Question

how pH changes with change in concentration of H plus and minus ions​

Answer 1

Answer:

convenient.

Sørensen devised the convenient units of pH (puissance d'hydrogen), strength of hydrogen, or pOH (puissance d'hydroxyl) to overcome this difficulty. pH is therefore defined as the logarithm to the base 10 of the reciprocal of the hydrogen ion concentration or the negative value of the logarithm to the base 10 of the hydrogen ion concentration.

pH=log1(H+)=−log(H+)pOH=log1(OH−)=−log(OH−)

But we have said that (H+)(OH−) = Kw

Therefore (pH)(pOH)= −log Kw

Now if (H+)=10−7

pH = log 10−7 = pH 7.0

or (H+) = 10−2

pH = log 10−2 = pH 2.0

It follows therefore that:

A neutral solution has a pH of 7.0

An acid solution has a pH less than 7.0

An alkaline solution has a pH greater than 7.0

Now if we consider 0.1N(M) HCl:

This acid completely dissociates and the (H+) concentration is 10−1 moles per litre.

Therefore pH = 1.0.

Likewise 0.1N(M) NaOH completely dissociates and the (OH−) concentration is 10−1 moles per litre.

Since

(H+)(OH−)=10−14(H+)=10−13

Therefore pH = 13.0.

In the case of 0.15N(M) HCl, its (H+) concentration is 1.5 × 10−1 moles per litre

log(H+) 0.15=ˉ1.1761−1+0.1761=−0.8239pH=0.8239

As weak acids and bases are not completely dissociated, the concentration of ions and hence ‘acidity’ and ‘alkalinity’ are less.

For example:

the pH of 0.1M or 0.1N acetic acid=pH 2.9pH of 0.1M or 0.1N ammonia=pH 11.1

Find the pH of 0.1 M(N) acetic acid, when the degree of dissociation is 2.0 per cent.

(H+)=0.020×10−1=0.0020 g moles per litre