Question

Calculate the amount of $N{H}_3$ and $N{H}_{4}Cl$ required to prepare a buffer solution of pH = 9.0, when the total concentration of buffering reagents is 0.6 moles/L. Given $p{K}_b$ of $N{H}_3$ = 4.7

• A. 1 mole/L
• B. 0.2 mole/L and 0.4mole/L
• C. 0.05 mole/L and 0.55 mole/L
• D. 3 mole/L

Answer 1

The correct option is B

0.2 mole/L and 0.4mole/L

pOH = $p{K}_b$ + log$\frac{\left[salt\right]}{\left[base\right]}$

Let the conc. of salt be ‘x’ moles/L

Conc. of Base = (0.6 – x) moles/L

5 = 4.7 + log$(\frac{x}{0.6}–x)$

0.3 = log$(\frac{x}{0.6}–x)$

${10}^{0.3}=\frac{x}{(0.6-x)}2=\frac{x}{(0.6-x)}2(0.6-x)=x$

X = 0.4

$\therefore$ Conc. of salt = 0.4 moles/L

$\therefore$​ Conc. of base = 0.2 moles/L