Hi everyone,
Can someone please explain this question. I thought it would be A but the answer is C.
Hi Hawraa,
We can rule out option A because nitrous acid will partially dissociate to produce H
+ ions and NO
2- ions. The presence of H
+ ions will reduce pH (noting that pH = -log
10[H
+], so greater [H
+] means lower pH). Option B is ruled out for a similar reason.
Now, to discern between C and D we look at pKa. Since nitrous acid has a higher pKa than chlorous acid, this means that nitrous acid is a weaker acid - so its conjugate base (NO
2-) is a stronger base. This means a solution containing NO
2- ions has a greater pH than a solution containing the same concentration of ClO
2 ions, so C must be the answer. Note also that Na
+ ions don't really react with water - it's the anion in this instance that governs pH.
So to sum up, C has a higher pH than A simply because there are no H
+ ions in its structure. C has the highest pH since its anion is the strongest base present.