Question

Is it correct to take molarity equal to the OH- ?
In order to find PH ...

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Answer 1

Explanation:

Yes it is correct to take molaritt = OH

Answer 2

Question :

4 gram NaOH is dissolved in 100ml of water. Find pH of solution so formed.

Answer :

Given :

• Mass of NaOH = 4g
• Volume of solution = 100 L

To find :

pH of solution

Formula used :

• Number of mole = mass ÷ molar mass
• Molarity = number of mole ÷ Volume of solution (in L)
• pH = - log [H+]

Important point :

• Molar mass of NaOH = 40g/mol
• pH + pOH = 14

Solution :

• Number of mole = 4/40 = 0.1 mole

$\sf \: NaOH \longrightarrow \: Na^ + \: + OH^ -$

➠ NaOH is strong base, it means it will be ionised completely

➠ Therefore, 1 mole of NaOH will give 1mole of OH- ion

➠ Therefore , 0.1 mole of NaOH will give 0.1 mole of OH- ion

☞ Molarity of OH- ion = 0.1 ÷ 100 = 0.001 M

➼ pOH = - log [OH-]

➼ pOH = - log (0.001)

➼ pOH = - log (1/1000)

➼ pOH = - (log 1 - log 1000)

➼ pOH = - (0 - 3)

➼ pOH = +3

✪ pH = 14 - pOH

✪ pH = 14 - 3

✪ pH = 11

ANSWER :

pH = 11

ADDITIONAL information :

Molarity of NaOH = mole of NaOH ÷ volume

Molarity of NaOH = 0.1 ÷ 100 = 0.001 M

Also , as shown above , molarity of OH- is also 0.001

✪ For base like NaOH , with n factor (or acidity) = 1 ;

✪ concentration of NaOH = Concentration of OH-