Which of the following observations would violate the law of constant proportions?
A sample of magnesium sulphide containing magnesium and sulphur in a 4:3 mass ratio.
The law of constant (definite) proportions says that a given pure compound always contains the same elements combined in the same fixed proportion by mass. For each compound we compare the mass ratio in the sample with the fixed ratio expected from atomic masses.
Ammonia NH3: N:H = 14 : 3×1 = 14:3. Sodium chloride NaCl: Na:Cl = 23:35.5. Water H2O: H:O = 2:16 = 1:8. All three match the expected ratios, so those samples obey the law.
Magnesium sulphide MgS: Mg:S = 24:32 = 3:4, not 4:3. A sample showing 4:3 has the wrong proportion for MgS and would therefore violate the law of constant proportions.
Hence, the answer is A sample of magnesium sulphide containing magnesium and sulphur in a 4:3 mass ratio.
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