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### Topic: unit conversion/density problem  (Read 9061 times)

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#### nikita

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##### unit conversion/density problem
« on: May 27, 2008, 05:39:15 PM »
Hi!  i am actually in college, but this is general chem and chap 1, so i decided to put it in here.  i am an adult going back to college, so hs chem was long ago for me.  i am working on unit conversion and densities, and i have a few problems that i am confused by, or maybe i just dont know where to start.

a 5.79 mg piece of gold is hammered into gold leaf of uniform thickness with an area of 44.6 cm^2.  what is the thickness of the gold leaf?

i understand 5.79mg to be the mass here.  i also guess that if i have the area, i am then looking for the h, in LxWxH?

i also guess that 19.3 grams is going to come into play here?

no, i am totally lost.  i think its because i may need a formula that i dont know.  i dont even know what to convert into what.  any pointers?  im not asking for the answer because i have that, maybe just a jumpstart, as i think i can figure it out if i just knew where to start.  thanks.

#### N3.Coaltion

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##### Re: unit conversion/density problem
« Reply #1 on: May 27, 2008, 05:51:53 PM »
Not sure if this will help at all but...

I believe since you know its gold you can find its density (You were right with 19.3 g/cc), and from there you have its density, mass, and area.

Then calculate its volume = mass/density.

Then finally its thickness = volume/area.

I think this is what needs to be done but I'm not entirely sure. Hopefully someone else with more knowledge comes around and checks it out. Hopefully I helped a little bit.

#### nikita

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##### Re: unit conversion/density problem
« Reply #2 on: May 27, 2008, 05:55:39 PM »
ok, i thought about it longer and decided to convert the mg mass to g and then i have my mass and density.

i get
19.3g = .00579g/v  and v then = 3.00 x 10^-4

so i have the volume and mass, how do i find the thickness?

#### Borek

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##### Re: unit conversion/density problem
« Reply #3 on: May 27, 2008, 05:57:44 PM »
You have volume - and you first instict of LxWxH was correct. Just LxW is already a surface which you are given.
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#### nikita

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##### Re: unit conversion/density problem
« Reply #4 on: May 27, 2008, 06:01:49 PM »
.0003 = 44.6x

.0003/44.6 = 44.6x/44.6

x = 6.73 x 10^-6cm^2

i know that is the right answer.  did i do this correctly, or did i make up a crazy way of doing it?

#### N3.Coaltion

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##### Re: unit conversion/density problem
« Reply #5 on: May 27, 2008, 06:06:02 PM »
Rounded I got the same result and yes I believe this is the correct approach...

*Edit* I'm not sure about college, but a lot of my teachers are picky about the showing of units and how they are simplified in the equations as well.

In this case it went from

Volume (cm3) = mass (g) / density (g/cm3)    [g cancel]

Thickness (cm) = volume (cm3) / area (cm2)  [which leaves you with cm]

As far as I can tell this is the correct method of choice for this problem.
« Last Edit: May 27, 2008, 06:16:19 PM by N3.Coaltion »

#### nikita

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##### Re: unit conversion/density problem
« Reply #6 on: May 27, 2008, 06:32:06 PM »
this is how i did it:

5.79mg x .001g/1mg = .00579g (canceling out mgs)
19.3 g = .00579g/v   (19.3 grams the density of gold)
v= .0003cm^2  (yes?)

so i took L x W x H for the volume formula with
v = .0003g

.0003g = 44.6cm^2    (LxW = 44.6, so i put in x for the height)  i really dont think that is the right way to do it, is it?
dividing, i get

6.73 x 10^-6g/cm^2 is the thickness?

i have a few more questions about other problems, but i guess its prudent to write a new thread.  oh, and thanks so much for the help thus far!