Question

for 20% (w/w) NaOH solution density is 2 g /mL. the molarity of the solution is ?ì​

Answer 1

Given :

• Concentration of NaOH solution = 20% (w/w)
• Density of solution = 2g/ml

To find :

Molarity of solution

Formula used :

• percentage (w/w) = mass of solute ÷ volume of solution

• Density of solution = mass of solution ÷ volume of solution

• Number of mole = Mass ÷ molar mass

• Molarity = Number of mole ÷ Volume of solution (in L)

Additional points :

(1) x % (w/w) means x gram solute is present in 100 gram of solution

(2) Molar mass of NaOH = 40 grams/mol

Solution :

Let 100 gram solution of 20% (w/w) NaOH is present,

This means that in 100 grams of solution 20 grams of NaOH is present

Therefore

• Mass of NaOH = 20grams
• Mass of solution = 100 grams

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Now , we know that density of given solution = 2 g/ml

➝ Volume of solution = Mass of solution ÷ Density of solution

➝ Volume of solution = 100 g ÷ 2g/ml

➝ Volume of solution = 50 ml

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Now ,

Molarity = Number of mole of solute ÷ Volume of solution (in L)

$\sf \implies \: Molarity \: = \dfrac{mass \: of \: solute}{molar \: mass} \times \dfrac{1}{volume \: of \: solution \: in \: litre} \\ \\ \\ \sf \implies \: Molarity \: = \dfrac{20}{40} \times \dfrac{1000}{50} \\ \\ \sf \implies \: Molarity \: = \bold{10M}$

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ANSWER :

Molarity = 10 M