Solution Preparation and Dilutions
Solutions are required for all functions in a laboratory setting. These reagents can be
prepared based on four sets of calculations: mass/volume concentrations, %
mass/volume concentrations, molarity concentrations, and dilutions of concentrated
solutions.
In this laboratory you will:
(a) learn proper metric conversions,
(b) learn the calculations required for each of the solution types above,
(c) determine the calculations for solutions required during the semester, and
(d) prepare the solutions for use throughout the semester.
Materials
Lab handout
Calculator
Procedure
Part A. Making Metric Conversions
When preparing solutions, metric units must match for the proper calculations to occur.
Use Table 1 to learn how metric units are properly converted.
Table 1. Commonly used metric units in the biotechnology field.
Units
1 liter (L)
1 meter (m)
1 gram (g)
Equivalent Values
1000 (103)
milliliters
(mL)
0.001 (10-3)
kilometer
(km)
0.001 (10-3)
kilogram (kg)
1,000,000 (106)
microliters (μL)
100 (102)
centimeters
(cm)
1000 (103)
milligrams
(mg)
1000 (103)
millimeters
(mm)
1,000,000 (106)
micrograms
(μg)
1,000,000 (106)
micrometers (μm)
1,000,000,000
(109) nanograms
(ng)
Fill in the conversion table (Table 1) in the Practice Problems section for Part A. Making
Metric Conversions at the end of this handout.
Solution Preparation 1
Part B. Making Solutions of Differing Mass/Volume Concentrations
Solutions are prepared with a certain mass of solute in a certain volume of solvent. Any
metric mass in any metric volume is possible, but the most common units of
mass/volume concentrations are as follows:
g/mL
g/L
mg/mL
μg/mL
μg/μL
ng/L
ng/μL
grams per milliliter
grams per liter
milligrams per milliliter
micrograms per milliliter
micrograms per microliter
nanograms per liter
nanograms per microliter
Although concentrations can be reported in any mass/volume units, these 7
mass/volume units are the most common in biotechnology applications.
To determine how to prepare a certain volume of a solution at a certain mass/volume
concentration, use the equation below. Convert units as necessary to make sure units
that are used can be cancelled out.
Mass/Volume Concentration Equation
concentration desired x total volume desired = mass of solute in the total volume
desired
(ex. g/mL)
(ex. mL)
(ex. g)
Ex. A technician needs 50 mL of 15 mg/mL pepsin solution for an experiment. Pepsin is
a protein-digesting enzyme that is produced and functions in the stomach. Using the
Mass/Volume Equation, the calculation would be as follows:
15 mg/mL x 50 mL = 750 mg = 0.75 g pepsin
The technician would add 0.75 g pepsin to a container and fill up to the 50 mL mark with
solvent (usually deionized water). Most balances weigh in grams, so the conversion
from mg to g was necessary.
Determine the calculations for the solutions in the Practice Problems section for Part B.
Making Solutions of Differing Mass/Volume Concentrations at the end of this handout.
Part C. Making Solutions of Differing % Mass/Volume Concentrations
A lab technician must be able to make any solution at any concentration or volume.
Most commonly, solutions are made with concentrations reported in one of these three
measurements:
Solution Preparation 2
Measurement
mass/volume
% (in mass/volume or volume/volume)
molarity (moles/liter)
Example
4 mg/mL salmon sperm DNA solution
2% sucrose solution
0.5 M TRIS solution
Technicians must be able to recognize which chemicals should be measured out, in what
amounts, and what math must be done to calculate these amounts. It takes practice to
consistently prepare solutions at the correct volume and concentration.
Preparation of % mass/volume solutions is presented here. Keep in mind that a 1%
solution contains 1 g of solute in a total volume of 100 mL.
Use the % Mass/Volume Equation shown below to calculate the mass of each solute
needed for a solution at some volume. Notice that as with preparing mass/volume
solutions, the math is relatively simple. Multiply concentration desired (in decimals)
with the volume needed (in mL). Make sure units that are used can be cancelled out
and convert units as necessary.
% Mass/Volume Concentration Equation
Step 1. Convert the % to a decimal
% (percent value) = decimal value of the g/mL (grams needed divided by 100 mL)
Step 2. Determine the amount needed
decimal (g/mL) of the % concentration x total volume desired (mL) = grams of solute
Ex. A technician needs 5 mL of 10% NaOH solution for an experiment. Using the %
Mass/Volume Equation, the calculation would be as follows:
Step 1. 10% = 10 g/100 mL = 0.1 g/mL
Step 2. 0.1 g/mL x 5 mL = 0.5 g NaOH
The technician would add 0.5 g NaOH to a container and fill up to the 5 mL mark with
solvent.
Determine the calculations for the solutions in the Practice Problems section for Part C.
Making % Solutions of Differing Mass/Volume Concentrations at the end of this handout.
Part D. Making Solutions of Differing Molarity Concentrations
The concentration of many solutions is reported as moles/liter (mol/L or M; the M is
spoken “molar”) or some function of those units. This concentration measurement is
called molarity. Molarity is sometimes a challenging concept to understand. However,
with your recently acquired solution preparation skills, you will see that making molar
solutions required only on extra calculation.
Solution Preparation 3
To understand how to make a solution of a given molarity, you must know what a
“mole” is. A mole of a compound is equal to 6.02 x 1023 molecules, but that is not really
a very useful number. So, in biotechnology, it is easier to use this definition: The unit “1
mole” is the mass, in grams, equal to the molecular weight (MW), also called “formula
weight” (FW), of the substance. The FW can be determined by using a Periodic Table or
by adding the atomic weights of the atoms in the molecule. An easy way, though, is to
just read the label of a chemical reagent bottle, which lists the “MW” or “FW.” The
molecular weight of NaCl is 58.5 atomic mass units (amu) since the Na atom weighs 23
amu, and a Cl atom weighs 35.5 amu.
Molarity concentrations are reported as the number of moles per liter (mol/L or M). If
the concentration is very low, then the concentration could be reported in
millimoles/liter (mmol/L or mM). If you wanted a 1-M NaCl solution, you would
measure out 1 mole of NaCl (58.5 g) and dissolve it in water to a total volume of 1 L.
This gives you 1 mole of NaCl per liter of solution, 1 M NaCl.
A liter of solution is a large volume for most research and development purposes. In
research and development labs, mL or μL quantities are usually used. To determine
how to mix up a smaller volume of a solution of some molarity, follow the example
below.
Multiply the volume desired (L) by the concentration (molarity) desired (mol/L), as you
did in the mass volume calculations. Then, multiply the result by the compound’s
molecular weight (g/mol) to account for measuring in moles, as in the following
equation:
Molarity Concentration Equation
volume x molarity x molecular weight = grams of solute to be dissolved in
wanted
desired
of the solute
solvent to the final desired volume
(L)
(mol/L)
(g/mol)
Convert smaller or larger units to these as necessary. The “L” units cancel out and the
“mol” units cancel out, leaving the mass in grams of the solutes needed to make the
solution.
Ex. A technician needs 50 mL of 0.5 M NaCl solution for an experiment. Using the
Molarity Concentration Equation, the calculation would be as follows:
0.05 L x 0.5 mol/L x 58.5 g/mol = 1.46 g NaCl
The technician would add 1.46 g NaCl to a container and fill up to the 50 mL mark with
solvent.
Solution Preparation 4
Determine the calculations for the solutions in the Practice Problems section for Part D.
Making Solutions of Differing Molarity Concentrations at the end of this handout.
Part E. Making Dilutions of Concentrated Solutions
Making dilutions of concentrated solutions is a common practice in a biotechnology lab.
A concentrated solution is generally called a “stock solution,” and the diluted solution is
called the “working solution.” Preparing a concentrated stock solution saves a lot of
time and is easier to store than large volumes of diluted working solutions. Making a
working solution simply required diluting some volume of stock solution to the
concentration needed.
The working concentration of a solution is represented as 1X. A concentrated solution
could be represented as 10X if it has 10 times the amount of solute per unit volume
compared with the working solution. A 50X stock has 50 times the concentration of
solute as a working solution. For example, an enzyme storage buffer may be used at a
concentration of 0.01 M TRIS. This is the working concentration of the TRIS solution
(1X). But because of shipping costs, a small amount of the enzyme buffer is shipped as a
10X solution with a concentration of 0.1 M TRIS. When the technician is ready to use
the buffer, it is diluted with deionized water down to 1X (0.01 M TRIS).
When a number of dilutions must be made, and each is proportionally the same dilution
as the one before, it is called a serial dilution. Doing a serial dilution makes sense for
many experiments when many samples of varying concentrations are needed. A serial
dilution is also useful for preparing very dilute solutions that are hard to make from
scratch, because the solute masses can be too small to measure on a balance.
Each succeeding sample is made with the
same ratio of sample and diluent as the one before
500 mL
1M NaCl
1 M NaCl
500 mL
0.5M NaCl
0.5 M NaCl
500 mL
0.25M NaCl
0.25 M NaCl
0.125 M NaCl
Each of the above dilutions is one part previous sample and one part solvent. This is
called a 1:2 dilution, or one part sample to two total parts.
If the technician needs 150 mL of the 0.1 M TRIS, then a dilution of the concentrated 1
M TRIS can be calculated. To figure out how to dilute something from a concentrated
solution, we use a simple ratio equation as shown in the following equation:
Solution Preparation 5
Diluting Concentrated Solutions Equation
C1V1 = C2V2
C1 = the concentration of the concentrated stock solution (the starting solution)
V1 = the volume to use of the stock solution in the diluted sample
C2 = the desired concentration of the diluted sample
V2 = the desired volume of the diluted sample
The C1V1 = C2V2 equation may be used with any concentration units (i.e., mass/volume,
%, or molar) as long as the units are the same on each side of the equation (for
canceling purposes). Using the equation with the scenario above in which the
technician needs 150 mL of the 0.1 M TRIS,
C1 = 0.1 M TRIS (the starting solution)
V1 = amount of the concentrate to use for the dilution
C2 = 0.01 M TRIS (the working solution)
V2 = 150 mL
(0.1 M)(V1) = (0.01 M)(150 mL)
V1 = (0.01 M)(150 mL)/(0.1 M)
V1 = 15 mL
Therefore, to make 150 mL of 0.01 M TRIS from the concentrated stock, measure out 15
mL of the concentrated 0.1 M TRIS stock and add 135 mL of deionized water to is and
mix well.
Determine the calculations for the solutions in the Practice Problems section for Part E.
Making Dilutions of Concentrated Solutions at the end of this handout.
Solution Preparation 6
Practice Problems
Part A. Making Metric Conversions
Table 1. Conversion Table
Measurement
Conversion
75.34 mg
g
4.3 mL
μL
440.3 mL
L
3.33 g
μg
0.34 g
mg
34.0 g
kg
0.004 L
mL
80.34 μL
mL
34 mg
μg
0.25 g
ng
Part B. Making Solutions of Differing Mass/Volume Concentrations
1. Describe how you would prepare 25 mL of a NaCl solution at a concentration of 2.5
g/mL.
2. Describe how you would prepare 2 L of a 0.5 g/mL dextrose solution.
Solution Preparation 7
Part C. Making Solutions of Differing % Mass/Volume Concentrations
1. Describe how you would prepare 200 mL of an 8% NaCl solution.
2. Describe how you would prepare 0.75 L of a 5% dextrose solution.
Part D. Making Solutions of Differing Molarity Concentrations
1. Describe how you would prepare 125 mL of a 10 M NaOH solution.
2. Describe how you would prepare 75 mL of a 0.1 M NaCl solution.
Solution Preparation 8
Part E. Making Dilutions of Concentrated Solutions
1. Describe how you would prepare 950 mL of a 1X CuSO4•5H2O solution from a 25X
CuSO4•5H2O stock.
2. Describe how you would prepare 50 mL of a 5 mM NaCl solution from a 1 M NaCl
stock.
Solution Preparation 9