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# Activity 3a Making Solutions of Differing Mass/Volume … - convert mg ml to molarity Lab 3: Making Solutions
Making solutions is a very common activity for lab workers in a biotechnology lab. Proper
solution making requires basic math skills, accurate measurement, and the ability to follow
instructions.
A solution is a homogeneous mixture of two or more substances. In a solution, the
solute is the substance that is dissolved in the solvent. Most of the time, the solvent will be
H2O, so if it is not otherwise specified, assume that you should dissolve the necessary amount of
solute calculated in H2O.
Activity 3a
Making Solutions of Differing Mass/Volume Concentrations
Purpose and Background
In this activity, you will learn how to calculate and make copper sulfate (CuSO4) solutions of
differing mass/volume concentrations.
Solutions are prepared with a certain mass of solute in a certain volume of solvent, similar to
the glucose solutions made in Activity 2c. Any metric mass in any metric volume is possible, but
the most common units of mass/volume concentrations are as follows:
g/mL grams per milliliter
g/L grams per liter
mg/mL milligrams per milliliter
?g/mL micrograms per milliliter
?g/?L micrograms per microliter
ng/mL nanograms per milliliter
ng/?L 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. The prefix "nano-" means one-
billionth. A nanogram is equal to 0.001 ?g, and there are 1000 ng in 1 ?g.
To determine how to prepare a certain volume of a solution at a certain mass volume
concentration, use the equation that follows.
Mass/Volume Concentration Equation:
(Volume desired) x (Concentration desired) = (mass of solute needed)
(for ex: mL) (for ex: g/mL) (for ex: g)
Note:
- the units in the numerator & denominator should cancel if the equation is set up properly.
- you must make sure that the units used in the calculation can be cancelled, and you may need
to convert units within the metric system if necessary.
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Example: You need to make 20 mL of a 40 mg/mL solution of glucose. How many grams of
glucose will you need to measure to make this solution?
(20 mL) (40 mg) = 800 mg but you want to know how many g, not mg,
mL so convert 800 mg to grams (= 0.8 g)
Notice how the mL units cancel out during multiplication so as to leave the answer in mg to
be weighed out. Since the balances measure in grams, it was necessary to convert mg to g.
To make this solution, 0.8 g of glucose is weighed and put into a graduated 50 mL tube.
Approximately 15 mL (~75 % of final volume) of solvent (deionized water or buffer) is added to
the tube, and the contents are mixed using the vortex. Then, additional solvent is added to
reach a total volume of 20 mL. The reason that not all 20 mL of solvent is added initially is that
the solute may take
up some space when Add ~75% of solvent
it dissolves in the volume; mix to dissolve;
solvent. Thus, if we add more solvent to reach
to the solute at once,
the final volume may
have in fact been
greater than 20 mL.
Procedure
1. Label all tubes
with the sample
name and
concentration,
and your 10 g of solute
group's initials.
2. Review the use of the balance and weigh boats before beginning.
3. Do the calculations necessary to prepare the solutions in Table 3.1 for tube numbers 1
through 5. Use the Mass/Volume equation to determine the mass of CuSO4 to measure in
order to give the correct concentration at the volume desired for each sample.
Table 3.1 CuSO4 Mass/Volume Solution Preparation
Tube CuSO4 Total Concentrat'n Calculation of Mass Needed
# solution Volume (mg/mL) (g)
1 5.0 mL of 300 mg/mL 5.0 mL 300 mg/mL 5.0 mLx300 mg =1500 mg = 1.5 g
mL
2 4.5 mL of 150 mg/mL
3 4.0 mL of 75 mg/mL
4 3.5 mL of 37.5 mg/mL
5 3.0 mL of 18.75 mg/mL
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4. Prepare the solutions in labeled 15 mL plastic tubes, using deionized water as the solvent.
Use the vortex on your bench to help dissolve the CuSO4. You may need to let the more
concentrated samples sit in the warm water bath for several minutes in order for the CuSO4
to dissolve fully.
5. Is the difference in concentration of the tubes obvious in one tube versus another? Compare
your tubes' colors and volumes to the standard "key" solutions prepared by the instructor. If
any of the volumes or colors are obviously wrong, try to identify where you may have made
an error. Then dump them out, and remake them.
6. Keep tube #1 for use in Activity 3d.
Activity 3b
Making Solutions of Differing % Mass/Volume Concentrations
Purpose and Background
In this activity, you will learn how to prepare solutions of specific % mass/volume
concentrations. You will prepare several % mass/volume solutions and use some of them as
testing reagents. The underlying rule for % mass/volume solutions is that a 1% solution contains
1 g of solute in a total volume of 100 mL.
Solutions of Differing % Mass/Volume Concentrations
% Mass/Volume Concentration Equation steps:
Rule: a 1% solution means 1 g solute in 100 ml solvent
Step 1: Convert the % to a decimal by dividing % by 100.
This changes the units to g/mL
Step 2: (same as in previous equation for mass/volume concentrations)
(Volume desired) x (Concentration desired) = (mass of solute needed)
(for ex: mL) (for ex: g/mL) (for ex: g)
Procedure
You will do the calculations for and prepare the following solutions:
? 5 mL of 10% NaOH solution *
? 5 mL of 5% CuSO4 solution
? 5 mL each of 5%, 2.5%, 1.25%, and 0.625% gelatin solutions
As an example, I will do the calculation for the preparation of 5 mL of 10% NaOH solution. I
will also prepare this solution for you, because NaOH is extremely caustic.
Step 1: Convert 10% to a decimal by dividing by 100: (10%) / 100 = 0.1 g/mL
Step 2: (5 mL) (0.1 g) = 0.5 g
mL
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How do you calculate molar concentration? How do you calculate molar concentration on a calculator? As mass / volume = molarity * molar mass , then mass / (volume * molar mass) = molarity . Substitute the known values to calculate the molarity: molarity = 5 / (1.2 * 36.46) = 0.114 mol/l = 0.114 M .

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