The Student Room Group

Chemistry A-level OCR transition metals

Please help with this question, I have no idea how to start it.

Question 2 (2118)
A student carries out an experiment to determine the percentage by mass of copper in an ore containing copper in its +2 oxidation state.
The student is provided with a sample of the copper ore, 1 moldm-3 potassium iodide, KI(aq), and 0.0200 mol dm 3 sodium thiosulfate, Na,S,0з.
The student's method is outlined below.
Step 1 Add an excess of warm nitric acid to 2.50 g of the ore.
The copper(I) compounds in the ore react, forming aqueous copper(Il) nitrate.
Step 2 Filter the mixture to remove the unreacted rock. Neutralise the filtrate.
Step 3 Add an excess of aqueous potassium iodide, KI(aq).
A precipitate of copper(1) iodide and a solution of iodine, I(aq), forms.
Step 4 Titrate the mixture from Step 3 using 0.0200 mol dm 3 sodium thiosulfate, Na,S,Oz in
the burette.
12(ag) + 25,0,2-(aq) -> 21-(aq) + 5,062-(aq)
26.55 cm° of 0.0200 moldm-3 Na,SO are required to reach the end point.
(a) In Step 1, the student observed that bubbles of gas were produced.
Suggest the formula of the copper(II) compound which reacted with HNO to form the gas, and write a full equation for the reaction.
(b) Write an ionic equation, including state symbols, for the reaction in Step 3.
(c) Suggest a suitable indicator for this titration and state the colour change at the end point in Step 4.
Colour from:
(edited 10 months ago)

Quick Reply