# DBT JRF 2022 — Question 10

Exam: DBT JRF 2022 · Section: A · Topic: Biochemistry › Buffer Preparation & Solution Chemistry · Difficulty: Medium

## Question

You are preparing a 500 ml solution containing 50 mM Tris (FW 120) buffer, 10 mM NaCl (FW 58.5), 5 mM EDTA (supplied solution is 500 mM). The quantity of each of the ingredients would be:

## Options

- **A.** 6.0 g Tris, 2.92 g NaCl, 5 ml EDTA
- **B.** 3.0 g Tris, 0.292 g NaCl, 5 ml EDTA
- **C.** 1.5 g Tris, 0.292 g NaCl, 5 ml EDTA
- **D.** 0.75 g Tris, 0.029 g NaCl, 5 ml EDTA

## Answer

Option B

## Explanation

Buffer preparation calculations for 500 mL: Using the formula: $\text{Mass} = \text{Molarity} \times \text{Volume (L)} \times \text{Molecular Weight (g/mol)}$ 1. Tris (FW = 120 g/mol, 50 mM): $m = 0.050\,\text{mol/L} \times 0.5\,\text{L} \times 120\,\text{g/mol} = 3.0\,\text{g}$ 2. NaCl (FW = 58.5 g/mol, 10 mM): $m = 0.010 \times 0.5 \times 58.5 = 0.292\,\text{g}$ 3. EDTA (from 500 mM stock, need 5 mM in 500 mL): Using $C_1V_1 = C_2V_2$ : $V_1 = \frac{5\,\text{mM} \times 500\,\text{mL}}{500\,\text{mM}} = 5\,\text{mL}$ Answer: 3.0 g Tris, 0.292 g NaCl, 5 mL EDTA (Option 2).

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