BACE Solution Preparation and Calculations 2 — Questions and Answers
Question 1: How much NaCl (MW = 58.44 g/mol) is needed to prepare 500 mL of a 0.9% (w/v) saline solution?
- 0.45 g
- 4.5 g (Correct answer)
- 9.0 g
- 0.9 g
Correct answer: 4.5 g
0.9% (w/v) means 0.9 g per 100 mL. For 500 mL: 0.9 g/100 mL x 500 mL = 4.5 g NaCl.
Weight/volume percent (% w/v) is defined as grams of solute per 100 mL of solution x 100%. To calculate mass for any volume: mass (g) = (% w/v / 100) x volume (mL) = (0.9/100) x 500 mL = 4.5 g. Weigh 4.5 g NaCl, dissolve in approximately 450 mL water in a 500 mL volumetric flask, then add water to the 500 mL mark.
Question 2: A laboratory stock solution of HCl is labeled '12 M'. How much stock is needed to make 100 mL of 1 M HCl?
- 1.2 mL
- 8.33 mL (Correct answer)
- 12 mL
- 120 mL
Correct answer: 8.33 mL
Using C1V1 = C2V2: (12 M)(V1) = (1 M)(100 mL), so V1 = 100/12 = 8.33 mL of stock.
The dilution equation C1V1 = C2V2 is fundamental for solution preparation. C1 = 12 M, C2 = 1 M, V2 = 100 mL, solve for V1: V1 = (C2 x V2)/C1 = (1 x 100)/12 = 8.33 mL. ALWAYS add concentrated acid to water (not water to acid) — 'AAA: Always Add Acid to water' — to prevent dangerous spattering from the heat of dilution.
Question 3: What does 'pH 7.4 with NaOH' typically mean when listed in a buffer preparation protocol?
- Add NaOH until the solution becomes colorless
- After dissolving all buffer components, measure pH and adjust to 7.4 by adding NaOH (to raise pH) or HCl (to lower pH) dropwise (Correct answer)
- Prepare the buffer using NaOH as the sole pH-determining component
- Add a fixed amount of 7.4 mL of NaOH to the solution
Correct answer: After dissolving all buffer components, measure pH and adjust to 7.4 by adding NaOH (to raise pH) or HCl (to lower pH) dropwise
Buffer protocols require pH adjustment after dissolving all components, adding NaOH to raise pH to the target value, with HCl available to correct any overshoot below target.
Buffer preparation requires: (1) dissolve all components in approximately 80% of the final volume of water, (2) measure the current pH with a calibrated pH meter, (3) add NaOH solution to raise pH or HCl to lower pH, (4) when within 0.2 pH units of target, add adjusting agent very slowly dropwise to avoid overshoot, (5) add water to reach total volume after pH is correct.
Question 4: A protocol requires a '10x PBS' stock solution. What concentration of NaCl would you expect in the stock, compared to the 1x working solution that contains 137 mM NaCl?
- 13.7 mM (one-tenth the working concentration)
- 1,370 mM (137 mM x 10) (Correct answer)
- 137 mM (same as working — stocks are not more concentrated)
- 274 mM (2x the working concentration)
Correct answer: 1,370 mM (137 mM x 10)
A 10x stock is 10 times more concentrated than the 1x working solution; 137 mM x 10 = 1,370 mM NaCl in the stock.
A 10x PBS stock contains all PBS components at 10-fold concentration; diluting 1 part stock with 9 parts water yields 1x PBS. NaCl in 1x PBS = 137 mM; NaCl in 10x PBS = 1,370 mM. Concentrated stock solutions reduce storage space and preparation time. Some buffers precipitate at high concentration and require warming before use.
Question 5: When preparing a calibration standard curve for a Bradford assay, why must standards be prepared in the same buffer as the protein samples?
- To ensure the standard proteins denature at the same temperature as the samples
- Because buffer components can affect the Bradford dye color reaction, potentially causing systematic error if standards and samples are in different buffers (Correct answer)
- To allow both standards and samples to be autoclaved together for sterility
- Buffer-matched standards are only required for mass spectrometry not colorimetric assays
Correct answer: Because buffer components can affect the Bradford dye color reaction, potentially causing systematic error if standards and samples are in different buffers
Buffer components (detergents, reducing agents, chaotropes) can interfere with the Bradford dye reaction; using matched buffers for standards ensures that any interference is constant across all measurements.
Bradford reagent is sensitive to detergents (SDS, Triton X-100) and basic compounds (Tris, NaOH) that can shift the Coomassie dye's equilibrium absorbance. If samples are in Tris buffer but standards are in water, the standard curve will be inaccurate for the samples. Solution: prepare BSA standards in the same lysis/sample buffer. Alternatively, use the Bradford micro format with diluted samples where buffer components are below interference threshold.
Question 6: What is the molar concentration of a glucose solution prepared by dissolving 18.02 g of glucose (MW = 180.2 g/mol) in water to a final volume of 1 liter?
- 0.01 M
- 0.1 M (Correct answer)
- 1.0 M
- 10 M
Correct answer: 0.1 M
Moles of glucose = 18.02 g divided by 180.2 g/mol = 0.1 mol. Concentration = 0.1 mol / 1.0 L = 0.1 M.
Molarity (M) = moles of solute / liters of solution. Step 1: calculate moles = mass (g) / molecular weight (g/mol) = 18.02 / 180.2 = 0.1 mol. Step 2: divide by volume in liters = 0.1 mol / 1.0 L = 0.1 M. This is a 100 mM glucose solution. For reference, blood glucose is approximately 5 mM and typical cell culture media contains 25 mM glucose (DMEM high glucose).
BACE Solution Preparation and Calculations 2
How much NaCl (MW = 58.44 g/mol) is needed to prepare 500 mL of a 0.9% (w/v) saline solution?