Transcript Document

Molecular Formula
Calculations
Combustion vs. Weight Percent
• CxHy + (x + y/4) O2  x CO2 + y/2 H2O
• CxHyOz + (x + y/4 - z) O2  x CO2 + y/2 H2O
Combustion Analysis
• CxHy + O2  x CO2 + y/2 H2O
• CxHy + (x + y/4) O2  x CO2 + y/2 H2O
Molecules with oxygen in their formula are more difficult to solve for
Oz knowing the respective masses of CxHyOz sample, CO2 and H2O.
• CxHyOz + (x + y/4 - z) O2  x CO2 + y/2 H2O
Combustion Problem
Erythrose is an important mono-saccharide that is used in
chemical synthesis. It contains Carbon, Hydrogen and Oxygen.
Problem:
Combustion analysis of a 700.0 mg sample of erythrose
yielded 1.027 g CO2 and 0.4194 g H2O.
(MM = 120.0 g/mol) Calculate the molecular formula.
Erythrose Combustion Solution
mol C x MM of C
Mass fraction of C in CO2 =
=
mass of 1 mol CO2
= 1 mol C x 12.01 g C/ 1 mol C = 0.2729 g C / 1 g CO2
44.01 g CO2
Erythrose Combustion Solution
mol C x MM of C
Mass fraction of C in CO2 =
=
mass of 1 mol CO2
= 1 mol C x 12.01 g C/ 1 mol C = 0.2729 g C / 1 g CO2
44.01 g CO2
mol H x MM of H =
mass of 1 mol H2O
2 mol H x 1.008 g H / 1 mol H
=
= 0.1119 g H / 1 g H2O
18.02 g H2O
Mass fraction of H in H2O =
Erythrose Combustion Solution
0.2729 g C
Mass (g) of C = 1.027 g CO2 x 1 g CO2
= 0.2803 g C
0.1119 g H
Mass (g) of H = 0.4194 g H2O x 1 g H2O = 0.04693 g H
Calculating the mass of O:
Mass (g) of O = Sample mass -( mass of C + mass of H )
= 0.700 g - 0.2803 g C - 0.04693 g H = 0.37277 g O
Calculating moles of each element:
C = 0.2803 g C / 12.01 g C/ mol C = 0.02334 mol C
H = 0.04693 g H / 1.008 g H / mol H = 0.04656 mol H
O = 0.37277 g O / 16.00 g O / mol O = 0.02330 mol O
C0.02334H0.04656O0.02330 = CH2O formula weight = 30 g / formula
120 g /mol / 30 g / formula = 4 formula units / cmpd = C4H8O4
Molecules with oxygen in their formula are much easier to solve
for Oz knowing the percent of C, H and O.
• CxHyOz + (x + y/4 -z) O2  x CO2 + y/2 H2O
Weight Percent Problem
A sample is usually sent to a commercial laboratory for analysis. The
analytical results provide the respective % of each of the elements in
the sample, in the case of erythrose: Carbon, Hydrogen and Oxygen.
Problem:
A 1.2000 g sample of an unknown sugar that was thought to be
erythrose was sent for analysis, the reported results were:
C 40.00%; H 6.71%; O 53.29%; MM = 120.11 g/mol
To solve: Let the analysis % values = mass in grams
Erythrose Weight % Solution
Let the weight % values = mass in grams of each element
Mass (g) of C = 40.00 g
Mass (g) of H = 6.71 g
Mass (g) of O = 53.29 g
Calculating moles of each element:
C = 40.00 g C / 12.01 g C/ mol C = 3.330 mol C
H = 6.71 g H / 1.008 g H / mol H = 6.657 mol H
O = 53.29 g O / 16.00 g O / mol O = 3.331 mol O
C3.330H6.657O3.331 = CH2O formula weight = 30.03 g / emp. formula
(MM= 120.11 g /mol) / 30.03 g / formula = 4 formula units / cmpd = C4H8O4