Question

Following four solutions are prepared by mixing different volumes of NaOH and HCI of different concentrations, pH of which one of them will be equal to 1 ?
A. $\;\;\;55mL \times \dfrac{M}{{10}}HCL{\text{ }} + 45ml{\text{ }} \times \dfrac{M}{{10}}NaOH$
B. $\;\;\;75mL \times \dfrac{M}{5}HCL + 25mL \times \dfrac{M}{5}NaOH$
C. $\;\;\;100mL \times \dfrac{M}{{10}}HCL + 100ml \times \dfrac{M}{{10}}{\text{ }}NaOH$
D. $\;\;60mL \times \dfrac{M}{{10}}HCI + 40mL \times \dfrac{M}{{10}}{\text{ }}NaOH$

Answer 1

Hint : Concentration is the abundance of a constituent divided by the total volume of a mixture. Several types of mathematical description can be distinguished: mass concentration, molar concentration, number concentration, and volume concentration.

Complete Step By Step Answer:
According to the question we will first find out the PH by finding their concentration.
Total volume = $75{\text{ }}mL + 25{\text{ }}mL = 100{\text{ }}mL$
We know that number of millimoles is given by
${\text{Number of millimoles}} = volume{\text{ in milliliter}} \times {\text{molarity}}$
On using the formula we get
Number of mmol of HCl $= 75{\text{ }}mL \times {\text{ }}\dfrac{M}{5}{\text{ }} = 15{\text{ }}mmol$
Number of mmol of NaOH $= 25{\text{ }}mL \times \dfrac{M}{5}\;{\text{ }} = 5{\text{ }}mmol$
$5$ mmol of NaOH will neutralize $5$ mmol of HCl.
$15 - 5 = 10$ mmol of HCl will remain. $$
$[{H^ + }] = [HCl] = \dfrac{{10\;mmol}}{{100\;mL}} = 0.1\;M$
$pH = - lo{g_{10}}\left[ {{H^ + }} \right]$
$pH = - lo{g_{10}}0.1\; = 1$
Similarly in all other cases we will find that the PH is not equal to 1.
Hence pH of $75mL \times \dfrac{M}{5}HCL + 25mL \times \dfrac{M}{5}NaOH$ is equal to $1.$
Therefore, the correct option is B.

Additional Information:
NaOH is used in making chemicals and soaps, in refining petroleum, and as a cleansing agent. Sodium hydroxide is very corrosive. It can cause irritation to the eyes, skin, and mucous membrane; an allergic reaction; eye and skin burns; and temporary loss of hair. Workers may be harmed from exposure to sodium hydroxide. The level of harm depends upon the dose, duration, and work being done. Sodium hydroxide is corrosive to all tissues. Concentrated vapors lead to serious damage to the eyes and respiratory system

Note :
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound Everyone knows that sodium hydroxide, NaOH, is a base. If a compound that contains an OH group also contains a metal ion, then it is a base.