Hence the assumption.
I would hate a multiple choice q. without the assumption::
[H+] = 0.0005 + y
[HSO4-] = 0.0005 - y
[SO4 2-] = y
0.0005^2 - y^2 = y * 10^-1.99
0 = y^2 + y * 10^-1.99 - 0.00000025
2y = -10^-1.99 + &sqrt;(10^-3.98 + 0.000001)
y = 2.437 * 10 ^-5
[H+] =...