Which pair of symbols below show different isotopes of the same element?

(A) 3918A and 3919R
(B) 6027X and 5928Y
(C) 126L and 146L
(D) 3717X and 3717X
(E) 32E and 31G

2019 U.S. National Chemistry Olympiad Local Section Exam Question 11

The concentration of sulfate ion in a solution is measured by precipitating the sulfate as BaSO4, filtering the precipitate on ashless filter paper, and heating the filter paper and precipitate in a tared crucible with a Bunsen burner. Which error will result in a sulfate concentration that is higher than the actual concentration?

(A) The empty crucible contains a few drops of water when it is tared.
(B) A glass fiber filter is used instead of ashless filter paper.
(C) Some fine precipitate is not captured by the filter.
(D) Some of the sulfate-containing solution spills before the BaCl2 solution is added.