Free BACE Preparing solutions Questions and Answers 2 — Questions and Answers
Question 1: What equipment is required to accurately prepare a 1.00 M NaCl solution in a volume of exactly 100 mL?
- A beaker, a top-loading balance, and a graduated cylinder
- A 100 mL volumetric flask, an analytical balance, and a magnetic stir plate (Correct answer)
- A 100 mL beaker, a triple beam balance, and distilled water
- A 250 mL Erlenmeyer flask, a digital scale, and a hot plate
Correct 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.
To prepare exactly 1.00 M NaCl in 100 mL: weigh 5.844 g NaCl on an analytical balance, dissolve in approximately 80 mL of distilled water in a 100 mL volumetric flask, then add water to exactly the 100 mL calibration mark. Beakers and graduated cylinders have ~5% error; volumetric flasks are rated to 0.1% or better. Using a beaker would give ~5 mL error in a 100 mL preparation — unacceptable for accurate molarity.
Question 2: What is the purpose of adding sterile water to a lyophilized (freeze-dried) enzyme powder when reconstituting it for laboratory use?
- To denature any contaminating proteins present in the lyophilized powder
- To dissolve and hydrate the enzyme restoring its active three-dimensional conformation and activity (Correct answer)
- To dilute the enzyme to below the concentration that causes autocatalytic degradation
- To lower the pH of the enzyme preparation to the optimal level for activity
Correct 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.
During reconstitution: (1) use the volume specified in the product sheet, (2) add liquid to the side of the vial without direct dispersion onto the powder to avoid foaming, (3) gently swirl or roll to dissolve — do not vortex vigorously, (4) allow time for complete dissolution, (5) aliquot into single-use amounts and freeze at -20°C to avoid repeated freeze-thaw cycles.
Question 3: 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?
- 0.2 mg/mL
- 0.04 mg/mL
- 0.008 mg/mL (Correct answer)
- 0.001 mg/mL
Correct 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.
Serial dilution: each step adds 1 part of the previous solution to 4 parts diluent (1:5 = 1/5 dilution factor). Starting at 1 mg/mL: after 1st = 0.2 mg/mL, after 2nd = 0.04 mg/mL, after 3rd = 0.008 mg/mL. The cumulative dilution factor after n steps = (1/5)^n = (1/5)^3 = 1/125. Serial dilutions are used for standard curves, plating bacteria for colony counting, and titrating viruses or antibodies.
Question 4: 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?
- Adding solvent to solute prevents air bubbles from forming in the solution
- Adding the solid first and then adding solvent while monitoring volume prevents overshoot of the target volume as dissolution can change the total volume (Correct answer)
- Adding solvent to solute is faster because the solid dissolves more quickly when solvent is added dropwise
- There is no difference — either order gives the same result if the final volume is checked
Correct 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.
When a solute dissolves, it occupies spaces between solvent molecules so the final volume is not simply the sum of volumes. Correct procedure: (1) weigh solute into the volumetric flask, (2) add approximately 80% of the final volume of solvent and mix to dissolve, (3) allow to equilibrate to room temperature, (4) add final solvent up to the calibration mark. If you add solute to exactly 100 mL of water, you will end up with slightly more or less than 100 mL, giving an inaccurate concentration.
Question 5: What does anhydrous mean when referring to a reagent and why does it matter for solution preparation?
- Anhydrous means the compound has been purified by recrystallization from water
- 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 (Correct answer)
- Anhydrous means the compound has been certified sterile and endotoxin-free for cell culture
- Anhydrous means the compound is a liquid that has had its water content reduced by distillation
Correct 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.
Many salts exist in both anhydrous and hydrated forms: Na2HPO4 (anhydrous, MW=141.96) vs Na2HPO4-7H2O (heptahydrate, MW=268.07). If a protocol calls for the anhydrous form but you use the heptahydrate without adjusting, you add less actual salt per gram weighed, giving a lower-than-intended concentration. Always verify the exact form and MW on your reagent bottle matches the protocol, or recalculate the mass needed.
Question 6: How do you prepare 100 mL of a 20% (v/v) ethanol solution from pure (100%) ethanol?
- Add 20 mL of 100% ethanol to a 100 mL volumetric flask and fill to the mark with water (Correct answer)
- Add 20 mL of water to 80 mL of 100% ethanol in a 100 mL flask
- Add 100 mL of ethanol to 20 mL of water and mix
- Dilute 100 mL of ethanol with enough water to make 20 mL total volume
Correct 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.
Volume/volume percent (% v/v) = (volume of solute / total volume) x 100. For 20% v/v in 100 mL total: 20 mL ethanol is needed. Add 20 mL of 100% ethanol to a 100 mL volumetric flask, add water to approximately 90 mL and mix, then bring to exactly 100 mL mark. Note: 70% ethanol is the standard laboratory disinfectant — more effective than 100% because water aids cell penetration and protein denaturation.
Free BACE Preparing solutions Questions and Answers 2
What equipment is required to accurately prepare a 1.00 M NaCl solution in a volume of exactly 100 mL?