Question

Given pH = 8.55 Find: [H3O+] and [OH-] Is this acidic, basic or neutral?​

Answer 1

• 
  `\rm [H_3O^(+)] = 10^(-8.55)\;M\approx 2.82* 10^(-9) \;M`, and
• 
  `\rm [OH^(-)] = 10^(-5.45)\; M\approx 3.55* 10^(-6)\;M`

This solution is likely to be basic.

Assumption: this solution is under room temperature, where
`K_w = 10^(-14)`.

Step-by-step explanation:

The concentration of hydronium ions
`\mathrm{H_3O^(+)}` in the solution can be found from the
`\mathrm{pH}` value. This relationship does not depends on temperature.

`\mathrm{[H_3O^(+)]} = 10^{-\mathrm{pH}} = 10^(-8.55)`.

The question states that for this solution,
`\rm [H_3O^(+)] = 8.55\;M`. Apply the relationship between
`\mathrm{[H_3O^(+)]}`,
`\mathrm{[OH^(-)]}`, and
`K_w`. Note that the value of
`K_w` is dependent on the temperature of the solution.

`\displaystyle \rm [OH^(-)] = \frac{\mathnormal{K_w}}{[H_3O^(+)]} = (10^(-14))/(10^(-8.55)) = 10^(-5.45)\; M`.

In other words,

• 
  `\rm [H_3O^(+)] = 10^(-8.55)\;M\approx 2.82* 10^(-9) \;M`, and
• 
  `\rm [OH^(-)] = 10^(-5.45)\; M\approx 3.55* 10^(-6)\;M` assuming that
  `K_w = 10^(-14)`.

`\rm [H_3O^(+)] < [OH^(-)]`.

In other words, this solution is basic.