Empirical formula
It is formula tells the simplest whole
number ratio of the atoms of different
element, present in a molecule.
Empirical formula
C6 H 12 O6
C8 H10 N4 O2
C6 H6
S2 Cl2
C12 H22 O11
C2 H2
C5 H 10 O5
H2O2
Calculation of empirical formula
Q1.A sample of copper metal weighing 2.50 g is
heated to form an oxide of copper. The final mass
of the oxide is 3.13 g. Determine the empirical
formula of the oxide.
Elements
Masses
Divide by Ar
Divide by smallest ratio
Simplest whole numbers
Empirical formula
Calculation of empirical formula
Q2. In the analysis of 2.500 g of an unknown
compound, it is found that 0.758 g are calcium, 0.530 g
are nitrogen and 1.212 g are from oxygen. Find the
empirical formula and the identity of this compound
Calculation of empirical formula
Q3. A 0.338 g sample of antimony was completely
reacted with 0.295 g of chlorine gas to form
antimony chloride. Determine the empirical
formula of the antimony chloride. Ar Sb 122
Calculation of empirical formula
Q4 A sample of the black mineral hematite, an oxide
of iron found in many iron ores, contains 34.97 g of
iron and 15.03 g of oxygen. What is the empirical
formula of hematite?
Calculation of empirical formula
Q5. Chemical analysis shows that citric acid
contains 37.51% C, 4.20% H, and 58.29% O.
What is the empirical formula?
Calculation of empirical formula
A sample of copper is heated in a crucible
to form black copper oxide, the following
data is collected
Mass of crucible = 35.75g
Mass of crucible + copper = 37.12g
Mass of crucible + copper oxide = 37.47g
a.Calculate mass of copper used
b.Calculate mass of oxygen that have
reacted with copper.
c. Find the empirical formula of copper
oxide
Calculation of Molecular formula
Q7 . What is the molecular formula of a
compound with a percent composition of 49.47%
C, 5.201% H, 28.84% N, and 16.48% O, and a
molecular mass of 194.2 amu?
n=
molecular mass
empirical formula mass
Molecular formula = n (empirical formula)
Q8 . Nicotine, an alkaloid in the nightshade family
Calculation
of
Molecular
formula
of plants that is mainly responsible for the
addictive nature of cigarettes, contains 74.02% C,
8.710% H, and 17.27% N. If molecular mass of
Nicotine is 162.3g. what is the molecular formula
of Nicotine?
Calculation of Molecular formula
Q9 . Methyl ethanoate is a pleasant smelling
chemical, composed of C, H and O. It has
following composition C = 48.65%, H = 8.11%,
O= 43.24% . Its relative molecular mass is 74.
Find the molecular formula of this compound.
Calculation of Molecular formula
Q10. A 1.00 g sample of one of these compounds
contains 0.040 g of hydrogen, 0.242 g of carbon and 0.718
g of chlorine.
(i) Calculate the empirical formula of this compound [2]
(ii) The relative molecular mass of the compound is 99
.Deduce the molecular formula of the compound. [1]
CH2Cl
C2H4Cl2
Calculation of Percentage composition
Ar and Mr or formula mass can used to calculate the
percentage by mass of an element in a compound.
% of an element by mass = Mass of element X 100
Mr of compound
Calculation of Percentage composition
1.
Calculate percentage composition of CH3COOH
Calculation of Percentage composition
2.
Calculate percentage by mass of Carbon in
C3H6O
3.
Calculate percentage by mass of each element in
C4H8O2
Calculation of Percentage composition
4. Formula of Sodium sulphate decahydrate is
Na2 SO4 . 10H2O
a. Calculate percentage by each element.
b. Calculate percentage of water .
Calculation of Percentage composition
4. Formula of Copper sulphate pentahydrate is
CuSO4 . 5H2O
a. Calculate percentage by mass of copper.
b. Calculate percentage of water .
C. Calculate percentage by mass of CuSO4