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**Hint:**Normality formula will be almost the same as that of molarity. But we take the number of equivalents for calculating normality whereas we take the number of moles for calculating molarity. Try to figure out what equivalents mean.

**Complete step by step answer:**

We are asked to calculate the normality of NaOH in the given solution. We know that the normality of a substance is the number of equivalents of substance present in a 1L volume of solution. Equivalents calculation will be different for different types of substances.

For bases equivalent weight = $\frac { molecular\quad weight\quad of\quad base }{ acidity\quad of\quad base } $

The acidity of a base means the number of ionizable hydroxide groups in the base.

NaOH has one ionizable hydroxide group.

The equivalent weight of NaOH = $\frac { molecular\quad weight\quad of\quad NaOH }{ acidity\quad of\quad NaOH } \quad =\quad \frac { 40 }{ 1 } \quad =\quad 40$

Normality of NaOH solution = $\frac { Weight\quad of\quad NaOH }{ equivalent\quad weight\quad of\quad NaOH } \times \frac { 1 }{ volume\quad of\quad solution\quad in\quad liters } $

Volume of solution = 1L(1000mL)

Weight of NaOH = 20.5 gm

Normality of NaOH = $\frac { 20.5 }{ 40 } \times \frac { 1 }{ 1 } \quad =\quad \frac { 20.5 }{ 40 } \quad =\quad \frac { 41 }{ 80 } \quad =\quad 0.5125N$

Therefore, the normality of NaOH is 0.5125N.

**Note:**

In this problem, normality is the same as that of molarity but in some cases where the equivalent weight will not be equal to molecular weight we need to calculate the correct number of equivalents. In redox reactions equivalent weight of a substance that undergoes oxidation or reduction will be the molecular weight of substance divided by the number of electrons transferred by one mole of the substance.

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