There are two isotopes of an unknown element, X19 and X21. The abundance of X19 is 11.79%. A weighted average uses the percentages of each isotope to scale their contribution to the total mass. Each isotope's contribution is the percentage (in decimal form) multiplied by the mass of the isotope. What is the contribution (in amu) to the weighted average from the X19 isotope, which has a mass of 19.00amu ? What is the percentage of the X21 isotope? What is the contribution (in amu) to the weighted average from the X-21 isotope, which has a mass of 21.00 am? What is the average atomic mass (in amu) of this element using four significant figures