How would you prepare a 500 mL of a 0.20 M solution starting with solid NaH2PO4*H2O?
The correct answer and explanation is :
Answer:
To prepare 500 mL of a 0.20 M solution of sodium dihydrogen phosphate monohydrate (NaH₂PO₄·H₂O), follow these steps:
- Calculate the required mass of NaH₂PO₄·H₂O:
- Molar mass of NaH₂PO₄·H₂O = 137.99 g/mol
- Desired molarity = 0.20 M
- Desired volume = 500 mL = 0.500 L
- Moles needed = Molarity × Volume
[
0.20 \text{ mol/L} \times 0.500 \text{ L} = 0.10 \text{ mol}
] - Mass required = Moles × Molar Mass
[
0.10 \text{ mol} \times 137.99 \text{ g/mol} = 13.80 \text{ g}
]
- Weigh the solid NaH₂PO₄·H₂O:
- Use an analytical balance to measure 13.80 g of NaH₂PO₄·H₂O accurately.
- Dissolve in distilled water:
- Transfer the weighed solid to a 500 mL volumetric flask or a beaker.
- Add approximately 300-400 mL of distilled water to dissolve the solid.
- Stir or swirl the solution until the solid dissolves completely.
- Dilute to the final volume:
- Once the solute is completely dissolved, transfer the solution to a 500 mL volumetric flask (if using a beaker).
- Add distilled water up to the 500 mL mark.
- Mix thoroughly by inverting the flask several times.
- Final Steps:
- Label the solution with “0.20 M NaH₂PO₄·H₂O” and store properly.
Explanation:
This procedure ensures accurate preparation of a 0.20 M NaH₂PO₄·H₂O solution. The key principle is using the molarity equation:
[
\text{Molarity (M)} = \frac{\text{moles of solute}}{\text{liters of solution}}
]
By calculating the moles required and converting it into grams using the molar mass, we determine the exact amount of NaH₂PO₄·H₂O to weigh. The volumetric flask ensures precision when diluting to the final volume, preventing concentration errors. Mixing thoroughly ensures homogeneity, providing a uniform solution for experimental use.