Question

8 gm of caustic soda (molar mass 40) is dissolved in water and solution is made to 800cm³. calculate the molarity of the solution

Answer 1

Answer:

Caustic soda = NaOH =40 grams

40g of NaOH dissolved in 1litre of water resulting solution xM

40g = 1000 x xM

So for 80g

$\frac{40 \times 800}{1000 \times 10 \times 1}$

32M

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Answer 2

In the given question

• Solute = Caustic soda
• Solvent = Water

Number of moles of solute = Mass of solute ÷ Molar mass

⇒ Number of moles of solute = 8 ÷ 40

⇒ Number of moles of solute = 0.2 moles

We know that

1 cm³ = 1 mL

So, Volume of solution = 800 cm³ = 800 mL = 0.8 litres

Molarity = Number of moles of solute ÷ Volume of solution

⇒ Molarity = 0.2 ÷ 0.8

⇒ Molarity = 0.25 M

∴ Molarity of solution = 0.25 M

REMEMBER:

• While finding out volume of solution note that it should always be in litres. Else multiply by 1000
• Molarity is a temperature dependent term.
• Number of moles of solute should be solved and not of solvent.