The ph of a solution is the negative logarithm of the molar concentration of hydronium ion, that is, ph=−log[h3o+] in neutral solutions at 25 ∘c, [h3o+]=10−7 m and ph=7. as [h3o+] increases, ph decreases, so acidic solutions have a ph of less than 7. basic solutions have a ph greater than 7. the hydroxide and hydronium ion concentrations are related by the the ion-product constant of water, kw , as follows: kw=1.0×10−14=[h3o+][oh−] in the same way as the ph, we can define the poh as poh=−log[oh−]. it follows from the kw expression that ph+poh=14.

1.

2. Acidic: pH < 7.00, neutral: pH = 7.00, basic: pH > 7.00

3. pH + pOH = 14.00 at room temperature

Explanation:

Firstly, it is true that pH is defined as the negative logarithm of the concentration of hydronium ions. A simple rule for any p-function in chemistry is to remember that p = -log. For example:

etc.

Secondly, neutral solutions are the ones which have equal concentrations of hydronium and hydroxide ions regardless of the temperature. At room temperature, specifically, for pure solutions:

Notice that applying the equation above:

This means neutral solutions at room temperature have a pH value of 7.00.

Now, let's say the concentration of hydronium increases to a value of:

Then:

Notice that an increase in the molarity of hydronium lead to a decrease in pH. Therefore, acidic solutions have a pH < 7.00, while basic solutions have a pH > 7.00. Neutral solution was described as the one having pH = 7.00.

Thirdly, ion-product constant of water is defined as:

The given value of:

Is only valid at room conditions. Now let's take the -log of both sides:

Apply the rule of logs:

We obtain:

That said, the equation is proved.