U1.2 · Mass Spectroscopy of ElementsPractice · 质谱与同位素
Lessons covered:U1.2 Mass Spectroscopy of Elements
- 1Multiple choice
The mass spectrum of a pure element is shown. Which element is it?
ExplanationEstimate the average atomic mass from the mass numbers and abundances: 63 × 0.6915 + 65 × 0.3085 ≈ 63.62. Atomic masses: Cu 63.55, Zn 65.38, Ga 69.72, Ge 72.63. The closest is Cu. Even the tallest peak (63) rules out Zn, Ga, and Ge.
- 2Numeric
The mass spectrum of silicon is shown. Use the mass numbers to estimate the average atomic mass of silicon.
amuExplanationThe average atomic mass is the abundance-weighted average of the isotopes: 28 × 0.9222 + 29 × 0.0469 + 30 × 0.0309 = 28.11. Using exact isotope masses gives 28.085, nearly the same. Mass 28 dominates, so the average is just above 28.
- 3Numeric
The mass spectrum of gallium (Ga, element 31) is shown. How many neutrons are in an atom represented by the peak at mass 71?
neutronsExplanationAll Ga atoms have 31 protons. Mass number = protons + neutrons, so neutrons = 71 − 31 = 40. The two peaks differ only in neutron count (the peak at 69 has 38).
- 4Multiple choice
The mass spectrum of bromine is shown; its average atomic mass is about 79.9. Which statement is supported by the spectrum?
ExplanationThe spectrum has peaks only at 79 and 81, with abundances of 50.7% and 49.3%, roughly half and half, so the average lands near 80. There is no peak at 80: no bromine atom has that mass. The average atomic mass isn’t the mass of any single atom. Isotopes have the same number of protons and differ in neutrons, and the peak at 79 is slightly taller.
- 5Multiple choice
The mass spectrum of boron is shown. Which best explains why boron’s average atomic mass is 10.81 rather than 10.5?
Explanation10.5 is the plain average of the two mass numbers, as if both isotopes were equally common. In fact ¹¹B is 80.1% and ¹⁰B only 19.9%: 10 × 0.199 + 11 × 0.801 = 10.80, closer to 11 (exact isotope masses give exactly 10.81).
- 6Multiple choice
The graph shows the three peaks from intact molecules in the mass spectrum of chlorine gas, (heights relative to the tallest peak). Which particle produces the peak at mass 72?
ExplanationChlorine has two stable isotopes, ³⁵Cl (75.76%) and ³⁷Cl (24.24%). A molecule of two atoms can only have mass 35 + 35 = 70, 35 + 37 = 72, or 37 + 37 = 74. The peak heights are about a² : 2ab : b² = 100 : 64 : 10; 70 is tallest because ³⁵Cl is the more common isotope. Two ³⁷Cl atoms give the smallest peak, at 74, and there is no stable ³⁶Cl in nature.
Your missed questions were saved automatically to your mistake notebook · 0
Your best score counts only full runs through the set, not retries of missed questions.