Salts part 2

Starter How many words can you think of that start an- ? What do they each mean?

Some definitions Anhydrous – literally means without water Hydrated – Hydration occurs when water reacts with a compound producing a single chemical product. Water of crystallisation – Many salts form dry crystals containing molecules of water. This water which does not wet the crystals is known as water of crystallisation.

Some compounds contain H 2 O in their struc- ture. These compounds are called hydrates. This is different from (aq) because the H 2 O is part of the molecule (not just surrounding it). The H 2 O can usually be removed if heated. A dot separates water: e.g. CuSO 4 5H 2 O is copper(II) sulfate pentahydrate. A greek prefix indicates the # of H 2 O groups.

What would you call this? SnCl 2 2H 2 O – tin(II) chloride dihydrate 1 - Na 2 SO 4 10H 2 O 2 - NiSO 4 6H 2 O 3 - sodium carbonate monohydrate 4 - barium chloride dihydrate 1 - sodium sulfate decahydrate 2 - nickel(II) sulfate hexahydrate 3 - Na 2 CO 3 H 2 O 4-BaCl 2 2H 2 O

Calculating the water of crystallisation 1. Measure the mass of the salt you begin with. 2. Heat the salt to make the water of crystallization vaporize, leaving the dehydrated salt. 3. Measure the mass of the salt you have left after the heating.

- The salt after heating is a dehydrated salt. You can calculate exactly what amount of substance you have with the help of the molar mass for the salt. - The mass lost during heating consisted of water of crystallization. This can be used to calculate the amount of water of crystallization contained in the salt. - If you divide the amount of water of crystallization with the amount of dehydrated salt, you will know how many units of water of crystallization exists per unit of salt.

Calculate the % of water in hydrated magnesium sulphate MgSO 4 ·7H 2 O – Relative atomic masses: Mg = 24, S = 32, O = 16 and H = 1 – relative formula mass = (4 x 16) + [7 x ( )] = 246 – 7 x 18 = 126 is the mass of water – so % water = (126 / 246) x 100 = 51.2%

Some practice Q1 – A student found that a sample of hydrated cobalt (II) chloride contained 5.2g of anhydrous cobalt (II) chloride and 4.32g of water. Calculate the formula of hydrated cobalt (II) chloride (Hint cobalt chloride = CoCl 2 ) Q2 – 13.9g of hydrated iron (II) sulphate crystals were heated, driving off the water of crystallisation, leaving 7.6 of anhydrous iron (II) sulphate. Calculate the formula of hydrated iron (II) sulphate. (Hint Iron sulphate = FeSO 4 )

Q3 – Hydrated barium chloride is heated. Its mass decreases from 30.5g to 26.0g. What is the formula of the hydrated salt? Q4 – Hydrated sodium carbonate, Na 2 CO 3 ·xH 2 O was found to contain 62.9% water by mass. Calculate the value of x Q5 – Potash alum has the formula K 2 SO 4 ·Al 2 (SO 4 ) 3 ·yH 2 O, calculate y, the number of moles of water of crystallisation given that the hydrated salt contains 45.57% water by mass

Answers Q1 – CoCl 2 ·6H 2 O Q2 – FeSO 4 ·7H 2 O Q3 – BaCl 2 ·2H 2 O Q4 – 10 Q5 - 24