<p>The neutralise completely 20 mL of 0.1 M aqueous solution of phosphorous acid&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><mfenced><mrow><msub><mi>H</mi><mn>3</mn></msub><mi>P</mi><msub><mi>O</mi><mn>3</mn></msub></mrow></mfenced></math>, the volume of 0.1 M aqueous KOH solution required is:</p>