<p>H has two natural isotopes of&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML" class="wrs_chemistry"><mmultiscripts><mi mathvariant="normal">H</mi><none/><mn>1</mn><mprescripts/><mn>1</mn><mrow/></mmultiscripts><mo>&#160;</mo><mi>and</mi><mo>&#160;</mo><mmultiscripts><mi mathvariant="normal">H</mi><none/><mn>2</mn><mprescripts/><mn>1</mn><mrow/></mmultiscripts></math>&nbsp;and O has two isotopes&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msup><mi>O</mi><mn>16</mn></msup><mo>&#160;</mo><mi>a</mi><mi>n</mi><mi>d</mi><mo>&#160;</mo><msup><mi>O</mi><mn>18</mn></msup></math>. Which of the following mol. mass of&nbsp;<math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>H</mi><mn>2</mn></msub><mi>O</mi></math>&nbsp;will not be possible?</p>