<p>What is the purity of impure&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>H</mi><mn>2</mn></msub><mi>S</mi><msub><mi>O</mi><mn>4</mn></msub></math>&nbsp;solution having density 1.8 g/mL if 50 mL of this solution requires 42.3 mL of 4.0 N NaOH:</p>