CHEMISTRY TEST ASSIGNMENT
The test is divided into two different section Total Marks: 50
Section 1: Numerical – 30 Marks
Section 2: Theoretical – 20
Marks Section 1 –
Numerical
1. We have about 10g of Iron (II) ammonium sulfate crystals. We mix these and make a
250cm3 acidified solution of this. Approximately 21.25cm3 of potassium dichromate
(VI) (0.02 mol dm-3) is needed for oxidation of 25cm3 of the solution made. Given the
formula of crystal as Fea(NH4)b(SO4)C. H2O , the value of a+b+c+x is?
Solution
The general formula of Iron (II) ammonium sulfate crystals is FeSO4. (NH4)2S04.
xH2O
This means that a is 1, b is 2, c is 1 and x is the water of crystallization.
Therefore, we are looking for the water of crystallization:
Moles of dichromate ions,
2-¿
Cr
2O7 ¿
Moles = molarity x volume / 1000
= 0.0200 x 21.25 / 1000 = 0.000425 mol
Moles of Iron (II) ammonium sulfate salt;
Moles of iron salt = mol Fe2+ titrated
= 6 x Cr2O72– = 6 x 0.000425 = 0.00255 mo