Verifying chemical mass statements for accuracy

Steps:

• Calculate relative formula mass for A: Na₂CO₃ = (2×23) + 12 + (3×16) = 106; 106 + 18x = 286 gives x = 10, correct.
• Calculate relative molecular mass for B: H₃PO₃ = (3×1) + 31 + (3×16) = 82, not 50, incorrect.
• Analyze C: Relative atomic mass is weighted average of isotope masses; for 20 and 22, midpoint average is 21 assuming equal abundance.
• Analyze D: Relative atomic mass defined as average isotope mass relative to hydrogen atom of mass 1.

Why B is correct:

• Relative molecular mass of H₃PO₃ is sum of relative atomic masses = 82, not 50, per standard formula M_r = Σ(A_r).

Why the others are wrong:

• A: Calculation exactly yields x=10 using standard atomic masses.
• C: Average of 20 and 22 isotopes is 21, as stated.
• D: Definition matches basic comparison to hydrogen atom mass of 1.

Final answer: B