 
Algebra/no response yesterday
I have a few problems I need you to take a look at and tell me if I arrived at the right answer. 1. I am to use the intercepts to graph the equation x+3y=6 I used the formula y=mx+b using 3 and 6 for x I got these as my ordered pairs … 
trig
I got the following question marked wrong and I was wondering if someone could tell me why I got it wrong. The question is of the 6 trigonometric ratios, if only cosine and secant are positive, the angle is in which quadrant? 
English
1. Some said yes and others said no. 2. Some said yes and the others said no. 3. Some said yes and some said no. 4. Some said yes, others said no, and the others were silent. 5. Some said yes, some said no, and the rest were silent. … 
Math
I don't understand how u can get 3454 from mutiplying 1,100(3.14) When i did it i got 1414 The question: is 2(3.14r2 + 2(3.14)rh 2(3.14)(10)2 + 2(3.14)(10)(45) and when i got done with the promblem the answer in book said approximately … 
Chemistry
@DrBob222 I got the last question that I posted wrong and I solved it the same way. Could you show me the answer and how you got it?