Question

The molarity of a solution that is composed of 80 g of sodium hydroxide dissolved in 2.0 L of solution equals (A) 1.0 M (B) 2.0 M (C) 4.0 M (D) 40 M

Answer 1

Answer- The above question is from the chapter 'Some Basic Concepts of Chemistry'.

Molarity: Number of moles of solute present in 1 litre of solution is called molarity.

Unit of Molarity = Molar (M)

•When volume is given in litres,

Molarity = Number of moles of solute ÷ Volume of solution in litres

•When volume is given in millilitres,

Molarity = (Number of moles of solute × 1000) ÷ Volume of solution in ml

•When mass % and density is given,

Molarity = (Mass % × Density × 10) ÷ Molar mass of Solute

Given question: The molarity of a solution that is composed of 80 g of sodium hydroxide dissolved in 2.0 L of solution equals

(A) 1.0 M

(B) 2.0 M

(C) 4.0 M

(D) 40 M

Answer: Given mass of NaOH (m) = 80 g

Molar mass of NaOH (M) = 1 [Na] + 1 [O] + 1 [H] = 23 + 16 + 1 = 40 g

Number of moles = m/M = 80/40 = 2 moles

Molarity = Number of moles of solute ÷ Volume of solution in litres

Molarity = 2 ÷ 2

Molarity = 1 M

∴ (A) 1.0 M is the right answer.

Answer 2

Molarity is defined as the number of moles of solute per volume of solution in meters.

Moles = mass/molar mass of compound

⇒Moles = 80 / 40 [ Molar mass of NaOH = 40 ]

⇒ Moles = 2

Volume of solution = 2 L

⇒ Molarity = moles / volume of solution

⇒ Moles = 2/2 = 1 M

So option (A) is correct.