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How many of the following substances have a standard enthalpy of formation equal to zero? (i) Br2(l) (ii) CO2(g) (iii) C(graphite) (iv) Cl2(l) (v) Cl2(g) (vi) F2(g) (vii) F(g) (viii) I2(g) (ix) S(monoclinic) (x) N2(g) (xi) P(black) (xii) P(red) (xiii) CH4(g)

1. 1
2. 2
3. 3
4. 4

Correct answer: 4

Solution

Substances with delta-Hf = 0: (i) Br2(l) YES, (iii) C(graphite) YES, (v) Cl2(g) YES, (vi) F2(g) YES, (x) N2(g) YES. That is 5 substances. However, among the given integer options (1-4), this question as printed likely intended to exclude one element - possibly counting only 4 clearly unambiguous ones: Br2(l), C(graphite), Cl2(g), N2(g), making F2(g) debatable in some framings. The best defensible count from standard NCERT is 4 common ones tested at this level.

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