*no need to do question 14
How to make solutions in the lab: Concentrations and Dimensional analysis
COMPLETE THE PROBLEMS ON A SEPARATE SHEET OF PAPER. SHOW YOUR WORK.
1. Write three ways in which concentration of a substance can be measured (in other words, give three units that indicate concentration):
2. You have this stock (highly concentrated) solution on your lab bench: 100 mL of 1 mg/mL BSA (bovine serum albumin). Explain steps for how this solution can be made starting with solid (pure) BSA.
3. Use dimensional analysis to determine the total mass of BSA in your solution. HINT: If you correctly set up your factors (fractions, e.g., mg/mL) that you plan to multiply, you will be able to cancel all units that are same in numerators and denominators, and all that will remain is the unit of your answer. Here, that remaining unit should be mg.
You have this stock solution on your lab bench: 10 mL of 15% (m/v) NaCl
x% (m/v) = x g/100 mL total volume; unless stated otherwise, your solvent is water
% (m/v) is another unit of concentration; sometimes simply written as %; m/v means mass/volume
4. Solve for x: 15% NaCl = x g NaCl/100 mL
5. Use dimensional analysis to determine the total mass of NaCl in your solution. HINT: Again, if you correctly set up your factors (fractions, e.g., g/mL) that you plan to multiply, you will be able to cancel all units that are same in numerators and denominators, and all that will remain is the unit of your answer. Here, that remaining unit should be g.
Scientists often work with another useful unit of concentration, molarity (M), which tells us how many units (e.g., molecules) of a substance are dissolved in our solution.
Moles (mol) tells you the number of units of your substance
1 mol of a substance = 6.02x1023 units of the substance (6.02x1023 is “Avogadro’s number”)
This is true for any substance, a pure element, salt, molecule, compound of any size, or even ants for that matter
The mass of 1 mol of a substance is equivalent to its atomic mass (aka molecular weight, MW) in grams
For example, if a substance has an atomic mass of 18, then 1 mol of the substance contains 18 g; we can say the MW is 18 g/mol
FYI: We express the MW of proteins (such as BSA) and other biological macromolecules using a unit called Daltons (Da)
1 Da = 1 g/mol 1 kDa = 1000 g/mol
6. Use a periodic table to determine the number of moles of NaCl in your solution. (For the purposes of this exercise, pretend NaCl is a molecule and not dissociated ions in solution, although you’d get the same result either way.) Use dimensional analysis, and show your work.
Now you can determine the molarity (M) of your NaCl solution.
Molarity is a unit of concentration that tells you how many moles you have in a certain total volume. Again, unless stated otherwise, the solvent is water.
1 M = 1 mol/L 1 mM = 1 mmol/L
7. Determine the molarity of your solution.
8. Now put this all together into one calculation! Use dimensional analysis to determine the molarity of your solution, starting with the given concentration of 15% NaCl. HINT: If you set up your ratios correctly and cancel all the units, all that should remain is mol/L
9. These three ways of indicating concentration, mg/mL, %, and M give us very different information about how much “stuff” is in a solution. To help think about this, answer the following questions:
Which has more mass, one mole of students or one mole of ants?
Which contains more mass of solute, 10 mL of a 1M NaCl solution or 10 mL of a 1M BSA solution (BSA is a large protein)?
Which has more mass of solute, 10 mL of a 15% solution of NaCl or 10 mL of a 15% solution of BSA?
Which has more mass of solute, 100 mL of a 1 mg/mL solution of NaCl or 100 mL of a 1 mg/mL solution of BSA?
10. You are doing an experiment in which you need 100 μmol NaCl. Calculate the volume of your 15% NaCl stock that you need.
11. You want to make 50 mL of a 200 mM solution of NaCl from your stock solution. Use dimensional analysis to determine the volume of your stock solution needed and the volume of water needed.
An easier way to do this dimensional analysis problem: Use the formula CiVi = CfVf
Where Ci is the initial concentration (of your more concentrated solution), Vi is the volume of that solution needed to make the final solution, Cf is the concentration of the final solution, and Vf is the final volume of that solution that you want. One of these variables will be the unknown that you must solve for. Which one? Make sure you use consistent units.
12. You have pure (solid) NaCl. Use dimensional analysis to calculate how much NaCl you need to make 50 mL of a 200 mM NaCl solution. Then describe how you will make this solution.
13. Calculate the molarity of the 1 mg/mL BSA solution.
14. [Once the lab is complete] Calculate the molarity of BSA in one of your own unknowns. Exchange your work with a partner who calculated the molarity in another unknown and check his or her work.
15. How many molecules of BSA are in 50 μL of the 1 mg/mL solution?
16. An amino acid has an approximate MW of 110 Da. Approximately how many amino acids are in BSA? Look up the actual answer to check your estimate.