LAB 5-MASS to MOLE to
MASS
STOICHIOMETRY
• OVERVIEW:
• At this time, your product is drying and therefore you
cannot measure the EXPERIMENTAL or MEASURED
mass in grams. The water would add mass and would
cause you to overestimate the mass of your product,
PbCr2O7(s). Please note water is not in the reaction, it is
the solvent and does not interact chemically. You can
however calculate your THEORETICAL YEILD, the mass
of product you SHOULD have collected in your filter
paper cone.
OVERVIEW CONTINUED:
• The calculation is performed in three steps
1. CONVERT THE GIVEN OR KNOWN TO
MOLES.
2. RATIO MOLES OF GIVEN TO MOLES OF
OBJECTIVE.
3. CONVERT MOLES OF OBJECTIVE TO
GRAMS, THEN COMPARE IT TO WHAT YOU
ACTUALLY COLLECTED AND MEASURED
IN LAB.
FLOWCHART OF THE THEORETICAL CALCULATION
Step # 1
CONVERT REACTANT(KNOWN) TO MOLES
MOLES KNOWN = MASS (GRAMS)
MOLAR MASS
Step #2
MOLE RATIO
(FROM REACTION)
KNOWN
. COEFFICIENT KNOWN = MOLES KNOWN
OBJECTIVE . COEFFICIENT OBJ.
MOLES OBJ. (X)
Step #3
CONVERT KNOWN REACTANT TO MOLES
MOLES OBJ. =
MASS (GRAMS) (X)
MOLAR MASS
Step # 1
CONVERT KNOWN REACTANT Pb(NO3)2 TO MOLES
MOLES KNOWN = MASS used
=
1.13g
= 0.0034 mol
MOLAR MASS
331.15 g/mol
Pb(NO3)2
Step #2 MOLE RATIO
(COEFFICIENT, from reaction)
KNOWN : = COEFFICIENT KNOWN = MOLES KNOWN
OBJECTIVE:
COEFFICIENT OBJ.
MOLES OBJ. (X)
Pb(NO3)2
PbCr2O7(s).
=
1
1
= 0.0034 mol = X = 0.0034
X
mol
Step # 1 CONVERT OBJECTIVE TO MASS IN GRAMS.
MOLES OBJ. = MASS (GRAMS) = 0.0034 = X grams = 1.438g
PbCr2O7
MOLAR MASS
423.11 g/mol
CALCULATION OF MOLAR
MASS of K2Cr2O7
LIST
SUBSCRIPTS
MULTIPLY
By atomic mass
INSERT
ATOMIC MASS
FROM PERIODIC TABLE
LIST
ELEMENTS
HERE
K
2 X 39.09 g/MOL
=
78.18 g
Cr
2 X 51.99 g/MOL
=
103.98 g
O
7 X 15.99 g/MOL
=
111.93 g
294.09 g/mol
CALCULATION OF MOLAR
MASS of Pb(NO3)2
LIST
SUBSCRIPTS
MULTIPLY
By atomic mass
INSERT
ATOMIC MASS
FROM PERIODIC TABLE
LIST
ELEMENTS
HERE
Pb
1 X 207.2 g/MOL
=
N
2 X 14.00 g/MOL
=
28.00g
O
6 X 15.99 g/MOL
=
95.95 g
The number of O
atoms is 3 times 2
The number of N
atoms is 1 times 2
207.2 g
331.15 g/mol
CALCULATION OF MOLAR
MASS of PbCr2O7
LIST
SUBSCRIPTS
MULTIPLY
By atomic mass
INSERT
ATOMIC MASS
FROM PERIODIC TABLE
LIST
ELEMENTS
HERE
Pb
1 X 207.2 g/MOL
=
207.2 g
Cr
2 X 51.99 g/MOL
=
103.98 g
O
7 X 15.99 g/MOL
=
111.93 g
The number of O
atoms is 7
The number of Cr
atoms is 2
423.11 g/mol