A 2.00 g ore sample is made up to 250 cm³, and the solution contains 0.0035 mol/dm³ copper. What percentage of the ore is copper?

The ore is about 2.8% copper. The working is 0.0035 × 0.250 = 0.000875 mol, × 63.5 = 0.0556 g, then 0.0556 ÷ 2.00 × 100 = 2.8%.

How is the aspirin content of a tablet found by colorimetry?

The aspirin is hydrolysed with sodium hydroxide to form salicylate ions. Iron(III) ions are added to give a violet complex whose absorbance is measured.

Why must aspirin be hydrolysed before the iron(III) test?

Iron(III) forms the violet complex with the phenol group of salicylate. Aspirin's phenol group is blocked as an ester until hydrolysis frees it.

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Analytical Chemistry Techniques

Colorimetric analysis of copper ore, aspirin and iron

3 things to know
A2 9

Overview

Copper in ore with ammonia and a worked percentage, aspirin with iron(III) after hydrolysis, iron in aluminium foil with thiocyanate, their equations, accuracy and improvements.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Key term

How is copper ore prepared for colorimetric analysis?

Key term

Write the equation for the formation of the deep blue copper–ammonia complex.

Key term

Why is ammonia added before measuring copper by colorimetry?

Questions from this subtopic

Each one with the model answer underneath.

1
How is copper ore prepared for colorimetric analysis?
Model answer
The ore is dissolved in nitric acid to release copper(II) ions. Excess ammonia solution is then added to form a deep blue complex.
2
Write the equation for the formation of the deep blue copper–ammonia complex.
Model answer
[Cu(H₂O)₆]²⁺ + 4NH₃ → [Cu(NH₃)₄(H₂O)₂]²⁺ + 4H₂O. The deep blue complex absorbs far more strongly than the pale blue aqua ion.
3
Why is ammonia added before measuring copper by colorimetry?
Model answer
The ammonia complex is much more intensely coloured than copper(II) in water. The analysis becomes more sensitive to small amounts of copper.
4
A 2.00 g ore sample is made up to 250 cm³, and the solution contains 0.0035 mol/dm³ copper. What percentage of the ore is copper?
Model answer
The ore is about 2.8% copper. The working is 0.0035 × 0.250 = 0.000875 mol, × 63.5 = 0.0556 g, then 0.0556 ÷ 2.00 × 100 = 2.8%.
5
How is the aspirin content of a tablet found by colorimetry?
Model answer
The aspirin is hydrolysed with sodium hydroxide to form salicylate ions. Iron(III) ions are added to give a violet complex whose absorbance is measured.
6
Why must aspirin be hydrolysed before the iron(III) test?
Model answer
Iron(III) forms the violet complex with the phenol group of salicylate. Aspirin's phenol group is blocked as an ester until hydrolysis frees it.

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