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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Cotc on June 02, 2005, 08:49:24 AM

Title: pH of solutions
Post by: Cotc on June 02, 2005, 08:49:24 AM

im having some trouble with the following questions. if anyone can help it will be much appreciated.

1. "what is the pH of a solution 0.16 M in acetic acid and 0.15 M in sodium acetate? Ka for acetic acid 1.75e-5"

this is what ive done: CH3COOH + H2O --> H30+ + CH3COO-

1.75e-5 = (0.15+x)(x)/(0.16-x) therefore x = 3.73e-5 (solve for x using quadratic formula)

so pH = -log(3.73e-5) = 4.43

im a bit confused though. im not sure if ive done everything correctly???

2. "1.90 g of potassium cyanide is dissolved in water and the solution made up to 250mL in a volumetric flask. what is the pH of the solution? Ka for HCN = 4.90e-10"

the mass is confussing me in this question. i dont know where to start.

please *delete me*

Title: Re:pH of solutions
Post by: Borek on June 02, 2005, 09:40:39 AM

1. It is a buffer solution. Use Henderson-Hasselbalch equation.

2. Convert mass to moles. This is a simple case of weak acid salt hydrolysis.