What is the concentration of hydronium ions in a solution with a pH = 3.9?

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Answer 1 · 2016-06-25T22:17:12

If #pH# #=# #3.9#, then #[H_3O^+]# #=# #10^(-3.9)*mol*L^-1#.

By definiion,
#pH# #=# #-log_(10)[H_3O^+]#, and #pOH# #=# #-log_10[HO^-]#.

In water, under standard conditions #pH+pOH=14#.

Both #pH# and #pOH# measure the extent of the equilibrium:

#2H_2O rightleftharpoons H_3O^+ + HO^-#

At #298*K#, the dissociation constant of water,

#K_w# #=# #[H_3O^+][HO^-]# #=# #10^-14#

The #p# means take the logarithm to the base 10, and mulitply this by #-1#.

And thus #pK_w# #=# #-log_10K_w# #=# #-log_(10)10^(-14)# #=# #14# #=# #pH+pOH#.