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Water of Crystallisation Questions

1) The sample of hydrated calcium sulphate, CaSO4ŸxH2O, has a relative formula mass of 172. By calculating the masses, x is determined to be 2. 2) The hydrated salt with the empirical formula CaN2H8O10 is found to have x=4 waters of crystallization in its dot formula of CaN2O6Ÿ4H2O. 3) A hydrated carbonate of an unknown Group 1 metal has the formula X2CO3Ÿ10H2O and a relative formula mass of 286. Calculating the mass of sodium determines the Group 1 metal is sodium

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Water of Crystallisation Questions

1) The sample of hydrated calcium sulphate, CaSO4ŸxH2O, has a relative formula mass of 172. By calculating the masses, x is determined to be 2. 2) The hydrated salt with the empirical formula CaN2H8O10 is found to have x=4 waters of crystallization in its dot formula of CaN2O6Ÿ4H2O. 3) A hydrated carbonate of an unknown Group 1 metal has the formula X2CO3Ÿ10H2O and a relative formula mass of 286. Calculating the mass of sodium determines the Group 1 metal is sodium

Uploaded by

Atulya Bharadwaj
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Water of Crystallisation Questions

1) A sample of hydrated calcium sulphate, CaSO4ŸxH2O, has a relative


formula mass of 172. What is the value of x?
40 + 32 + (4x16) + n(2 + 16) = 172 So: 136 + 18n = 172 -> n = 2

2) A hydrated salt is found to have the empirical formula CaN2H8O10.


What is its dot formula?
CaN2H8O10 – how many times can H2O go into this? The H8 means only 4 times.
Therefore, there are 6 O’s left over. These join to the first part. CaN2O6Ÿ4H2O.

3) A hydrated carbonate of an unknown Group 1 metal has the formula


X2CO3Ÿ10H2O and is found to have a relative formula mass of 286. What is
the Group 1 metal?
2x + 12 + 48 + 20 + (10 x 16) = 286 So: 2x + 240 = 286 -> x=23. Answer: Na/Sodium

4) A teacher told a student that the amount of hydrated salt in lansfordite


was 0.030 mol, and that the amount of water lost on heating was 0.15 mol.
Calculate the value of x in the formula MgCO3ŸxH2O?
0.15 / 0.03 = 5 So: there are 5 times more moles of water. Answer: x=5

5) 11.25 g of hydrated copper sulphate, CuSO4ŸxH2O, is heated until it


loses all of its water. Its new mass is found to be 7.19 g. What is the value of
x?
CuSO4ŸxH2O = 11.25g
CuSO4 = 7.19g
xH2O = 4.06g (minus 7.19g from 11.25g)

63.5 + 32 + 64 = 159.5 à 7.19g / 159.5 = 0.04508 Moles of CuSO4


2 + 16 = 18 à 4.06g / 18 = 0.2256 Moles of H2O
0.2256 / 0.04508 = 5.006 times more H2O. Therefore x=5.

6) 13.2 g of a sample of zinc sulphate, ZnSO4.xH2O, was strongly heated


until no further change in mass was recorded. On heating, all the water of
crystallisation evaporated as follows: ZnSO4ŸxH2O è ZnSO4 + xH2O.
Calculate the number of moles of water of crystallisation in the zinc sulphate
sample given that 7.4 g of solid remained after strong heating.
ZnSO4 = 65 + 32 + 64 = 161
H2O = 2 + 16 = 18

7.4g / 161 = 0.045963 Moles of ZnSO4


13.2g – 7.4g = 5.8g
5.8g / 18 = 0.3222 Moles of H2O
0.3222 / 0.045963 = 7 times more H2O. x=7

7) A sample of hydrated magnesium sulphate, MgSO4ŸxH2O, is found to


contain 51.1% water. What is the value of x?
100 – 51.1 = 48.9
24 + 32 + 64 = 120
48.9 / 120 = 0.4075 Moles of MgSO4

51.1 / 18 = 2.8389 Moles of H2O


2.8389 / 0.4075 = 6.967 = 7 moles of H2O. x=7.

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