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Chromium (III) Sulfate Formula is Cr2(SO4)3. It is a blue-grey coloured solid inorganic compound. Chromium sulfate is insoluble in water in its anhydrous form. However, it is soluble in water and alcohol when hydrated. Chromium(III) complexes generally have a slow ligand exchange. Aqueous Chromium sulfate changes from blue/violet to green when heated to a sufficiently high temperature. The color change is because of the slow dissociation of water ligands and coordination of sulfate ions.
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Key Terms: Chromium Sulfate, Chromium III Sulfate, Sulfate, Inorganic Compound, Molecule, Valency, Cation, Ligands, Ions, Alcohol, Temperature
Chromium III Sulfate Structure
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The ratio of chromium to sulfate in this inorganic compound is 2:3. The percentages of individual elements of Cr2(SO4)3 (dichromium trisulfate) are Cr - 26.52%, O - 48.95% and S - 24.53%.
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Chromium III Sulfate Formula
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- The formula of Chromium III Sulfate is Cr2(SO4)3. It is an ionic compound where Chromium (Cr) is the metal with Cr+3 cation and sulfate is the SO4-2 anion.
- Chromium III has a valence of 3 and Sulfate has a valence of 2, we will need 2 chromium and 3 sulfate ions to balance 6 valence ties.
- So, the Charge carried by the chromium cation Cr+3 will be written as the subscript of the sulfate anion SO4-2 and the Charge carried by the sulfate anion SO4-2 will be written as subscript for the chromium cation Cr+3 .
- The sulfur and oxygen atoms share a covalent bond. So, polyatomic ion SO4-2 will be placed inside parenthesis. Cr is placed outside the parentheses as it is not a polyatomic ion.
- So, to arrive at the formula of Chromium III Sulfate, we will interchange the charges of ions Cr+3and SO4-2 to get Cr2 and (SO4)3. When we write the two together we get Cr2(SO4)3.
Chromium III Sulfate Properties
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The properties of Chromium II Sulphate are as follows:
| Chromium III Sulfate Properties | |
|---|---|
| Chemical Formula | Cr2(SO4)3 |
| Appearance | Bluish gray crystalline solid |
| Density | 3.10 g/cm3 (anhydrous) 1.86 g/cm3 (penta decahydrate) 1.709 g/cm3 (octa decahydrate) |
| Molar Mass | 392.16 g/mol 608.363 g/mol (dodecahydrate) 716.45 g/mol (octadecahydrate) |
| Melting Point | 90 °C |
| Boiling Point | >700 °C (decomposes to chromic acid) |
| Solubility | Soluble in water and alcohol |
- Chromium III sulfate is a blue-gray or violet-gray amorphous solid.
- It turns green when it is subjected to heat.
- It is hygroscopic in nature.
Chromium III Sulfate Production
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Chromium III Sulfate is produced as a byproduct in the process of Jones Oxidation during which a solution of sodium dichromate or potassium dichromate is dissolved with sulfuric acid to oxidize alcohols.
Chromium(III) sulfate can also be obtained by treating chromium III hydroxide with dilute sulfuric acid.
Chromium III Sulfate Uses
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- It is extensively used in the leather industry during the process of tanning.
- It is used in textile dyes particularly for dyeing of khaki cloth.
- It is also used in green paints, ceramic glazes and inks.
- It is used in chrome plating to decorate and protect metals.
- It serves as electrolyte to obtain chromium metal
Things to Remember
- The molecular formula of Chromium III Sulfate is Cr2(SO4)3.
- Chromium III Sulfate is an inorganic ionic compound.
- It is very harmful to the environment and a major health hazard.
- It is corrosive in nature and categorized as an irritant.
- Chromium III Sulfate is non combustible.
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Sample Questions
Ques. What type of compound is Chromium III Sulfate? Is it ionic or molecular? (2 Marks)
Ans. Chromium III Sulfate Cr2(SO4)3 is an ionic compound where Cr is the metal with Cr+3 cation and sulfate is the SO4-2 anion. The sulfur and oxygen atoms are inside brackets indicating a covalent bond between them. Cr is without parenthesis which indicates it is not a polyatomic ion.
Ques. What is the oxidation number of Cr in the Cr2(SO4)3? (3 Marks)
Ans. Suppose the oxidation number of Cr is taken as x
Since Cr2(SO4)3 is a molecule, it’s oxidation number will be 0.
Substituting the values of oxidation numbers in the formula as below:
O = -2, S = +6 and Cr = x
Therefore the equation for Cr2(SO4)3 will be
2x + [(+6)⋅(3)+(−2)⋅(4)⋅(3)]=0
2x+(+18)+(−24)=0
2x+(−6)=0
2x=+6
⇒x=+3
Hence, the oxidation number of the Cr in Cr2(SO4)3 is +3.
Ques. Why does chromium iii sulfate change to green color when heated? (2 Marks)
Ans. Chromium(III) complexes generally have a slow ligand exchange. When an aqueous solution of this compound is heated, it changes its color to green as a result of the slow dissociation of water ligands and coordination of sulfate ions.
Ques. Calculate Gram Formula Mass of Chromium III Sulfate. (5 Marks)
Ans. Let us look at the individual elements of Chromium III Sulfate molecule.
Cr2(SO4)3 = Cr2(S1x3O4x3)
No. of atoms of each element present in this ionic compound are given below:
Chromium III (Cr+3) ions = 2
Sulfide (S-2) ions = 3
Oxygen (O) atoms = 12
Gram Atomic Mass of these individual elements are :
Chromium, Cr = 52.00 g/mol
Sulfur, S = 32.06 g/mol
Oxygen, O= 16.00 g/mol
So, Gram Formula Mass (GFM) for Cr2(SO4)3 will be calculated as below:
GFM for Cr2(SO4)3 = (2 x 52) + (3 x 32.06) + (12 x 16) = 104.0+ 96.18 + 192.0 = 392.1 g/mol
Ques. How to determine the formula of Chromium III Sulfate? (5 Marks)
Ans. To determine the formula for Chromium III Sulfate we will use the criss-cross method wherein we interchange the charges carried by the cation and anion present in the ionic compound.
Chromium III has a valence of 3 and Sulfate has a valence of 2, we will need 2 chromium and 3 sulfate ions to balance 6 valence ties.
So, the Charge carried by the chromium cation Cr+3 will be written as the subscript of the sulfate anion SO4-2 and the Charge carried by the sulfate anion SO4-2 will be written as subscript for the chromium cation Cr+3 . The polyatomic ion SO4-2 will be placed inside parenthesis.
So, to arrive at the formula of Chromium III Sulfate, we will interchange the charges of ions Cr+3and SO4-2 to get Cr2 and (SO4)3. When we write the two together we get Cr2(SO4)3.
Ques. What is the difference between Gram Formula Mass (GFM) and Gram Molar Mass (GMM)? (1 Mark)
Ans. There are two types of compounds namely Molecular and Ionic. Molecular Compounds form molecules, which have a Gram Molar Mass. Ionic Compounds form Formula Units, which have a Gram Formula Mass.
Ques. Calculate the number of sulfate ions in 39.2 g of chromium sulfate. (2 Marks)
Ans. Gram molecular mass of Cr2(SO4)3 = 392.6 g
No. of moles in 39.2 g of Cr2(SO4)3 = 39.2/392.6 = 0.1 mol
1 mol of Cr2(SO4)3 contains = 3 x 6.023 x 1023 sulfate ions
So, No. of Sulfate ions in 0.1 mol of Cr2(SO4)3 will be= 0.1 x 3 x 6.023 x 1023
= 1.8069 x 1023
Ques. What are the uses of chromium sulfate? (3 Marks)
Ans. The uses of chromium sulfate are as follows:
- It is extensively used in the leather industry during the process of tanning.
- It is used in textile dyes particularly for dyeing of khaki cloth.
- It is also used in green paints, ceramic glazes and inks.
- It is used in chrome plating to decorate and protect metals.
- It serves as electrolyte to obtain chromium metal
Ques. Write the percentage of each element in chromium iii sulfate. (2 Marks)
Ans. The percentages of individual elements of Chromium III Sulfate - Cr2(SO4)3 are Cr - 26.52%, O - 48.95% and S - 24.53%
Ques. Write the formula of Chromium III Sulfate. (1 Mark)
Ans. The formula of Chromium III Sulfate is Cr2(SO4)3.
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