A chemical reaction that gives off heat is most likely to be

A chemical reaction that gives off heat is most likely to be exothermic.

The answer is Sodium Sulfate = Na2SO4  

Explanation:

Molar mass of sulfate = 1 (S) + 4 (O) = 1 (32) + 4 (16) = 32 + 64 = 96  

Molar mass of sodium sulfate = 2 (23) + 96 = 46 + 96 = 142  

% of Sulfate = (96/142)*100 = 67.6%  

Percent mistake in Studen A,  

(I) % mistake = (67.6 - 68.6)/67.6 = 1.48  

(ii) % mistake = (67.6 - 66.2)/67.6 = 2.07  

(iii) % mistake = (67.6 - 67.1)/67.6 = 0.74  

For understudy B  

(I) % mistake = (67.6 - 66.7)/67.6 = 1.33  

(ii) % mistake = (67.6 - 66.6)/67.6 = 1.48  

(iii) % mistake = (67.6 - 66.5)/67.6 = 1.63  

Sutdent An is some how exact.  

Understudy B is exact however not precise.