Mass Measurements In Chemistry

Calculating Formula Weight

• I CAN calculate the formula weight (molar mass) of a compound from it’s formula.

We’ve talked about the mass of atoms in terms of one or two….but these masses are very small.

To be practical for laboratory work, a standard of measure had to be derived so that mass quantities could be easily measured.

To that end, scientists created a unit called the ATOMIC MASS UNIT used to measure the mass of subatomic particles.

The mass of a proton or neutron was assigned a value of 1 atomic mass unit (amu).

To create a system of practical measurement, scientists used the atom CARBON-12 [6 P+ and 6 No] and assigned it a mass of 12 AMU.

Then they calculated the number of atoms required to make a mass of 12 grams of Carbon-12.

They found it took 6.022 x 1023 atoms of C-12 to have a mass of 12 grams. In turn, each element’s atomic mass required 6.022 x 1023 atoms to equal the same mass in grams.

FOR EXAMPLE

1 atom of Oxygen 16 weighs 16 AMU .

6.022 x 1023 atoms of O-16 weighs 16 grams!

This number became very important in chemistry is known as:

Avagadro’s Number

The quantity of matter containing Avagadro’s Number of particles, such as atoms, molecules or formula units, is known as a:

Mole

When working in a lab setting, the MOLE is used to determine quantities of chemicals.

Mass Calculations and Chemical Formulas

• Chemical Formulas represent the ELEMENTS and the NUMBER OF ATOMS of each that compose a compound.– Subscripts in formulas relate to the element

immediately IN FRONT of them.– If a formula contains a PARENTHESIS followed by a

subscript, the subscript multiplies ALL elements inside.

• EXAMPLE

Ca3(PO4)2

3 atoms Ca 2 atoms P

8 atoms O

Calculating Formula Weight (Molar Mass)

• Formula Weight (Molar Mass) of an element or compound is the MASS in GRAMS of ONE MOLE of the element or compound.

• For an element, the FW is the ATOMIC MASS from the periodic table, or for a particular ISOTOPE.

• For a compound, the FW is found by counting the number of each kind of atom.

• Multiply the number of atoms X the Atomic Mass Number of the element (from the PT).

• Add the results and round to significant figures.

Example

• What is the formula weight of Calcium Carbonate?• CaCO3

From Formula From Periodic Table

1 mole Ca X 40.08 g = 40.08 g1 mole C X 12.011 g = 12.011 g3 moles O X 15.9994 g = + 47.9982 g ________________

= 100.0892 g = 100.09 g/mole

Hydrates• Certain ionic compounds trap water molecules in

their structure when they form. • These are known as HYDRATES.• Hydrates are written in this manner:• Compound Formula Dot Number of Water Molecules

• CuSO4 5H2O• Hydrates are named as:• Compound Name, a Prefix for the number of water molecules,

then the word Hydrate.• When calculating the formula weight of a hydrate, just include

the MASS OF THE NUMBER OF WATER MOLECULES in the total.

Practice Problem• What would be the formula weight of

Magnesium Chlorate Pentahydrate?• Formula = Mg(ClO3)25H2O• 1 mole Mg x 24.305 g = 24.305 g• 2 moles Cl x 35.453 g = 70.906 g• 6 moles O x 15.999 g = 95.994 g• 10 moles H x 1.0079 g = 10.079 g• 5 moles O x 15.999 g = 79.995 g • 281.279 g/mole

PRACTICE PROBLEMS• Complete the following FORMULA WEIGHT (MOLAR MASS)

calculations ON YOUR OWN PAPER!

• 1. Find the FORMULA WEIGHT of Aluminum Sulfate, Al2(SO4)3.

 

• 2. What is the MOLAR MASS of Glucose, C6H12O6?

 • 3. Find the FORMULA WEIGHT of:

Sodium Potassium Permanganate Tetrahydrate

NaK(MnO4)2 4H2O

 • 4. What is the MOLAR MASS of Ammonium Phosphate Dihydrate.