Question

What combination of substances will give a buffered solution with $pH5.05$??
(Assuming each pair of substance is dissolved in $11$ water)
($K_b$ for $N{H}_3=1.8\times {10}^{-5}$ and $K_b$ for $C_5{H}_{5}N=1.7\times {10}^{-9}$)

• A. $1.0molN{H}_3$ and $1.5mol$ $N{H}_{4}Cl$
• B. $1.5molN{H}_3$ and $1.0mol$ $N{H}_{4}Cl$
• C. $1.0mol{C}_5{H}_{5}N$ and $1.5mol$ $C_5{H}_{5}NHCl$
• D. $1.5molN{H}_3$ and $1.5mol$ $C_5{H}_{5}NHCl$

Answer 1

The correct option is C $1.0mol{C}_5{H}_{5}N$ and $1.5mol$ $C_5{H}_{5}NHCl$

$C_5{H}_{5}N+H_{2}O=C_5{H}_{5}NH++O{H}^{-}$

initial concentration

0.2 0.3

at equilibrium

0.2-x 0.3+x x

$\left[C_5{H}_5\right]=1/5=0.2$ M

$\left[C_5{H}_{5}N{H}^{+}\right]=1.5/5=0.3$ M

$1.7\times {10}^{-9}=(0.3+x)\left(x\right)/0.2-x$

$x=1.13\times {10}^{-9}=\left[O{H}^{-}\right]$

pOH =8.95

pH =14 - pOH = 5.05

C is the answer