UCSB Science Line
Sponge Spicules Nerve Cells Galaxy Abalone Shell Nickel Succinate X-ray Lens Lupine
UCSB Science Line
How it Works
Ask a Question
Search Topics
Our Scientists
Science Links
Contact Information
I was wondering if you could please show me how to do problems with mole. I really don't get it at all. I have a worksheet that asks how to determine the mass of a certain amount of atoms and the equivalents and how many atoms are in certain amounts of elements. I just really don't get it and I was wondering if you could please give me a few examples. Thanks!
Answer 1:

Dont worry, I also had a lot of problems with at first.

First things first, pull out your Periodic Table. Find the atomic weight (probably in the top left corner.) This Atomic weight is the weight of 1 mole of atoms of that kind. For instance look at N (Nitrogen), you will see the atomic mass is 14.00674 grams. So that means if you had 1 mole of nitrogen atoms, it would weight, 14.00674 grams.

So if you have a fraction of a mole, say for instance, just multiply * 14.00674 grams to obtain the weight, 7.00337 grams. (I used the equation below)

Fraction of a mole * Atomic weight = weight or mass you have

Now we have to get what fraction of a mole you have.
To start, you need know that all a mole means is you have 6.022 x 10^23 atoms of that type. So if I tell you I have a mole of nitrogen, then I have 6.022 x10^23 nitrogen atoms. That ugly number is called Avogadros Constant, NA. Scientist use NA just so they dont have to keep writing that long number over and over again.

To get the fraction of a mole you have, take the number of atoms you have, and divide it by 6.022 x 10^23. (Use equation below)

Number of atoms you have / 6.022 x 10^23 = Fraction of a mole

After you do that, plug it into the first above equation and that should give you your answer.

Answer 2:

1 mole = 6.03 x 10^23 particles. One mole of any element has a mass in grams that is equal to its atomic number, and has exactly 6.02 x 10^23 atoms - however because the atoms of each element have different sizes and weights, then the volume that each one occupies is different. Also, some elements are solids, liquids, or gases at room temperature, so they will also occupy different volumes.
The trick to working with moles is to understand that you are dealing with different units, and you have to translate between them. We call this "units factoring" because it involves setting up ratios of the units and cancelling...
For example: money. The smallest amount of money you can have is a penny.
You can't have half a penny. But you can translate every amount of money into its equivalent number of pennies. The standard is that 1 dollar = 100 pennies. So, 10 dollars = 10 dollars x 100 pennies/dollar = 1000 pennies.
Similarly, half a dollar = 0.5 dollar x 100 pennies/dollar = 50 pennies.
And, working backwards, if someone gives you 50 pennies and asks you how many dollars you have, you can probably do this in your head, but let's write it out for the sake of illustrating the point: 50 pennies x 1 dollar/100 pennies = 50/100 = 0.5 dollar.It works just like this for doing mole fraction problems. If one mole of carbon, for example, has a mass of 12 grams, then 12 grams of carbon contain (12)(6.03 x 10^23) atoms. How many atoms are in 3 grams of carbon? Well...
(3 grams of carbon) x (1 mole of carbon/12 grams ) =3/12 = 1/4 of a mole of carbon.
Then... ( 1/4 of a mole) x (6.02 x 10^23 atoms/mole) = approximately 1.5 x 10^23 atoms.
If you have a compound like H2O (that should be a sub script, but I can't do subscripts in email) then one mole of water contains 6.02 x 10^23 MOLECULES of water. But each molecule of water contains 2 H and 1 O atom = 3 atoms, so there are approximately 1.8 x 10^24 atoms in a mole of water.

Answer 3:

In having trouble with the notion of what a mole is and how to use the concept, you are not alone. Many people have problems understanding what a mole is.
A mole is a collective term. Just as one says, a bunch of grapes, a pride of lions, or better still, a dozen cookies, one could say, a "mole of atoms", or a "mole of molecules". A mole is a collection of Avogadro number of things. The things could be atoms or molecules.
They could also be horses. A mole of horses would be Avogadro number of horses (a lot actually !).Avogadro number is 6.023 x 10^(23).
A mole of H atoms contains 6.023 x 10^(23) H atoms.
A mole of H2 molecules contains 6.023 x 10^(23) H2 molecules or 2 moles of H atoms (since each molecules has 2 atoms).
A mole of CaCO3 (calcium carbonate) has one mole (Avogadro number) of Ca atoms, one mole of C atoms and 3 moles of O atoms.
So what is special about the Avogadro number.
It is just a simple multiplicative constant that relates atomic weights to the simpler, more familiar measure of the gram (g).
Look up a periodic table of the elements. Under each element entry, you will usually find the atomic mass. For example, under Oxygen, you find the atomic weight is 16.00. This means that 16.00 g of oxygen has Avogadro number of oxygen atoms. Carbon is 12.01. So a 12.01 g sample of caron has Avogadro number of carbon atoms.
If we take the CaCO3 example, the formula weight is the sum of the atomic masses. We find that for CaCO3, the formula weight is
Ca (40.08) + C (12.01) + 3O(16.00) = 100.1 100.1 g of CaCO3 is one mole of CaCO3. We can prepare it by reaction one mole of CaO weighing 56.08 g, with one mole of CO2 weiging 44.01 g.If we preferred to work with ounces, instead of g, and wished to related the atomic weight to ounces, then the value that we would chose for the Avogadro number would be different.

Click Here to return to the search form.

University of California, Santa Barbara Materials Research Laboratory National Science Foundation
This program is co-sponsored by the National Science Foundation and UCSB School-University Partnerships
Copyright © 2015 The Regents of the University of California,
All Rights Reserved.
UCSB Terms of Use