CAIE A-Level Chemistry AS 2.3.4 Terms Anhydrous Hydrated and Water Questions

Practise using anhydrous and hydrated salt terminology and calculating water of crystallisation from formulae and relative molecular masses.

Exam points

Use the terms anhydrous, hydrated and water of crystallisation when determining formulae of hydrated salts.

[Maximum number: 3]

Species such as $\mathrm{NH}_{4}{ }^{+}, \mathrm{CO}_{3}{ }^{2-}$ and $\mathrm{PO}_{4}{ }^{3-}$ are examples of molecular ions.

There are many naturally occurring hydrated compounds that contain the anion $\mathrm{PO}_{4}{ }^{3-}$ .

Struvite is a soft hydrated mineral with $M_{\mathrm{r}}=245.3$ . The anhydrous form of the mineral has the formula $\mathrm{NH}_{4} \mathrm{MgPO}_{4}$ .

Calculate the number of molecules of water of crystallisation in struvite.
Give your answer to the nearest integer. Show your working.

number of molecules of water of crystallisation =

(molecular mass of $\mathrm{NH}_{4} \mathrm{MgPO}_{4}$ ) = 137.3

$\left(245.3-M_{r}\left(\mathrm{NH}_{4} \mathrm{MgPO}_{4}\right) \div 18.0\right)$ =6

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CAIE A-Level Chemistry AS 2.3.4 Terms Anhydrous Hydrated and Water Questions question 1