Question

The pH of N/50 sodium hydroxide is-
A.12.0
B.2.0
C.1.0
D.13.0

Answer 1

Please keep two things in mind in order to solve this type of problem:
-firstly, Normality of any acid or base is equal to its molarity multiplied by the number of hydrogen ions or hydroxide released by 1 mole of an acid or a base respectively.
-And second is that pH= 14 – pOH.

Complete step by step answer:
We will now solve the question step by step:
Remember the ionic equilibrium for NaOH is:
${\text{NaOH}} \to {\text{N}}{{\text{a}}^{\text{ + }}}{\text{ + O}}{{\text{H}}^{\text{ - }}}$
Which means 1 mole of NaOH releases one mole of hydroxide ion.
And, hence, Normality of NaOH =${\text{Molarity X 1}}$, i.e. Molarity = normality. Therefore, Molarity of NaOH is 0.1.
Which also means [$OH^-$] = 0.1 M.
As we know, [${{\text{H}}^{\text{ + }}}$][${\text{O}}{{\text{H}}^{\text{ - }}}$] = ${\text{1}}{{\text{0}}^{{\text{ - 14}}}}$and we have the value of [OH-], therefore putting that into this equation we will have:
[${{\text{H}}^{\text{ + }}}$] =${{{\text{1}}{{\text{0}}^{{\text{ - 14}}}}}}{{{\text{0}}{\text{.1}}}}$ i.e. [${{\text{H}}^{\text{ + }}}$] = ${\text{1}}{{\text{0}}^{{\text{ - 13}}}}$M
Now, pH = -log[${{\text{H}}^{\text{ + }}}$] = -log${\text{1}}{{\text{0}}^{{\text{ - 13}}}}$= 13
In this way we can conclude that the pH of N/10 sodium hydroxide is 13.

Hence, option D is the correct answer.

Additional information:
You should know that pH scale is used to measure how acidic/basic a particular substance is. The range pH scale varies from 0 to 14; with 0 being most acidic and 14 being most basic. A compound is said to be neutral i.e. neither acid nor basic if its pH is 7.
pH stands for power of hydrogen. In it H is in capital form because it is the symbol of element hydrogen. Soren Sorenson is the inventor of pH scale.
Water has a pH of 7 while the pH of milk is 6.6. It is interesting to note that our blood has a pH of 7.4

Note:
Alternative method: We have [${\text{O}}{{\text{H}}^{\text{ - }}}$] =0.1 M; which can give us pOH = -log [${\text{O}}{{\text{H}}^{\text{ - }}}$] = -log0.1 =1.
Now, pH =14-pOH. Therefore, pH = 14-1 = 13.
Thus, the pH of N/10 NaOH is 13, option D is correct.

The value of pH falls with rise in temperature but this never means water which is neutral at room temperature will become acidic on rising temperature. pH scale is always calibrated using a deionised water sample and that value of pH is taken as a neutral point. For instance at 100oC pH of water is 6.14 and hence all the substances with pH below 6.14 are said to be acidic and with pH above 6.14 are said to be basic.