Question

Calculate the pH of precipitation of phenytoin from a 1% aqueøus phenytoin sodium at 25 degrees C (the solubility of phenytoin in water at 25 degrees C is 1 g/10000ml water, m.w. of phenytoin is 252.27 , pKa 8.1 , m.w. of phenytoin sodium is 275.27) 2. Calculate the pH of precipitation of ephedrine from a 6.5% solution of ephedrine HCl at 25 degrees C. (the solubility of ephedrine in water at 25 degrees C is 1 g/20ml water, m.w. of ephedrine is 165.24 , pKb 4.64, m.w. of ephedrine HCl is 201.74) 3. Calculate the pH of precipitation of a 3% aqueous sulfacetamide sodium monohydrate solution at 25 degrees C. (the solubility of sulfacetamide in water at 25 degrees C is 1 g/150ml water, m.w. of sulfacetamide is 214.25 , pKa 5.87 , m.w. of sulfacetamide sodium monohydrate is 254 ) 4. Calculate the pH of precipitation of a 12% scopolamine HCl aqueous solution at 25 degrees C. (the solubility of scopolamine in water at 25 degrees C is 1 g/9.5ml water, m.w. of scopolamine is 303.36 , pKb 5.8, m.w. of scopolamine HCl is 339.86 ) 5. The dissociation constant of ethanol amine is 2.77×10−5 at 25∘C. Calculate its pKb value.

Answer 1

In chemistry, calculating the pH or pOH of solutions depends on whether the solute is a strong or weak acid or base, and involves using specific formulas such as the Henderson-Hasselbalch equation for buffers, or -log[H3O+] for strong acids. Calculating the pKb for a weak base requires using the formula pKb = -log(Kb).

Step-by-step explanation:

pH and pOH Calculations

To calculate the pH of various solutions, we need to apply different equations based on the solution's nature such as the Henderson-Hasselbalch equation for buffer solutions, the formula pH = -log[H3O+] for strong acids, and corresponding equations for strong bases, taking into account the complete ionization in water.

For example, the pH calculation for a strong acid like HCl would involve finding the hydrogen ion concentration directly from the molarity of the solution since HCl ionizes completely. Conversely, for a weak acid or base, we must use its dissociation constant to find the pH.

The last question regarding the dissociation constant asks for the calculation of the pKb value. The pKb can be found using the formula pKb = -log(Kb), so for ethanol amine with a Kb of 2.77×10−5, the calculation would be pKb = -log(2.77×10−5), which we would compute with a calculator.