Question

Determine the pH of an aqueous buffer solution (250 mL) comprised of dissolving 5.2812 g of Tris (B) (FW = 121.14 g/mol) and 10.2500 g of Tris-HCl (BH+) (FW = 157.60 g/mol) if pKa (BH+) = 8.07?

(a) 8.24 (b) 7.90 (c) 8.54 (d) 7.82 (e) 8.01

Answer 1

The pH can be calculated using the Henderson-Hasselbach equation:

$pH=pKa+log\left ( \frac{[B]}{[BH^{+}]} \right )$

We need to calculate the concentrations of the species, which can be done using:

$M=\frac{\frac{m}{m_{molar}}}{V(L)}$

For B:

$[B]=\frac{\frac{5.2812g}{121.14g/mol}}{0.250L}=0.174M$

For BH⁺:
$[BH^{+}]=\frac{\frac{10.2500g}{157.60g/mol}}{0.250L}=0.260M$

And finally, we can calculate:

$pH=8.07+log\left ( \frac{0.174M}{0.260M} \right )=7.90$