SKILL #3 MOLAR MASS OF A COMPOUND
OBJECTIVE: to calculate the grams of mass that corresponds to one mole (6.02 x
1023 particles) of a substance.
WHAT YOU NEED:
A) Mass numbers from the periodic table.
B) The molecular formula of the compound.
PROCEDURE
A) Using subscripts, calculate the number of each element in the compound.
NOTE – ignore reaction coefficients for this calculation.
a.
A subscript defines the number of each element written immediately
before it.
b. A subscript defines and is distributed through each parenthesis that
immediately precedes it.
EX- for Pb(CO3)2 there are 2 carbons, six
oxygen and one lead.
B) IN A CHART STRUCTURE multiply each mass number by the number of
each element. (do NOT assess significant figures yet).
C) Add each elements mass product to the others, this is the molar mass.
D) Use the rules of addition of significant figures to round your answer.
Element
Pb
C
O
number
1
2
6
mass number (grams)
207.2
12.0111
15.9994
subtotal
207.2
24.0222
95.9964
+
297.2186
Since the last operation is addition and the number with the fewest decimal
places (207.2) has one decimal place, your molar mass should be rounded to
one decimal place, which is 297.2