Isotopic combinations of carbon and chlorine in C₂H₅Cl Steps:

• Base mass with two ¹²C, five ¹H, ³⁵Cl: 2×12 + 5×1 + 35 = 64
• One ¹³C substitution adds +1: possible mass 65 (with ³⁵Cl) or 67 (with ³⁷Cl)
• ³⁷Cl substitution adds +2: possible mass 66 (with two ¹²C) or overlaps at 66/67 with ¹³C
• Distinct masses from these combinations: 64, 65, 66, 67 Why C is correct:
• Four distinct masses arise from the primary isotopic variants (0 or 1 ¹³C, and ³⁵Cl or ³⁷Cl), per standard mass spectrometry patterns for such compounds. Why the others are wrong:
• A. 2: Counts only Cl isotopes, ignoring ¹³C contribution.
• B. 3: Misses one combination, like the +1 from ¹³C with ³⁵Cl.
• D. 5: Includes rare two-¹³C case (mass 68), not standard for primary possibilities. Final answer: C