Question

A newly discovered element, Y, has two naturally occurring isotopes. 90.3 percent of the sample is an isotope with a mass of 267.8 u, and 9.7 percent of the sample is an isotope with a mass of 270.9 u. What is the weighted average atomic mass for this element? (2 points)

268.1 u
269.0 u
269.4 u
270.6 u

Answer 1

Add together the products of each isotope and its fractional amount (by mass).
If 90.3 percent, or 0.903 as a decimal fraction, of the element has a mass of
267.8 amu, then that amount contributes 267.8 times 0.903, or 241.8234 units
to the weighted average.
The remaining 9.7 percent, or 0.097 as decimal fraction, of mass 270.9 amu,
contributes 270.9 times 0.097, or 26.2773 units to the weighted average.
Those are the only two components, so add them together.
241.8234 + 26.2773 = 268.1007 or 268.1 to 4 significant digits.
Answer choice a is your best answer.

Answer 2

Thank you so much. I am starting to connect the dots.