Finding the Mass of a compound

1
Finding the Mass of a compound
2
Formula Mass

• Add up all the atomic masses in the compounds
  formula
• Also called molecular mass if the compound is
  covalently bonded (forms molecules)
• (Remember, ionic compounds have no molecules)

3
Formula mass

• of NaCl
• Look up atomic mass of Na 23 amu
• Look up atomic mass of Cl 35.3 amu

4
Formula mass

• of NaCl
• Add the atomic masses together
• 23 amu 35.5 amu 58.5 amu

5
Formula mass

• of K2CO3
• This time you have
• two K
• one C
• three O

6
Formula mass

• of K2CO3
• Multiply the atomic mass by the number of atoms
  of that element in the formula
• K 39.1 x 2 78.2 amu
• C 12.0 x 1 12.0 amu
• O 16.0 x 3 48.0 amu

7
Formula mass

• of K2CO3
• Now add the total mass for each element in the
  compound
• K 39.1 x 2 78.2 amu
• C 12.0 x 1 12.0 amu
• O 16.0 x 3 48.0 amu
• 
  ________
• Formula Mass 138.2 amu

8
Now you try it for

• Mg (NO3)2

9
Now you try it for

• Mg (NO3)2
• Element of atoms
• Mg 1
• N 2
• O 6

10
Now you try it for

• Mg (NO3)2
• Element of atoms Atomic mass
• Mg 1 24.3 amu
• N 2 14.0 amu
• O 6 16.0 amu

11
Now you try it for

• Mg (NO3)2
• Mg 1 x 24.3 24.3 amu
• N 2 x 14.0 28.0 amu
• O 6 x 16.0 96.0 amu

12

• Mg (NO3)2
• Mg 1 x 24.3 24.3 amu
• N 2 x 14.0 28.0 amu
• O 6 x 16.0 96.0 amu
• _______
• 148.3 amu

13
Gram Formula Mass (GFM)

• is the formula mass expressed in grams instead of
  in amu
• This is a much more useful and practical unit
• Follow the same steps as formula mass, except
  your answer will have units of grams, not amu

14
Formulas of Hydrates

• A HYDRATE is a an ionic compound that has water
  molecules attached to its crystal structure in
  the solid phase.
• These water molecules are weakly attached to the
  crystal, and are easily removed if the crystals
  are heated. Once the water is removed, the
  compound is called ANHYDROUS (without water).

15
Formulas of Hydrates

• Na2CO3 10 H2O
• This is a sodium carbonate crystal with ten water
  molecules attached to every one sodium carbonate
• The is the symbol that attaches the ten
  water molecules to the sodium carbonate. Think
  of it as a

16
Finding GFM of Hydrates

• Na2CO3 10 H2O
• Type of atoms atomic mass
• Na 2 23
• C 1 12
• O 3 16
• H2O 10 2 16 18

17
Finding GFM of Hydrates

• Na2CO3 10 H2O
• Na 2 x 23 46
• C 1 x 12 12
• O 3 x 16 48
• H2O 10 x 18 180
• 286 g

18
You try it!

• Find the GFM of the hydrated crystal
• CaSO4 2H2O

19
You try it!

• CaSO4 2H2O
• Ca 40 x 1 40
• S 32 x 1 32
• O 16 x 4 64
• H2O 18 x 2 36
• 172 g