Inorganic chemistry
 Calculating the number of atoms in a
given amount of a compound.
 Calculating the Frequency and
Wavelength of an Electromagnetic
Wave,.
 Calculating the energy of a photon.
Assistance Lecturer Amjad Ahmed Jumaa
www.soran.edu.iq
1
Calculating the number of atoms in a given amount of a compound:
convert from grams of compound to moles of a particular atom to
number of atoms. Let's try an example.
Example:
How many carbon atoms are present in (50.3 g) of (C2H6).
Solution:
We started this problem in the above example, when we calculated the
moles of ethane in (50.3 g ethane). To continuo, we need two additional
conversion factors. One should represent the mole ratio between
moles of( C ) atoms and moles of ethane molecules. The other
conversion factor needed is Avogadro's number.
www.soran.edu.iq
Step(1): the two conversion factors needed are:
You should come up with the following strategy:
Grams of C2H6 → moles of C2H6 → moles of C → atoms of C.
Step (2):
? C atoms = 50.3 g C2H6 x
= 2.01 x 1024 C atom.
www.soran.edu.iq
x
Calculating the frequency and wavelength of an electromagnetic wave:
All types of electromagnetic radiation move through a vacuum at a speed
of about (3.00 x 108 m/s), which is called the speed of light( c), speed is
an important property of a wave traveling through space and is equal to
the product of the wavelength and the frequency of the wave. For
electromagnetic waves :
c = λv …………………….. (1).
Equation (1) can be rearranged as necessary to solve for either the
wavelength (λ) or the frequency (v).
Example:
A certain (AM) radio station broadcasts at a frequency (6.00 x 102 kHz).
What is the wavelength of these radio wave in meters.
www.soran.edu.iq
Solution:
Step(1): solve the equation (1) algebraically for the wavelength (λ).
c = λv
……………………(2)
Step (2): since the speed of light has units of (m/s). We must convert the frequency
from units of (kHz) to (Hz)(s-1).
6.00 x 102 kHz x
= 6.00 x 105 Hz = 6.00 x 105 s-1.
Step (3): calculate the value of (λ) by substituting the known quantities into
equation (2).
λ=
www.soran.edu.iq
= 5.00 x 102 m.
Example (2):
What is the frequency of light that has a wavelength of (665) (nm).
Solution:
Step (1): solve equation (1) algebraically for the frequency (v).
c = λv
…………………..(3)
Step (2): since the speed of light has units of (m/s). we must convert
the wavelength from units of (nm) to (m).
665 nm x
= 6.65 x 10-7 m.
Step (3): calculate the value of (v) by substituting the known
quantities into equation (3).
www.soran.edu.iq
= 4.51 x 1014 s-1 = 4.51 x 1014 Hz
v=
Calculating the energy of a photon:
atoms and molecules could emit( or absorb) energy only in discrete
quantities.
Planck gave the name (quantum) to the smallest quantity of energy
that can be emitted (or absorbed) in the form of electromagnetic
radiation. The energy (E) of a single quantum of energy is given by:
E = hν …………………..(4)
h is Planck's constant = 6.63 x10-34 J.s
ν is the frequency of radiation.
www.soran.edu.iq
Example:
The yellow light given off by a sodium vapor lamp has a wavelength of (589nm),
what is the energy of a single photon of this radiation.
Solution:
Step (1): you are given wavelength in this problem. Equation (4) shows
the relationship between energy and frequency. However, there is a
relationship between frequency and wavelength (see equation 3).
Substituting for the frequency in equation (4), we have:
………5
Step (2): since the speed of light is in units of (m/s), we must convert the
wavelength from units of (nm) to (m).
www.soran.edu.iq
x
= 5.89 x 10-7 m.
Step (3): Calculate the value of (E) by substituting the known quantities into
equation (5).
E=
www.soran.edu.iq
= 3.38 x 10-19 J.