Biology Answers

Question 12: The last step in the protocol is to autoclave the 10x PBS solution. Why do you think this step is important? Look up the definition of autoclave if you are unsure what it means.

Question 11: The PBS protocol above says to adjust pH to 7.4 with HCl. What does this imply on the pH of 10x PBS before adjusting the pH, would it be greater or smaller than 7.4?

The recipe for PBS says to dissolve compounds in 800 ml of water, adjust the pH to 7.4, then add water up to 1 liter. The final pH should still be 7.4, because the pH of buffer solutions remains stable when they are diluted as long as the concentration of its constitutive acid and base is not too low.

Question 10: Why do you think the protocol does not say to dissolve compounds directly in 1 liter of water?

Question 9: What should be done before measuring an unknown pH of a solution using a pH meter?

Question 8: If you had the choice between a 1 liter beaker and a 1 liter graduated cylinder, which one should you use to measure volumes with maximal precision when making 1 liter of PBS?

Question 7: If you have excess reagent on the weighing boat, what should you avoid doing and why?

Question 6: What is the first thing to do after putting a weighing boat on the balance?

Question 5: As previously stated, the concentration of NaCl, KCl and Phosphate in working strength 1 x PBS is 0.137M NaCl, 0.012M Phosphate, 0.0027M KCl, pH 7.4 How do they compare to the concentrations you calculated for 10x PBS?

Question 4: Using periodic table found in your textbook, calculate (to 2 decimal places) the molecular mass for each of the compounds used to make PBS.

Create the following table and fill it in with the mass of each component required to make 1 liter of 10 x PBS (the recipe for 10x PBS is below question 2) and their final molar concentration in the buffer calculated as described above.

Compound formula Molecular mass (in g/mol) Mass of compound per liter of 10x PBS (in g) Molar concentration (in mol/l)
NaCl
KCl
Na2HPO4
KH2PO4

Question 3: Which ions are being produced by this process, assuming that each of the chemical compounds dissociate into their constituent parts once they are dissolved in water?

Preparation of the correct buffer is key to any good biological experiment and it is important that you understand how to calculate the mass of each chemical required to make that buffer and what the resulting concentration of those constituents will be in moles per liter.
Your text book explains that moles are just a way to express the amount of a substance, such that one mole is equal to 6.02 x 1023 particles of that substance. T