Question

If 200 mL aqueous solution of 10 g NaOH is prepared, find the molarity of the resulting solution.

• A. \(1.25\,M\)
• B. \(1.5\,M\)
• C. \(1.66\,M\)
• D. \(12.5\,M\)

Hint:

For molarity problems, always convert volume into litres before substituting into the formula \(M=\frac{n}{V}\).

Answer 1

The Correct Option is A

Concept: Molarity is defined as the number of moles of solute dissolved in one litre of solution. \[ M=\frac{\text{Number of moles}}{\text{Volume in litre}} \] To calculate molarity, we first determine the number of moles from the given mass and molar mass.

Step 1: Calculate the molar mass of sodium hydroxide. \[ NaOH = 23+16+1 \] \[ =40\,g\,mol^{-1} \]

Step 2: Calculate the number of moles present in 10 g NaOH. \[ n=\frac{\text{Mass}}{\text{Molar Mass}} \] \[ n=\frac{10}{40} \] \[ n=0.25 \text{ mol} \]

Step 3: Convert volume into litres. \[ 200\,mL=\frac{200}{1000} \] \[ =0.2\,L \]

Step 4: Calculate molarity. \[ M=\frac{0.25}{0.2} \] \[ M=1.25 \] Therefore, \[ \boxed{M=1.25\,M} \] Hence, the correct option is \[ \boxed{(a)} \]