Group 15 Elements: Exercises and Case studies

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Chapter: Essentials of Inorganic Chemistry : Group 15 Elements

Case studies 1. Phosphate solution for rectal use 2. Forensic test for arsenic



1. Oral phosphate preparations contain typically monobasic sodium phosphate, and/or disodium phosphate. Determine the chemical formulae of these three compounds.


2. Draw the Lewis structure of the following molecules and determine the oxidation state of phosphorus:

(a)        H3PO4

(b)       PCl3

(c)        Na2HPO4


3. Determine the oxidation state of arsenic in the following molecules:

(a)        As2O3

(b)       H3AsO4

(c)        AsF5


4. A typical IV solution for the treatment of hypophosphataemia contains 100 mmol/l phosphate, 19 mmol/l potassium ions and 162 mmol/l sodium ions. Calculate the amount of phosphate, potassium and sodium ions present in 500 ml. Express your answer in grams.


Case studies


1. Phosphate solution for rectal use

Your pharmaceutical analysis company has been contacted by an important client and asked to analyse a batch of phosphate solutions for use in enemas. The description of your brief states that you are supposed to analyse the APIs in this solution following standard quality assurance guidelines.

Typical analysis methods used for quality purposes are based on titration reactions. A certain amount of solution is diluted with water. A known amount of this solution is then titrated with sodium hydroxide or hydrochloric acid depending on the API. Phenolphthalein and methyl red solutions are typically used as indicators.

(a)    Research the APIs typically present in a solution used for enemas and describe their chemical structure. Describe the type of titration suggested. Describe the chemical structure and mode of action of the indicators.

(b)   Formulate the relevant chemical equations.

(c)    Phosphate enemas typically contain 12.8 g sodium dihydrogen phosphate dihydrate and 10.24 g dis-odium phosphate dodecahydrate and water made up to a 128 ml solution.

For the determination of the sodium dihydrogen phosphate dehydrate content, 20 ml of this solution is diluted with 80 ml of water and titrated with 0.5 M sodium hydroxide solution using phenolphthalein as indicator. The following amounts of sodium hydroxide are used:

25.0 ml 25.6 ml 25.4 ml

For the determination of the disodium hydrogen phosphate dodecahydrate content, 50 ml of this solu-tion is titrated with 0.5 M hydrochloric acid using methyl red as indicator. The following amounts of hydrochloric acid are used:

22.0 ml 22.3 ml 22.4 ml

Calculate the amount of sodium dihydrogen phosphate and disodium hydrogen phosphate present in your sample. Express your answer in grams and moles.

(d)   Critically discuss your result in context with the stated value for the API.

(e)    Research the typically accepted error margins.


2. Forensic test for arsenic

Arsenic compounds are known for their toxicity and are often used in poisons. In order to obtain forensic evidence, specific tests were developed for the qualitative analysis of arsenic-based compounds. The so-called Marsh test was published in 1836 and is based on the reduction of As3+ in the presence of Zn.

(a)    Research the conditions needed for the Marsh test.

(b)   Formulate the relevant reduction and oxidation equations.

(c)    Is it possible to use the Marsh test as a quantitative test?


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