FREE BACE Preparing solutions Flashcards

What equipment is required to accurately prepare a 1.00 M NaCl solution in a volume of exactly 100 mL?

Answer: A 100 mL volumetric flask, an analytical balance, and a magnetic stir plate

A volumetric flask provides exact final volume, an analytical balance provides accurate mass measurement — both are required for a 1.00 M concentration with proper significant figures.

What is the purpose of adding sterile water to a lyophilized (freeze-dried) enzyme powder when reconstituting it for laboratory use?

Answer: To dissolve and hydrate the enzyme restoring its active three-dimensional conformation and activity

Lyophilization removes water from biological materials for stable dry storage; reconstitution with sterile water restores the aqueous environment needed for proper protein folding and enzymatic activity.

A protocol requires a 1:5 serial dilution series starting with a 1 mg/mL protein stock. What is the protein concentration of the third dilution in the series?

Answer: 0.008 mg/mL

Each 1:5 dilution divides the concentration by 5. Starting at 1 mg/mL: 1st dilution = 0.2 mg/mL, 2nd = 0.04 mg/mL, 3rd = 0.008 mg/mL.

When preparing a solution of a solid reagent, why should you add solvent to the solute rather than adding solute to a pre-measured volume of solvent?

Answer: Adding the solid first and then adding solvent while monitoring volume prevents overshoot of the target volume as dissolution can change the total volume

Dissolving solids changes volume. By dissolving first in less solvent then adjusting to final volume, you avoid overshooting the target volume.

What does anhydrous mean when referring to a reagent and why does it matter for solution preparation?

Answer: Anhydrous means the compound contains no water of hydration — important because hydrated forms have higher molecular weights and using the wrong form will give incorrect concentrations

Anhydrous reagents contain no incorporated water molecules, while hydrated forms include water molecules in their crystal structure, giving a different molecular weight — using the wrong form without correcting the calculation produces incorrect concentrations.

How do you prepare 100 mL of a 20% (v/v) ethanol solution from pure (100%) ethanol?

Answer: Add 20 mL of 100% ethanol to a 100 mL volumetric flask and fill to the mark with water

A 20% v/v solution requires 20 mL of ethanol per 100 mL total volume. Add 20 mL of ethanol to a 100 mL volumetric flask and fill to the 100 mL mark with water.