Answer :
Below is a detailed step-by-step explanation for the problem.
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Part (a): Preparation of a 0.5 M NaOH Solution in 1000 mL
1. Calculate the Number of Moles Required
To prepare a 0.5 M solution in 1000 mL (which is 1 L), the number of moles needed is obtained using the relation
[tex]$$\text{moles} = \text{Molarity} \times \text{Volume}$$[/tex]
Thus,
[tex]$$\text{moles} = 0.5 \, \text{M} \times 1 \, \text{L} = 0.5 \, \text{moles}.$$[/tex]
2. Determine the Mass of Pure NaOH Needed
Using the molar mass of sodium hydroxide ([tex]$\text{NaOH}$[/tex]), which is approximately 40 g/mol, the mass of pure [tex]$\text{NaOH}$[/tex] required is
[tex]$$\text{Mass} = \text{moles} \times \text{Molar mass} = 0.5 \, \text{moles} \times 40 \, \text{g/mol} = 20 \, \text{g}.$$[/tex]
3. Adjust for the 96% Assay of the Pellets
Since the sodium hydroxide pellets are only 96% pure, you need to account for the purity by dividing the mass of pure [tex]$\text{NaOH}$[/tex] by the assay (expressed as a decimal). Thus, the required mass of pellets is
[tex]$$\text{Mass of pellets} = \frac{20 \, \text{g}}{0.96} \approx 20.83 \, \text{g}.$$[/tex]
4. Weigh the Pellets
Weigh approximately [tex]$20.83 \, \text{g}$[/tex] of sodium hydroxide pellets using an analytical balance.
5. Dissolve the Pellets
Transfer the weighed pellets into a beaker and add a small amount of deionized water to dissolve the sodium hydroxide completely.
6. Transfer and Dilute
Once the pellets are fully dissolved, transfer the solution into a 1000 mL volumetric flask. Then, dilute the solution with deionized water until the bottom of the meniscus reaches the 1000 mL mark. Ensure to mix thoroughly so the solution is homogeneous.
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Part (b): Explanation of the Dilution Equation
The dilution equation is given by
[tex]$$C_1 V_1 = C_2 V_2,$$[/tex]
where:
- [tex]$C_1$[/tex]: Initial concentration of the stock (concentrated) solution.
- [tex]$V_1$[/tex]: Volume of the stock solution that is taken for dilution.
- [tex]$C_2$[/tex]: Final concentration of the diluted solution.
- [tex]$V_2$[/tex]: Final total volume of the diluted solution.
This equation ensures that the number of moles of solute remains constant before and after the dilution.
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Part (c): Chemical Hazard of Water and First Aid Measures
Hazard: Hot water can cause scald burns.
First Aid: If exposed to scald burns from hot water, immediately cool the affected area with cool or lukewarm water for at least 10–15 minutes. Remove any wet or contaminated clothing and seek medical assistance if necessary.
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This completes the detailed step-by-step solution.
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Part (a): Preparation of a 0.5 M NaOH Solution in 1000 mL
1. Calculate the Number of Moles Required
To prepare a 0.5 M solution in 1000 mL (which is 1 L), the number of moles needed is obtained using the relation
[tex]$$\text{moles} = \text{Molarity} \times \text{Volume}$$[/tex]
Thus,
[tex]$$\text{moles} = 0.5 \, \text{M} \times 1 \, \text{L} = 0.5 \, \text{moles}.$$[/tex]
2. Determine the Mass of Pure NaOH Needed
Using the molar mass of sodium hydroxide ([tex]$\text{NaOH}$[/tex]), which is approximately 40 g/mol, the mass of pure [tex]$\text{NaOH}$[/tex] required is
[tex]$$\text{Mass} = \text{moles} \times \text{Molar mass} = 0.5 \, \text{moles} \times 40 \, \text{g/mol} = 20 \, \text{g}.$$[/tex]
3. Adjust for the 96% Assay of the Pellets
Since the sodium hydroxide pellets are only 96% pure, you need to account for the purity by dividing the mass of pure [tex]$\text{NaOH}$[/tex] by the assay (expressed as a decimal). Thus, the required mass of pellets is
[tex]$$\text{Mass of pellets} = \frac{20 \, \text{g}}{0.96} \approx 20.83 \, \text{g}.$$[/tex]
4. Weigh the Pellets
Weigh approximately [tex]$20.83 \, \text{g}$[/tex] of sodium hydroxide pellets using an analytical balance.
5. Dissolve the Pellets
Transfer the weighed pellets into a beaker and add a small amount of deionized water to dissolve the sodium hydroxide completely.
6. Transfer and Dilute
Once the pellets are fully dissolved, transfer the solution into a 1000 mL volumetric flask. Then, dilute the solution with deionized water until the bottom of the meniscus reaches the 1000 mL mark. Ensure to mix thoroughly so the solution is homogeneous.
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Part (b): Explanation of the Dilution Equation
The dilution equation is given by
[tex]$$C_1 V_1 = C_2 V_2,$$[/tex]
where:
- [tex]$C_1$[/tex]: Initial concentration of the stock (concentrated) solution.
- [tex]$V_1$[/tex]: Volume of the stock solution that is taken for dilution.
- [tex]$C_2$[/tex]: Final concentration of the diluted solution.
- [tex]$V_2$[/tex]: Final total volume of the diluted solution.
This equation ensures that the number of moles of solute remains constant before and after the dilution.
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Part (c): Chemical Hazard of Water and First Aid Measures
Hazard: Hot water can cause scald burns.
First Aid: If exposed to scald burns from hot water, immediately cool the affected area with cool or lukewarm water for at least 10–15 minutes. Remove any wet or contaminated clothing and seek medical assistance if necessary.
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This completes the detailed step-by-step solution.
