Atomic Mass

A weighted average mass of the atoms of an element. (Assuming that carbon-12 isotope is exactly 12)

(example: Atomic mass of Cl=35.45 is calculated from two isotopes. Cl-35 and Cl-36)

Write the equation to calculate the Atomic Mass of an element.

Atomic Mass of X =

(m_x1)(%_x1)/100% + (m_x2)(%_x2)/100% + etc.

(etc. for each isotope of X)

m_x1, m_x2, m_XN= atomic masses for wach isotope of element

%_x1 %_x2, %_XN= percent composition of each isotope

Molecular Mass

Found by adding all the atomic masses of an element.

EXAMPLE: water-H₂O

molecular mass = 18

2(1) + 16 = 18

  H₂      O

Avogadro's Number

The number of atoms or formula units in "X" grams of an element or molecule where "x" is the atomic or molecular mass.

(Always equal to 6.02X10²³)

Mole

6.02 x 10²³ items; can be anything... Atoms, elephants or marshmallows!

• The number of atoms in one mole (atomic mass in grams) of a monoatomic element.
• The number of formula units in one mole (formula mass in grams) of an ionic compound.
• The number of molecules in one mole (formula mass in grams) of a molecular substance.

For any substance, write a general formula to convert from...

Moles to Grams

g = (n)(MM)

n = moles

MM = atomic or molecular mass

g = grams

For any substance, write a general formula to convert from...

Grams to Moles

n = g/MM

n = moles

MM= atomic or molecular mass

g = grams

For any substance, write a general formula to convert from...

Moles to the number of particles

(n)(6.02 x 10²³) = P

n= number of mles

P= number of particles

For any substance, write a general formula to convert from...

Number of particles to moles

n = P/6.02 x 10²³

n= number of moles

P = number of particles

How many grams in 5 moles of ammonia (NH₃)?

g = (n)(MM)

   = (5)(17)

   = 85 grams

How many moles in 50 grams of hydrogen gas (H₂)?

n = g/MM

= 50/2 = 25 Moles

How many molecules in 0.5 moles of CuNO₂?

(n)(6.02 x 10²³) = P

P = (0.5)(6.02 x 10²³)

= 3.01 x 10²³ molecules