[dirknathan]

I've given this one some good though, and I can't come up with the correct answer.  It is mind boggling, I would really appreciate to see how it is completed.

During a severe episode of air pollution, the concentration of lead in the air was observed to be 3.03 ug Pb/m³ .

How many atoms would be present in a 0.500 - L sample of this air (the approximate lung capacity of a human adult)?



It seems simple enough, but between conversions and simply finding the atoms I can't get it down.
Thanks for your time.

[vex]

You'll need to add units to the quantities in your question, otherwise nobody will be able to help you.

[dirknathan]

ahh, the copy and paste left them out, fixed.

[vex]

Okay, thanks. 

So this is what's known as a dimensional analysis problem, and it's entirely about knowing how to convert between units. Let's think about this.

1) What units do you have in your problem, and what do they represent? (mass, time, etc.)
2) What conversions might you need? Most of the conversions in this problem are just manipulating the metric system, but I think I know the one you're getting tripped up on: how can I get the number of atoms in a given amount of substance? Think about the definition of a mole...

Good luck!

[dirknathan]

mL to m³?

I've got that down.  I literally scribbled an entire page of conversions.  I get lost in relation of the concentration of Pb and .500 L of air.  I am also am mixed inbetween "Do I use the mass of Pb?" or leave it out.

I believe you could go mg/m³.  But, again, I'm frusturated and at a loss.

P.S. this isn't for a grade, but I simply want to understand.

[vex]

You'll need to use mass of Pb to get the answer. The crux of the question is, "How many atoms of Pb are in 0.5 L of air that has the given concentration of Pb?"

[dirknathan]

I simply don't understand how to put it all together.

This is more of a trick problem for us so far.  All the other work given to us was super, super simple.  This blows the rest away, and I assume it's easy.  But I just dedicated 30 minutes of configuring into and and I can't get the correct answer.  The teacher even gave us the gateway and I am still clueless.  I took 2 chemistry classes in highschool, took zero my senior year though, I've slept since then.

[Borek]

Perhaps thinking in terms of proportions will make it simpler for you? The first question that you are trying to solve is: if 1 m³ of air contains 3.03 μg Pb, how much Pb is in 0.5 L? Then it will be a matter of converting mass of Pb to atoms.

The question listed above can be written as

[tex]\frac {3.03 \mu g} {1 m^3} = \frac x {0.5 L}[/tex]

Once you solve the proportion you will still have mixed units (which is a reason why I have omitted units of x, to not suggest anything), so it is not yet the end of the story - but hopefully at least you will know now what it is that you are trying to do?

[dirknathan]

All my teachers ask the class to answer questions on the board whilst learning.  I'm the type that needs to see the entire problem worked out to understand the process.  I don't get the relation, and all along I feel like it comes down to a ratio.  I can't concept this until I see it.

This online program is only giving me one more try.  But thanks for the input.

[dirknathan]

4.40*10¹⁵?

I tried again.  I only get one for try though.  That answer look warm?

[Borek]

I got 4.41 as well, so you are definitely on the right track - but your order of magnitude is off.

[dirknathan]

4.41x10¹⁸

I had an answer with 4.41 earlier today, but I don't remember the magnitude.

[dirknathan]

I'm off to work, I have to have this entered in by midnight.