Waters of Crystallisation Calculations

Question 1

How do you calculate simple waters of crystallisation (from Mr)?

Answer 1

1) Subtract the Mr of anhydrous salt from the total Mr

2) Divide remaining Mr by Mr of water (18) to find the moles of water

Question 2

Simple Waters of Crystallisation Calculation Example

Answer 2

If molar mass of Na2SO4. 𝑥H2O is 322.1 gmol^-1 . How many waters of crystallisation are present?

Na2SO4 = 142.1 (Mr) H2O = 18 (Mr)

322.1 - 142.1 = 180

𝑥H2O = 180 -> 18𝑥 = 180
𝑥 = 10

10 waters of crystallisation are present

Question 3

How do you calculate simple waters of crystallisation (from Mr and Mass)?

Answer 3

1) Minus know value of one from the total value of the overall mass in order to get the other value
2) Divide the mass by the Mr to get moles for each value
3) Divide each answer by the lowest answer from step 2
(ie. empirical formula method)

Question 4

Waters of Crystallisation Calculation Example

Answer 4

A 15.00g of hydrated salt, NaSO4. 𝑥H2O was heated and 7.05g of water was removed. value of 𝑥?

Na2SO4 = 142.1 (Mr) H2O = 18 (Mr)

15. 00 - 7.05 = 7.95g (Na2SO4)
16. 95 / 142.1 = 0.05594… 7.05 / 18 = 0.39166…
17. 05594… / 0.05594 = 1
18. 039166… / 0.05594 = 7