A STEPWISE METHOD FOR SOLVING WORD PROBLEMS IN CHEMISTRY
Dr. Carson Prevatte, Motlow State Community College
Note to instructor:
This work was created to facilitate an extracurricular, interactive Science Workshop to
supplement lecture by breaking down the process of solving word problems into a stepwise,
“failsafe” strategy and providing ample time for questions and self or group work. The total
duration of the Workshop is intended to be 90 minutes with 60 minutes for the delivery of the
workshop and the remaining 30 minutes for questions and small group work.
The Workshop could be delivered at any point during a semester of General Chemistry I or II
by using example word problems coinciding with those being covered at that point in the
semester. This is not to say that the problem-solving process changes, but different aspects of
the problem-solving process may be more prominent at different points in General Chemistry.
The examples employed were selected for students in the first few weeks of General
Chemistry 1 when students are first learning to solve word problems requiring the application
and rearrangement of mathematical equations.
STEP 1: List the variables mentioned in the question and underline the unknown property.
Each variable is a property of the physical system.
STEP 2: Reference the equations reference sheet to identify the equations that have these
variables represented by their symbols in the equation.
STEP 3. Isolate the unknown variable before plugging in any values for the variables.
Option 1: If you identified only one equation, rearrange the equation to isolate the unknown
variable (EXAMPLE 1).
Option 2: If the two equations that have a common variable that was not in the question,
rearrange both equations to isolate the common variable and set both equations equal to each
other, e.g., if a = x and b = x, then a = b (EXAMPLE 2). This creates a new equation that you
can use to solve for the unknown variable.
STEP 4: Plug in the values and units of the known variables then calculate the value and units
of the unknown variable. If the units do not match, convert the variables’ units to match the
constants’ units before plugging into the equation. Enter numbers in scientific notation into your
calculator with parenthesis to enforce the correct order of operations and make it easier to
track of your calculation.
EXAMPLE 1: What is the wavelength of a photon with a frequency of 30.0 GHz?
STEP 1: List the variables mentioned in the question and underline the unknown property.
Each variable is a property of the physical system.
The variables are wavelength (λ) and frequency (ν)
STEP 2: Reference the equations reference sheet to identify the equations that have these
variables represented by their symbols in the equation.
Temp in °F = ( 9°F / 5°C ) ´ (temp in °C ) + 32°F
K = °C + 273.15
1 lb = 453.6 g
2.54 cm = 1 in
1 Angstrom (Å) = 10-10 m
1 mol = 6.022 * 1023
c = λν, c = 3.0 x 108 m/s
E = hν, h = 6.63 x 10-34 J*s
STEP 3. Isolate the unknown variable before plugging in any values for the variables.
Option 1: If you identified only one equation, rearrange the equation to isolate the unknown
variable.
c = λν  c / ν = λ
STEP 4: Plug in the values and units of the known variables then calculate the value and units
of the unknown variable. If the units do not match, convert the variables’ units to match the
constants’ units before plugging into the equation. Enter numbers in scientific notation into your
calculator with parenthesis to enforce the correct order of operations and make it easier to
track of your calculation.
Converting Gigahertz (GHz) to inverse seconds (s-1):
30.0 GHz x (109 Hz / 1 Hz) x (1 s-1 / 1 Hz) = 30.0 x 109 s-1 or 3.00 x 1010 s-1,
Calculating wavelength (λ):
c/ν = λ  (3.0 x 108 m/s) / 30.0 x 109 s-1 = 1.00 x 10-2 m
EXAMPLE 2: What is the energy of a photon with a wavelength of 450.0 nm?
STEP 1: List the variables mentioned in the question and underline the unknown property.
Each variable is a property of the physical system.
The variables are energy (E) and wavelength (λ).
STEP 2: Reference the equations reference sheet to identify the equations that have these
variables represented by their symbols in the equation.
Temp in °F = ( 9°F / 5°C ) ´ (temp in °C ) + 32°F
K = °C + 273.15
1 lb = 453.6 g
2.54 cm = 1 in
1 Angstrom (Å) = 10-10 m
1 mol = 6.022 * 1023
c = λν, c = 3.0 x 108 m/s
E = hν, h = 6.63 x 10-34 J*s
STEP 3. Isolate the unknown variable before plugging in any values for the variables.
Option 3: If the two equations that have a common variable that was not in the question,
rearrange the equation that contains the unknown variable to isolate the unknown variable.
Rearrange the other equation to isolate common variable between the two equations. Next, set
both equations equal to each other, e.g., if a = x and b = x, then a = b. This creates a new
equation that you can use to solve for the unknown variable.
E = hν Does not require rearrangement since energy (E) is isolated.
c = λν  c / λ = ν
Substituting c / λ for ν into E = hν yields E = h(c / λ)
STEP 4: Plug in the values and units of the known variables then calculate the value and units
of the unknown variable. If the units do not match, convert the variables’ units to match the
constants’ units before plugging into the equation. Enter numbers in scientific notation into your
calculator with parenthesis to enforce the correct order of operations and make it easier to
track of your calculation.
Converting nanometers (nm) to meters (m):
450.0 nm x ( 10-9 m / 1 nm ) = 450.0 x 10 x 10-9 m or 4.500 x 10-7 m
Calculating energy (E):
E = h(c / λ) = 6.63 x 10-34 J*s x ( 3.0 x 108 m/s / 4.500 x 10-7 m) = 4.42 x 1019 J