Serial Dilutions

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Making Molar Solutions and Dilutions

After performing this lab, the student should be able to:
• Calculate grams of solute and correctly prepare a molar solution. • Prepare parallel and serial dilutions using C1V1 = C2V2 • Distinguish a parallel dilution from a serial dilution. • Determine whether a parallel or serial dilution should be used in a given situation. • Use a microcentrifuge to make a pellet.

Making Molar Solutions and Dilutions
A major job of any biotechnician is the preparation of solutions. Accuracy is of the utmost importance. An incorrectly prepared solution can destroy months of hard work or cost companies thousands of dollars. Therefore, several safeguards are in place in most SOPs (Standard Operating Procedures) to ensure that mistakes are minimized. First, all calculations are written down in the lab notebook, even though a calculator is used to do the arithmetic. Second, important calculations are double-checked by another person (and sometimes triple-checked). Third, the exact mass or volume of each reagent used in making the solution is recorded in the lab notebook. Fourth, this information and more, is recorded on a data sheet and on a label on the bottle itself.

Weight per volume is the simplest way of expressing a concentration. This is often used for small amounts of chemicals and specialized biological reagents. For example, enzyme concentrations are often given as weight per volume-- 1 mg/µL DNA.

Percents may be:
1) weight per volume percent which is the grams of solute per 100 mL of solution (for solids in liquids) 2) volume percent, the mL solute per 100 mL solution (for liquids in liquids) 3) weight percent which is the grams of solute per 100 g solution (for solids in solids) Note that in all cases a 100 mL (or 100 g) solution is used since percent means “out of 100”. Molarity is by far the most commonly used solution calculation in the lab. Molarity is equal to the number of moles of a solute that are dissolved per liter of solution. It is written as M. In order to calculate molarity, you need to know moles. Moles are a standard number of molecules, but since different molecules have different weights, we need to know how much a molecule weighs—its molecular weight (MW) or formula weight (FW). The FW may be obtained from the bottle of reagent or by adding up the atomic weights of each of the atoms in that molecule (remember to add the weight of any associated water molecules indicated in the chemical formula). Formula: grams (to be measured) = FW x M x L Don’t forget to convert mL to L. Example: To prepare 100 mL of 1 M NaOH (FW 40.0).

g = FW x M x
g = 40 g/mole x 1 mole/liter x 0.100 L
g = 4
measure 4 g of NaOH pellets.
How many grams of NaCl would you need to prepare 500 mL of a 1 M solution. The gram formula weight of sodium chloride is 58.44 g/mol.

Each group needs: Class Shares:
5 small beakers or flasks (150 – 250 mL)
25 or 50 mL graduated cylinder
10 mL graduated cylinder OR
10 mL pipet and filler or bulb
100 – 1000 µl micropipet OR
1 mL pipet OR
disposable transfer pipette calibrated to 0.5 mL
glass stirring rod OR
stir plate or stirring hot plate and stir bar
test tubes (4)
test tube rack
microcentrifuge tubes (3)
microcentrifuge tube rack
weigh boats (2 per group)
masking tape or labeling tape
permanent marker
Calcium chloride – CaCl2 • 2H20 (F.W. 142.9)
Magnesium sulfate – MgSO4 • 7H2O (F.W. 246.48)
1 M HCl
vortexer (optional)
pH meters and pH standard buffers for calibration OR
universal pH indicator strips
Procedure *denotes step must be checked by instructor before going on PART A: MAKING MOLAR SOLUTIONS
1. *Calculate the number of grams needed to make 20 mL of a 2 M solution of calcium chloride (F.W. 142.9). Formula weights can vary depending...
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