Monday, November 16, 2009

Moles and Atoms: relationships, molecules.. etc?

Okay, so today we had a quiz in chemistry, and it had very confusing problems, asking how may atoms in like 3 mol of a compound, or atoms of H in 3 moles of NH4, etc. Also, even molecules came up. I was astounded, and dumbfounded.

Like this problem:

(NH4)2 S

How many atoms of N are there in a mole of that compound, and the choices were like, well I don't remember how many moles, but the choices:

A: 1.2 x 10^23

B: 6.0 x 10^23

C. 3.0 x 10^23

D. 2

How would you do problems such as these?

The wording was particularly tricky, and I know that 1 mole of an element consists of avogadro''s number of that element in atoms??

Moles and Atoms: relationships, molecules.. etc?
Well, your last statement is partially correct. A mole of a compound contains Avogadro's number (which is 6.022 x 10²³) molecules. For example, 1 mole of CO2 would contain 6 x 10²³ molecules of CO2. These problems are fairly easy, since we know how many molecules there are in 1 mole of a substance, and we know from the formula how many atoms of each element are in each molecule.

The first problem for example:

How many atoms of hydrogen are in 3 moles of ammonia (NH3)?

3 moles of ammonia is equal to 3 x 6.022 x 10²³ molecules of ammonia, which works out as 1.8066 x 10^24 molecules. Each molecule contains 3 hydrogen atoms, as denoted by the formula, so there are 3 x 1.8066 x 10^24 atoms of hydrogen.

This is a total of 5.4198 x 10^24 atoms.

The next problem:

How many nitrogen (N) atoms are there in 1 mole of ammonium sulfide (NH4)2S?

1 mole of ammonium sulfide contains 6.022 x 10²³ molecules - no multiplication required. Each molecule of ammounium sulfide contains 2 atoms of nitrogen, so the answer is 2 x 6.022 x 10²³, which equals 1.2044 x 10^24 atoms of nitrogen, which is what I assume A. was supposed to say.

Hope I've helped you get your head around this type of problem - you'll see a lot more of them.
Reply:you've got your choices wrong. because the answer is (2)(6.02X10^23)

there are 6.02X10^23 molecules of ammonium sulfide in a mole of ammonium sulfide.

if there are two Nitrogen atoms in each molecule, then that means that there are (2)(6.02X10^23) atoms of Nitrogen in a mole of ammonium sulfide
Reply:1 mole (NH4)2S x 2 atoms of N x 6.02 x 10^23 atoms in 1 mole of nitrogen =

answer: 1.2 x 10^23

* what i did was multiplied it with 2 atoms of N (becuase in the formula there are 2 nitrogens) then i multiplied it by 6.02 x 10^23 (because there are that many atoms in one mole)

Reply:It might help you if you simplify more complicated formulas like this:

(NH4)2S = N2 H8 S

Now, this can represent a single molecule that has a ratio of 2 : 8 : 1 atoms. Or it can also represent a mole of the compound that has a ratio of 2 : 8 : 1 moles of atoms.

I don't think you remembered the choices correctly, since the number of nitrogen atoms in one mole of compound will be 2 moles. And, what is a mole? Of course, it is 6.02 X 10^23 particles.

So 2 moles of atoms will then be 2 X (6.02 X 10^23) which is 1.204 X 10^24 atoms.

Don't let Avogadro's Number intimidate you, It is just a multiple unit like dozen or gross (12 dozen). Yes, it is an incredibly huge number. But why to we need it? Of course, we use it to terrify beginning chemistry students .... no, just kidding.

Avogadro's number is needed since we can't physically measure infinitesmally small atoms and molecules. And the unique feature of Avogadro's number (or moles) of atoms is that their mass in grams are the masses on the periodic table. And we can weigh grams, but no scales can ever be produced that can weigh a single atom or molecule.


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