The mass of potassium hydroxide required to make 300.ocm3 of 0.4 mol$dm^{-3}$ solution is {KOH = 56.0}

Step 1: Analyze the question We need the mass of KOH that must be dissolved to make 300.0 cm³ of a 0.4 mol/dm³ solution.

Step 2: Concept

• Moles = concentration × volume (in dm³)
• Mass = moles × molar mass

Step 3: Solution

Convert the volume to dm³: 300.0 cm³ ÷ 1000 = 0.300 dm³

Calculate the moles of KOH: n = C × V = 0.4 × 0.300 = 0.12 mol

Calculate the mass: Mass = n × M = 0.12 × 56.0 = 6.72 g

Step 4: Answer The mass of KOH required is 6.72 g, which is option C.

Tip: Always convert cm³ to dm³ first by dividing by 1000. A quick check is to find the mass for 1 dm³ (0.4 × 56 = 22.4 g), then scale it down: 22.4 × 0.3 = 6.72 g. Wrong options like 26.88 g and 13.44 g come from forgetting to convert units or from slipping on the multiplication.