Magnesium arsenide oxide

In a further method the sample is heated in a sealed tube for 5 min with a mixture (3 + 1) of magnesium and magnesium oxide, which converts all the arsenic into magnesium arsenide On decomposition by dilute sulphuric acid, arsine is evolved and is absorbed in a 0.5% solution of silver diethyldithiocarbamate (AgDDC) in pyridine. The colour produced has an absorption maximum at 560 nm and is proportional to concentration up to 20 fig of arsenic in 3 ml of solution. Alternatively, the arsine is oxidized by bromine and determined iodimetrically. 

Write a molecular formula for each compound, and indicate the oxidation state of the group 5A element in each formula (a) phosphorous acid, (b) pyrophosphoric acid, (c) antimony trichloride, (d) magnesium arsenide, (e) diphosphorus pent-oxide, (f) sodium phosphate. 

---