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  4. What is the pH of the NaOH solution when 0.04 g of it dissolved in water and made to 100 mL solution?

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Q. What is the $ pH $ of the $ NaOH $ solution when $ 0.04 \,g $ of it dissolved in water and made to $ 100\,mL $ solution?

EAMCETEAMCET 2015

Solution:

Molecular mass of

$NaOH =23+16+1=40$

Molarity $=\frac{\text { Mass of solute }(\text { in } g) \times 1000}{\text { Molecular mass of the solute }} \times$ Volume of solution (in L)

$=\frac{0.04 \times 1000}{40 \times 100}=10^{-2} $

$ OH ^{-} =10^{-2} mol / L$

So, $ pOH =-\log \left[ OH ^{-}\right]$

Hence, $ pOH =2$

As we know that,

or, $ pOH + pH =14$

$ pH =14-2=12 $