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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 .

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mass measurements in chemistry

Mass Measurements In Chemistry

Calculating Formula Weight


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.



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:


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

mass calculations and chemical formulas
Mass Calculations and Chemical Formulas
  • Chemical Formulas represent the ELEMENTSand 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.



3 atoms Ca

2 atoms P

8 atoms O

calculating formula weight molar mass
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 atomsX the Atomic Mass Number of the element (from the PT).
  • Add the results and round to significant figures.
  • What is the formula weight of Calcium Carbonate?
  • CaCO3

From Formula From Periodic Table

1mole CaX 40.08g = 40.08 g

1 mole C X 12.011 g = 12.011 g

3 moles O X 15.9994 g = + 47.9982g


= 100.0892 g

= 100.09 g/mole

  • 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
Practice Problem
  • What would be the formula weight of Magnesium Chlorate Pentahydrate?
  • Formula = Mg(ClO3)25H2O
  • 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.