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Chapter 9: Q11P (page 208)

A $0.0450 M$ solution of benzoic acid has a pH of $2.78$ . Calculate ${pK}_{a}$ for this acid

Short Answer

Expert verified

The ${pK}_{a}$ for benzoic acid is $= 4.20$

Step by step solution

01

Define pKa

${pK}_{a}$ is the number that describes the acidity of a particular molecule. The formula to find the ${pK}_{a}$ is

${pK}_{a} = - {logK}_{a}$

02

Reaction of dissociation

$C_{6} H_{5} {CO}_{2} H \overset{}{⇌} C_{6} H_{5} {CO}_{2}^{-} + H^{+} [C_{6} H_{5} {CO}_{2} H] = F - 10^{- 2.78} [C_{6} H_{5} {CO}_{2}^{-}] = [H^{+}] = 10^{- 278}$

03

Calculating the Ka of benzoic acid

$K_{a} = \frac{[C_{6} H_{5} {CO}_{2}^{-}] [H^{+}]}{[C_{6} H_{5} {CO}_{2} H]} K_{a} = \frac{{(10^{- 2.78})}^{2}}{0.0450 - 10^{- 2.78}} = 6.35 \times 10^{- 5}$

04

Calculating pKa of benzoic acid

Substituting the value of $K_{a}$ in the formula ${pK}_{a}$ we get

${pK}_{a} = - {logK}_{a} = - \log (6.35 \times 10^{- 5}) = 4.20$

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Most popular questions from this chapter

Calculate the pH of a solution prepared by mixing $0.0800 mol$ of chloroacetic acid plus $0.0400 mol$ of sodium chloroacetate in $1.00 L$ of water.

(a) First do the calculation by assuming that the concentrations of HA and $A^{-}$ equal their formal concentrations.

(b) Then do the calculation, using the real values of and in the solution.

(c) Using first your head, and then the Henderson-Hasselbalch equation, find the pH of a solution prepared by dissolving all the following compounds in one beaker containing a total volume of $1.00 L : 0.180 {molCICHCOO}_{2} H, 0.020 mol {CICHCHO}_{2} {Na}_{2} 0.080 {molHNO}_{3,}$ and $0.080 molCa {(OH)}_{2}$ . Assume that $Ca {(OH)}_{2}$ dissociates completely.

Explain how you would use a spreadsheet such as Figure 8-9 to find the and concentrations of all species if you know the values of the equilibrium constants and the formal concentration $F$

Which of the following bases would be most suitable for preparing a buffer of $pH 9.00$ ?

(i) ${NH}_{3}$ (ammonia, $K_{b} = 1.76 \times 10^{- 5}$ );

(ii) role="math" localid="1654764490555" $C_{6} H_{5} {NH}_{2}$ (aniline,role="math" localid="1654764515634" $C_{6} H_{5} {NH}_{2}$ );

(iii) $H_{2} {NNH}_{2} (hydrazine, K_{b} = 1.05 \times 10^{- 6}))$ );

(iv) $C_{6} H_{s} {NH}_{2}$ (pyridine, $K_{b} = 1.58 \times 10^{- 9})$ ).

Write the Henderson-Hasselbalch equation for a solution of formic acid. Calculate the quotient $[{HCO}_{2}^{-}] / [{HCO}_{2} H]$ at (a) $pH 3.000$ ; (b) $pH 3.744$ ; (c) $pH 4.000$ .

Find the pH of 0.050 M triethylammonium bromide.

See all solutions

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