[steph_r]

Hello everybody!Im having trouble understanding the MOLE concept.

I don't understand the difference between working out the following

(a)The number of atoms

(b)How many atoms

(c) The actual number of atoms

(d) The amount (in mol) of a sunstance

How do i know when to multiply the answer by Avogadro's constant?? and when to count the number of atoms (eg. NaCl = 2 atoms) and then multiply by Avogadros constant?? I'm confused

Help, in simple terms, would be greatly appreciated.

Thank u in advance

[english]

You question is quite broad.  Could you provide a sample question featuring these concepts?

[steph_r]

Sure.. (sorry about that)

(a) Calculate the number of atoms in 24g of Magnesium
(b) How many atoms are there in 0.100g of Methane?
(c) Calculate the actual number of carbon atoms present , if a sample of calcium carbonate, weighs 21.8g.

My question is, is there an easy way of distinguishing the difference between the different terms? Is there an easy way of knowing when to use Avogadro's number and the different formulas? Or is it something that needs practise?

Thank you in advance.

[english]

You're going from grams to atoms, meaning you're going to have to first convert to an appropriate unit before using Avagadro's number.

If you have 24 g of Mg, and you want to go to atoms, what should you do?  You cannot go directly from grams to atoms--there is no conversion factor connecting the two. 

Our only option if from grams to ___?  Then, you can go from that to atoms.  What's the missing link here?  We're going from a large unit of measure to a very small unit of measure.  When you go from big to small, in terms of units, what algebraic operation is in effect?  Multiply?  Divide?

Think of a more common example.  How do you go from days (big unit) to seconds (small unit)?

Unit conversions from mass to atoms always involves a fundamental constant (Avagadro's number).

[steph_r]

I understand what you mean and it has helped immensely, however when you say "our only option is from grams to ___?" what do you mean exactly? what's the missing link?

Thank you. (I wish teachers could give examples like yours. I never thought about it that way) 

[english]

What's the only other unit you can go to?  From grams to moles...right?  That's the missing link between grams and the constant.

Then you have your atoms.  From grams to atoms, is much like converting from hours to seconds.  25 g of Mg is how many moles?  How many atoms in this many moles?

An analogous way of saying 25 hours is how many minutes?  How many seconds are in this many minutes?

[steph_r]

I understand now. 
Thank you! I greatly appreciate your help.

[english]

You're welcome