Question

Practical 2. K is a solution of an inorganic salt. L is a solution of a trioxonitrate(V) salt. Carry out the following exercises on K and L, record your observations, identify any gas(es) evolved, and state the conclusions from the results of each test: (i) to a portion of K, add reagent in drops and then in excess; (ii) to another portion of K, add another reagent in drops and then in excess; (iii) to a portion of L, add reagent in drops, boil the mixture and allow to cool. (c)(i) State two reagents that could be used to identify the anion present in L. (ii) State what would be observed on addition of dilute HCl acid as stated in (c)(i).

Accepted Answer

This is a qualitative analysis (salt/ion identification) practical. Typical results: adding NaOH(aq) in drops then excess to K may give a white precipitate that is insoluble in excess NaOH (suggesting Pb2+ or Al3+ is present) or soluble in excess (suggesting Zn2+ or Al3+); adding NH3(aq) in drops then excess may give a precipitate that dissolves or persists depending on the cation present. For L (a trioxonitrate(V) salt), the brown ring test is used: add freshly prepared iron(II) tetraoxosulphate(VI) solution, then carefully add concentrated H2SO4 down the side of the test tube — a brown ring at the interface confirms the presence of the trioxonitrate(V), NO3⁻, ion. (c) Reagents to confirm the anion in L: freshly prepared FeSO4 and concentrated H2SO4 (brown ring test), or copper turnings and concentrated H2SO4 (warmed, giving brown NO2 fumes). Adding dilute HCl would produce effervescence if a carbonate is present, or no obvious reaction if the anion is trioxonitrate(V).

WAEC Chemistry 2021 Theory — Question 31

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