Question

3) 80 grams of NaOH is dissolved in 100ml solution. Calculate its molarity and also find the pH of that
solution

Answer 1

Answer:

M = 20 Molar and  pH = 14.

Explanation:

Molarity :

It is define as numbers of moles of solute present in unit amount of volume of solution in litre.

We have formula for molarity

$\displaystyle Molarity=\frac{Given \ mass}{Molar \ mass \times volume \ (mL)}\times1000$

Given :

Mass of solute = 80 g

Molar mass of solute ( NaOH ) = 40 g

Volume of solution = 100 mL

Now put these value in formula we get

$\displaystyle Molarity=\frac{80}{40\times100}\times1000 \\\\\displaystyle Molarity=\frac{ \cancel 80}{\cancel 40\times100}\times1000 \\\\\displaystyle Molarity=\frac{2}{100}\times1000 \\\\\displaystyle Molarity=\frac{2}{\cancel 100}\times \cancel 1000 \\\\\displaystyle Molarity=2\times10 \ mol \ L^{-1}\\\\\displaystyle Molarity=20 \ mol \ L^{-1}$

Thus we get molarity is 20 Molar.

Now finding pH

We have [ OH⁻ ] = 20

We know that

$\displaystyle [ H^+]=\frac{K_w}{[OH^-]} \ where \ K_w=10^{-14}$

put the value here

$\displaystyle \text{ [ H^+]=\frac{10^{-14}}{20}=5\times10^{-16} }$

We know that pH = - log  [ H⁺ ]

$\displaystyle pH=-\log[5\times10^{-16}]\\\\\displaystyle pH=16-\log5\\\\\displaystyle pH=16-0.7\\\\\displaystyle \text{ pH=15.3 Not possible}\\\\\displaystyle \text{Highest value of pH=14 }\\\\\displaystyle \text{So here pH will be 14}$

Thus we get pH = 14.