Challenge Your Exam Readiness With Our Free ACS General Chemistry Practice Questions

Average atomic mass is a weighted mean of the isotopic masses based on their fractional abundances, a core skill emphasized in ACS atomic structure objectives. Convert percentages to fractions and multiply each isotopic mass by its fraction, then sum: (0.199)(10.012 amu) + (0.801)(11.009 amu) ≈ 1.992 + 8.818 ≈ 10.810 amu. Reporting to four significant figures (consistent with given data) gives 10.81 amu. On the ACS exam, common pitfalls include averaging the two masses without weighting or rounding too early. Another frequent distraction is mixing up abundance with relative intensity on a mass spectrum; remember the y-axis is relative abundance, which maps to the fractional weights used here. Dimensional analysis helps confirm units carry through as atomic mass units. The correct result should be very close to the known average mass of boron, which has a similar isotopic pattern, reinforcing conceptual understanding of why natural samples rarely match integer mass numbers. Proper handling of significant figures and weighted averages demonstrates mastery of isotope problems expected in first-term general chemistry.