Chapter 16: Problem 31
How much \(\mathrm{NaOH}\) (in grams) is needed to prepare \(546 \mathrm{~mL}\) of solution with a \(\mathrm{pH}\) of 10.00 at \(25^{\circ} \mathrm{C} ?\)
Short Answer
Expert verified
You need 0.002184 grams of NaOH.
Step by step solution
01
Understanding the pH and pOH
First, we need to understand that the pH of a solution is related to its hydrogen ion concentration. Also, remember that at 25°C, the sum of pH and pOH is 14. Given that the pH is 10, we can calculate the pOH as follows:\(\text{pOH} = 14 - \text{pH} = 14 - 10 = 4.\)
02
Calculating OH- Concentration
The concentration of hydroxide ions, \([\mathrm{OH}^-]\), in a solution can be found using the formula:\([\mathrm{OH}^-] = 10^{-\text{pOH}}\)Substituting the known value of pOH:\([\mathrm{OH}^-] = 10^{-4} = 0.0001 \mathrm{~mol/L}\)
03
Calculating Moles of NaOH Required
To find how many moles of \(\mathrm{NaOH}\) are needed for the solution, use the formula:\(\text{Moles of NaOH} = [\mathrm{OH}^-] \times \text{Volume of solution in liters}\)Convert the volume from milliliters to liters:\(546 \mathrm{~mL} = 0.546 \mathrm{~L}\)Then calculate the moles:\(\text{Moles of NaOH} = 0.0001 \mathrm{~mol/L} \times 0.546 \mathrm{~L} = 0.0000546 \text{ moles}\)
04
Calculating Mass of NaOH
Calculate the mass of \(\mathrm{NaOH}\) using the formula:\(\text{Mass of NaOH} = \text{Moles of NaOH} \times \text{Molar mass of NaOH}\)The molar mass of \(\mathrm{NaOH}\) is approximately 40.00 \(\mathrm{g/mol}\). So:\(\text{Mass of NaOH} = 0.0000546 \text{ moles} \times 40.00 \mathrm{~g/mol} = 0.002184 \mathrm{~g}\)
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Key Concepts
These are the key concepts you need to understand to accurately answer the question.
pH Calculation
The pH of a solution indicates its acidity or basicity and is derived from the concentration of hydrogen ions. For any solution, \text{pH}\ is calculated as follows: \( \text{pH} = -\log [\text{H}^+] \). At room temperature (25°C), the sum of pH and pOH is always 14. This relationship helps us find the concentration of hydroxide ions when given the pH.
- Given a pH of 10, the pOH is calculated as \(\text{pOH} = 14 - \text{pH} = 4\).
Hydroxide Ion Concentration
Once the pOH is known, we can calculate the concentration of hydroxide ions, \[ \text{OH}^- \]. The formula used is \( [\text{OH}^-] = 10^{-\text{pOH}} \).
- For a pOH of 4, \( [\text{OH}^-] = 10^{-4} = 0.0001 \, \text{mol/L} \).
Molar Mass of NaOH
The molar mass is crucial when converting moles to grams. Sodium hydroxide (NaOH) consists of:
- Sodium (Na): approximately 23 \, \text{g/mol}
- Oxygen (O): approximately 16 \, \text{g/mol}
- Hydrogen (H): approximately 1 \, \text{g/mol}
Solution Concentration
Solution concentration tells us how much solute is present in a given volume of solvent. Here, we need NaOH to make a solution with a specific pH, which determines the hydroxide concentration.
- We calculated the required moles of NaOH using: \( \text{Moles of NaOH} = [\text{OH}^-] \times \text{Volume in liters} \).
- For a volume of 546 \, \text{mL} (or 0.546 \, \text{L}), the moles required were \( 0.0000546 \, \text{moles} \).