How much sodium hydroxide is required to make 250 cm3 of 0.1 molar solution? Na=23; 0=16; H=1]

Analyze the question: We need the mass of NaOH that must be dissolved to make 250 cm³ of a 0.1 M (0.1 mol/dm³) solution.

Concept:

• Molarity (M) = moles of solute ÷ volume in dm³
• Moles = Mass ÷ Molar mass
• So Mass = M × V (dm³) × Molar mass

Solution:

1. Molar mass of NaOH = 23 + 16 + 1 = 40 g/mol

2. Convert the volume to dm³: 250 cm³ = 250 ÷ 1000 = 0.25 dm³

3. Moles of NaOH needed: n = 0.1 × 0.25 = 0.025 mol

4. Mass of NaOH: Mass = 0.025 × 40 = 1.0 g

Answer: The mass needed is 1.0 g, which matches option D.

Tip: A quick shortcut is that 1 mol/dm³ of NaOH needs 40 g per 1000 cm³. For 0.1 M, that is 4 g per 1000 cm³. For 250 cm³ (a quarter of 1000 cm³), 4 ÷ 4 = 1 g. Option B (4.0 g) is the trap, as it is the mass for 1000 cm³ of 0.1 M solution, so always check the volume!