Carbonate System 125 ptsl Three manufacturers are producing carbonated bottled w
ID: 940371 • Letter: C
Question
Carbonate System 125 ptsl Three manufacturers are producing carbonated bottled water. Onc company introduces carbonate by bubbling carbon dioxide into water until Cr 104 M (equivalent to adding H CO, C 10 M). Another manufacturer adds sodium bicarbonate until Cr 10+ M (equivalent to adding HCO, until Cr 104 M), and the third company adds sodium carbonate unti CT 104 M. Using log concentration-pH diagram (or pC-pH diagram), estimate the pH of bottled water from each of the three manufacturers. Assume all Na is in the form of dissolved sodium ions.Explanation / Answer
In the first case carbon dioxide is dissolved in water . CO2 + H2O ----> H2CO3
the ka values of carbonic acid are ka1 = 4.3 x 10-7 and ka2 = 5.6 x 10-11
Since the seocnd ionisation is very less, we assume all the protons form first ionisation and thus
pH = 1/2 pka1 - 1/2 log C = 1/2- log (4.3x10-7) -1/2 log 10-4 = 5.18
In the second case , sodium carbonate is added with C= 10-4 M
This is a solution of amphiprotic salt, whose pH is given as 1/2[pka1 + pka2]
Thus pH = 1/2[ -log (4.3x10-7) - log 5.6 x10-11] = 8.309
In the third case sodium carbonate is added which is a salt of strong base and weak acid.
Then the pH = 1/2 pkw + 1/2 pka+ 1/2 log C
= 7 + 1/2 (6.36) +1/2 (4) =12.18