Question

The number of moles of NaCl in 3 litres of 3M solution is
(a) 1
(b) 3
(c) 9
(d) 27

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 number of moles of NaCl in 3 litres of 3 M solution is

(a) 1

(b) 3

(c) 9

(d) 27

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

3 M = Number of moles ÷ 3 L

3 × 3 = Number of moles

∴ Number of moles of NaCl = 9

Or

3 M means 3 moles in 1 litre of solution.

⇒ Number of moles in 3 litres solution = 3 × 3 = 9 moles

∴ Number of moles of NaCl = 9

(c) 9 is the correct answer.

Answer 2

Answer:

Molarity is equal to the number of moles of solute divided by the volume of solution in litres.

$\frak{Given}\begin{cases}\textsf{Molarity = 3M}\\\textsf{Solution = 3 litres}\end{cases}$

$\underline{\bigstar\:\textbf{According to the Question :}}$

$:\implies\sf Molarity=\dfrac{Number\:of\:Moles}{Volume\:of\: Solution}\\\\\\:\implies\sf 3=\dfrac{Number\:of\:Moles}{3}\\\\\\:\implies\sf 3\times3=Number\:of\:Moles\\\\\\:\implies\underline{\boxed{\sf Number\:of\:Moles=9}}$

⠀

$\therefore\:\underline{\textsf{Number of moles will be c) \textbf{9}}}.$