The Mole & Molar Mass

STOICHIOMETRY

The M ol e & M olar Mass A mole of any substance contains the same number of elementary particles as there are atoms in exactly 12 grams of the 12C isotope.

I.e.  In one mole, there are

6.022 x 1023 atoms.  Here's another way: 23 there are 6.022 x 10 atoms of carbon in 12 grams of carbon-12.

Let's say that clearly:

one mole of ANYTHING contains 6.022 x 23 10 entities.

The term entity can be applied to any particle:

An atom, an ion, an electron, or a molecule.

6.022 x 10

23

is called Av og adr o' s Num be r and has the symbol -1 N and a unit mol

For example:  One mole of H2O contains 6.022 x 1023

molecules  One mole of nails contains 6.022 x 1023 nails  One mole of Fe contains 6.022 x 1023 atoms  One mole of electrons contains 6.022 x 1023 electrons  One mole of SS 2 students contains 6.022 x 1023 poor, suffering (I mean happy, joyful) science students Get the idea?

Molar Mass When we weigh one mole of a substance on a balance, this is called a "molar mass" and has the units g/mol (grams per mole).

NOTE A molar mass is the weight in

grams of one mole. One mole contains 6.022 x 1023 entities.  Therefore, a molar mass is the mass in grams of 6.022 x 1023 entities.

Calculate molar mass?

Just calculate the molecular weight and put the unit "g/mol" after the number. Period!

QUIZ Calculate the molar mass of Al(NO3)3 [Al = 27, N = 14, O = 16] Answer:

213.00 g/mol

Practice Problems calculate the mass of one mole for the following:  CO2  CuCO3  Ca(C2H3O2)2  CaCO3

Mol e Con version s

Mass of the substance

Mole of a substance = Molar mass of the substance

Example calculate how many grams are in 0.700 moles of H2O2? [H2O2 = 34]

Soln: Given: 0.70 moles Molar mass of H2O2 = 34gmol-1 Problem: Mass = ? mole = mass Molar mass i.e. mass = mole x molar mass 0.7 x 34 = 23.8 grams

Practice Problems Calculate the grams present in:  0.200 moles of H2S [H2S = 34]  3.40 x 10-5 moles of Na2CO3

[Na2CO3 =106]

Answers (1) 6.82 gram (2) 3.60 x 10-3 gram

Calculating the Mass of One Molecule When calculating the mass of one molecule, do the following: 2. Calculate the molar mass of the substance 3. Divide it by Avogadro's Number [ check slide 7] a molar mass is the mass (in grams) of 6.022 x 1023 molecules.

Can you remember?

Now let’s go What is the mass of one molecule of H2O? Use slide7: 6.022 x 1023 molecules of water weigh 18g 1 molecule will weigh X molar mass X = 1 x 18 Avogadro's Number 6.022 x 1023 = 2.992 x 10-23 grams

Calculate the mass (in grams) of one molecule of CH3COOH

[CH3COOH = 60.06

g/mol.]

60.06

g/mol ÷

6.022 x 10 mol  = 9.973 x 10-23 g 23

-1

Practice Problems  Calculate the mass (in

grams) for one molecule of:

PCl5 F 2

CH4

How Many Atoms or Molecules? Recall one mole of ANYTHING

contains 6.022 x 10 entities.

23

Again let’s go  Example #1: 0.450 mole of Fe contains how many

atoms?  Soln  1 mole of Fe contains 6.02 X 1023 atoms  0.45 mole will contain 0.45 mol X 6.02 X 1023 atoms  1 mole  = 2.71 X 1023 atoms

Alternatively, use the graphic below:

DO It Yours elf . You can Do It!

One More Example Example #2: 0.450 gram of Fe contains how many atoms? [ Fe =56] Soln Can u use the graphic and do it yourselves? Try and let’s see! Step One: 0.450 g divided by 55.85 g/mol = 0.00806 mol Step Two: 0.00806 mol x 6.022 x

Prac tic e Prob lem s  Calculate the number of molecules

in1.058 mole of H2O  Calculate the number of atoms in 0.750 mole of Fe  Calculate the number of molecules in1.058 gram of H2O  Calculate the number of atoms in 0.750 gram of Fe [Fe = 56, H = 1, O = 16]

T ha t’s ju st it Hamid Waas’i 2009