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 N o ] 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 x atoms of C-12 to have a mass of 12 grams. In turn, each element’s atomic mass required x atoms to equal the same mass in grams.

FOR EXAMPLE 1 atom of Oxygen 16 weighs 16 AMU 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 Ca 3 (PO 4 ) 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? CaCO 3 From Formula From Periodic Table 1 mole Ca X g = g 1 mole C X g = g 3 moles O X g = g ________________ = g = 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 CuSO 4 5H 2 O 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(ClO 3 ) 2  5H 2 O 1 mole Mg x g= g 2 moles Cl x g= g 6 moles O x g= g 10 moles H x g= g 5 moles O x g= g g/mole

PRACTICE PROBLEMS Complete the following FORMULA WEIGHT (MOLAR MASS) calculations ON YOUR OWN PAPER! 1. Find the FORMULA WEIGHT of Aluminum Sulfate, Al 2 (SO 4 ) What is the MOLAR MASS of Glucose, C 6 H 12 O 6 ? 3. Find the FORMULA WEIGHT of: Sodium Potassium Permanganate Tetrahydrate NaK(MnO 4 ) 2  4H 2 O 4. What is the MOLAR MASS of Ammonium Phosphate Dihydrate.