Moles

Page 1

Basics:

- neutrons have a charge of 0 (they are neutral)

- protons have a +1 charge

- electrons have a -1 charge

- mass of an electron is 1/2000 a.m.u

- atoms always have the same number of protons and electrons , so the overall charge is 0

- number of protons= atomic number/ proton number

- number of protons AND neutrons= mass number/atomic mass

A (mass number)

X (element symbol)

Z (atomic number)

- atoms with the same number of protons but a different number of neutrons are called isotopes

Isotopes have identical chemical properties but different physical properties (eg: melting point and density)

- 1 amu is defined as 1/12 the mass of 1 carbon 12 isotope

- Relative atomic mass (A subscript r) is the weighted average of the isotopic masses of the element compared to the mass of one twelfth of a carbon 12 isotope

to calculate the Ar

1. put the percentage over 100

2. multiply that percentage by the mass number of that isotope

3. add however many isotopes there are and that's the Ar

- we can not weigh a single atom of an element using a balance. Can't even weigh thousands of atoms because a lab balance is not sensitive enough to do this.

- the MOLE is just the same concept as a dozen, a century.

Moles just indicate a very large number of any particles

- one mole of particles is 6 x 10^23 particles (Avogadro's number, N, NA)

ON CALCULATOR: 6 --> 2nd --> , --> 23

- 1 mole of hydrogen is 1.0g

(molar mass of hydrogen is 1.0g per mole)

ROAD MAP

# of particles ...........mass (g)

x or div by NA.......................... x or div by Molar mass

moles (mol)

Page 2

Diatomic elements: if the element name is given, assume that it is in diatomic form.

If the word "atoms" (oxygen atoms) is used, assume that is it single

"What is the mass of 2 moles of oxygen?" --REMEMBER THAT OXYGEN IS DIATOMIC. SO MUST x2 WHATEVER ANSWER YOU GET

For molecules, the relative molecular mass (Mr) can be calculated by

1. taking the Ar of each element and multiplying that with the number of atoms in that molecule

2. adding those up to get the Mr

Percentage composition of elements in compounds

1. put the total atomic number over the Mr

2. multiply by 100 and get the %

The empirical formula gives lowest whole number ratio of the different atoms in a sample of a compound

Eg: C2H4, C4H8, etc... they all have the same empirical formula CH2

The molecular formula requires the knowing the Mr

1. all you have to do is figure out how many more times the value of the Mr is from the Mr of the empirical formula. divide the big Mr by the small Mr and get how many more TIMES

2. then multiply this number on all of the individual elements

Calculating the EF and MF

1. list all the grams your given

2. put it over the Mr

3. you get the moles

4. put it over the lowest common denominator

5. you get a number

6. put those numbers back in and that's the empirical formula

7. divide the Mr by the Mr of the empirical formula

8. use what you got and plug it back in to the formula

9. you get the MF

- Btw: calculating it by the percentage is the same thing.