Solution Preparation and Calculations Flashcards

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?

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.

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?

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).

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?

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.

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?

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.

Which of the following is the most critical final step before finalizing the volume of a buffer solution prepared from solid components?

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.

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?

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.