9701 MCQ Solution 53

Question 53: [Inorganic > Period 3]

A mixture of the oxides of two elements of the third period is dissolved in water. The solution is approximately neutral.

What could be the constituents of the mixture?

A  Al2O3 and MgO

B  Na2O and MgO

C  Na2O and P4O10

D  SO3 and P4O10

Reference: Past Exam Paper -June 2003 Paper 1 Q13

Solution 53:

Answer: C.

A cannot be answer because MgO when dissolved in water will dissolve slightly and form an alkaline solution of approximately 9 pH while on the other hand Al2O3 is insoluble therefore won’t dissolve and won’t have any effect in the mixture therefore solution left is slightly alkaline and not neutral

B cannot be answer because when Na2O dissolve in water it will form a strong alkaline solution of pH 14 and the other hand MgO will dissolve slightly forming a solution of pH 9 the solution left is strongly alkaline and nowhere near neutral.

C is the answer because when Na2O dissolve in water it will form a strong alkaline solution of pH 14 because it is a strong base whereas in the other hand P4O10 is a strong acid and when dissolved in water it forms a pH of 1 so the final solution is neutral because the strong base Na2O will neutralize the acid resulting in a neutral solution at the end.

D cannot be answer because both SO3 and P4O10 are strong acids and will each form a pH of 1 resulting in a very acidic solution at the end.