Answer to Question #156609 in Physical Chemistry for nimrod

Question

An environmental chemist needs a carbonate buffer of pH 10.0 to study the effects of the acid rain on limestone-rich soils. How many grams of Na₂CO₃ must she add to 1.5L of freshly prepared 0.20M NaHCO₃ to make the buffer? k_a of HCO₃^- is 4.7 x 10^-11.

Answer 1

pH= 10.0

[NaHCO3] = 0.20M

Ka= 4.7 x 10^-11

pKa= -log Ka

pKa= -log (4.7 x 10^-11) = 10.33

Using Henderson Hasselbalch equation:

pH= pKa + log[Na2CO3]/[NaHCO3]

10.0 = 10.33 + log [Na2CO3]/0.20

Log[Na2CO3]/0.20 = 10.0 - 10.33 = -0.33

[Na2CO3]/0.20 = 10^-0.33

[Na2CO3] = 0.20 x 0.47 = 0.094M

Moles of Na2CO3= C x V

= 0.094M x 1.5L

= 0.141mole

Molar mass of Na2CO3= 106gmol^-1

Mass= mole x molar mass

= 0.141 x 106 = 14.95g