BACE - Biotechnician Assistant Credentialing Solution Preparation and Calculations Questions and Answers 1 — Questions and Answers
Question 1: A technician needs to prepare 500 mL of a 0.5 M Tris-HCl buffer. The formula weight of Tris is 121.14 g/mol. How many grams of Tris should be weighed out?
- 121.14 g
- 60.57 g
- 30.29 g (Correct answer)
- 500 g
Correct answer: 30.29 g
To calculate the mass of solute needed, use the formula: Mass (g) = Molarity (mol/L) x Volume (L) x Formula Weight (g/mol). First, convert the volume from mL to L: 500 mL = 0.5 L. Then, plug in the values: Mass (g) = 0.5 mol/L x 0.5 L x 121.14 g/mol = 30.285 g, which is rounded to 30.29 g.
Question 2: A biotechnician is asked to make a 1:10 dilution of a concentrated stock solution to a final volume of 100 mL. Which of the following correctly describes the procedure?
- Mix 1 mL of stock solution with 99 mL of diluent.
- Mix 10 mL of stock solution with 100 mL of diluent.
- Mix 90 mL of stock solution with 10 mL of diluent.
- Mix 10 mL of stock solution with 90 mL of diluent. (Correct answer)
Correct answer: Mix 10 mL of stock solution with 90 mL of diluent.
A 1:10 dilution means 1 part of the stock solution is mixed with 9 parts of the diluent for a total of 10 parts. To get a final volume of 100 mL, you would divide the total volume by the total number of parts (100 mL / 10 parts = 10 mL/part). Therefore, you need 1 part stock (10 mL) and 9 parts diluent (90 mL).
Question 3: A lab protocol requires the preparation of a 2% (w/v) agarose gel solution in a total volume of 150 mL. How much agarose powder is needed?
- 2.0 g
- 0.3 g
- 3.0 g (Correct answer)
- 30.0 g
Correct answer: 3.0 g
A weight/volume (w/v) percentage solution is defined as grams of solute per 100 mL of solution. A 2% (w/v) solution means 2 g of solute in 100 mL of solvent. To find the amount needed for 150 mL, you can set up a proportion: (2 g / 100 mL) = (x g / 150 mL). Solving for x gives: x = (2 g * 150 mL) / 100 mL = 3.0 g.
Question 4: You need to prepare 250 mL of a 1X working solution from a 20X stock solution. How much of the 20X stock solution is required?
- 10.0 mL
- 12.5 mL (Correct answer)
- 20.0 mL
- 25.0 mL
Correct answer: 12.5 mL
This is a dilution calculation that can be solved using the formula C1V1 = C2V2, where C1 is the initial concentration, V1 is the initial volume, C2 is the final concentration, and V2 is the final volume. Plugging in the values: (20X)(V1) = (1X)(250 mL). Solving for V1 gives: V1 = (1X * 250 mL) / 20X = 12.5 mL.
Question 5: Which of the following is the most critical final step before finalizing the volume of a buffer solution prepared from solid components?
- Heating the solution to ensure all solutes are dissolved.
- Allowing the solution to sit for at least one hour.
- Vortexing the solution vigorously for five minutes.
- Adjusting the pH to the desired value. (Correct answer)
Correct answer: Adjusting the pH to the desired value.
When preparing a buffer, after dissolving the solid components in a volume of solvent less than the final desired volume, the pH must be checked and adjusted. The pH can change as components dissolve. The final volume is brought up to the mark only after the pH has been correctly set.
Question 6: A technician prepares a solution by dissolving 58.44 grams of NaCl (Formula Weight = 58.44 g/mol) in enough water to make a final volume of 1.0 L. What is the molarity (M) of this solution?
- 0.1 M
- 0.5 M
- 1.0 M (Correct answer)
- 2.0 M
Correct answer: 1.0 M
Molarity is defined as moles of solute per liter of solution. First, calculate the number of moles of NaCl: Moles = Mass (g) / Formula Weight (g/mol) = 58.44 g / 58.44 g/mol = 1.0 mole. Since the final volume is 1.0 L, the molarity is 1.0 mole / 1.0 L = 1.0 M.
BACE - Biotechnician Assistant Credentialing Solution Preparation and Calculations Questions and Answers 1
A technician needs to prepare 500 mL of a 0.5 M Tris-HCl buffer.
The formula weight of Tris is 121.14 g/mol.
How many grams of Tris should be weighed out?