CAIE A-Level Chemistry 2.3.4 Hydrated and Anhydrous Compounds

Practise using anhydrous, hydrated and water of crystallisation correctly in formulae and heating data.

Exam points

identify a hydrated salt as containing water of crystallisation in a fixed formula ratio
identify the anhydrous substance as the solid remaining after complete water removal without decomposition
use the dot formula salt·xH2O and distinguish bound crystal water from an aqueous solution

[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

All question bank results loaded

2.3.4—Terms: anhydrous, hydrated and water question 1