What is the Relative molecular mass if ZnSO4.7H2O?
What is the relative molecular mass of ZnSO4·7H2O? In this formula, does the dot (·) indicate addition or multiplication?
4 Answers
Feb 03, 2025
The dot between the compound and the hydrate (7H20) just indicates that it is a hydrated crystal. So, just add accordingly. Your answer should be about 287.56 g/mol.
To consider the masses of molecules measured in grams, for example, would be to deal inconveniently with extremely small numbers. Rather, the mass of a molecule is compared with that of an atom of carbon-12. The relative atomic mass of carbon-12 is taken to be 12. Relative masses have no units because they have cancelled in their calculation. A relative molecular mass can be calculated easily by adding together the relative atomic masses of the constituent atoms. For example, ethanol, CH3CH2OH, has a Mr of 46.
The ‘.’ is normally placed in the centre of the line as in ‘ ∙ ‘
ZnSO4∙7H2O. meaning the hydrate molecule contains 7 molecules of water of crystallisation.
Simply adding all the atomic masses of the elements, will give the Molecular mass of the hydrate.
(As a matter of interest..If you take the mol.mass of the ZnSO4∙7H2O and then divide the mol.mass of the 7H2O by the total mol. mass and multiply this by 100%, you’ll find the Percent of water in the hydrate…as follows..
ZnSO4 Mol mass = 65 + 32 + (4 x 16) = 145g/mol.
7H2O mol mass = (7 x 2 x 1) + (7 x 16) = 126g/mol.
Total = 271g/mol.
126g/mol / 271g/mol x 100% = 46.5% water in the hydrate.
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