Alum Molar Mass (With Calculations)

Alum Molar Mass

Molar mass of Alum [KAl(SO4)2 · 12H2O] is 474.372 g/mol.

Well, now you have come to know the molar mass of Alum.

But how can you get this value?

Let me show you the calculation to get the molar mass of Alum [KAl(SO4)2 · 12H2O].

You can also refer to this one minute video which will show you the simple steps to calculate the molar mass of any compounds.

Alum [KAl(SO4)2 · 12H2O] Molar Mass Calculation

If you have a periodic table with you, then you can easily calculate the molar mass of Alum [KAl(SO4)2 · 12H2O].

Because the molar mass of any molecule (or compound) can be calculated by simply adding the molar masses of individual atoms.

Now here we have to find the molar mass of Alum.

So for that, have a look at the periodic table given below.

You can see the molar mass value of all the atoms from this periodic table.

Now in Alum [KAl(SO4)2 · 12H2O], there is Potassium atom, Aluminum atom, Sulfur atoms, Oxygen atoms and Hydrogen atoms.

So let’s look at the molar mass of Potassium, Aluminum, Sulfur, Oxygen and Hydrogen from the above periodic table.

You can see that;

The molar mass of Potassium is 39.098 g/mol. [1]

The molar mass of Aluminum is 26.982 g/mol. [2]

The molar mass of Sulfur is 32.06 g/mol. [3]

The molar mass of Oxygen is 15.999 g/mol. [4]

The molar mass of Hydrogen is 1.008 g/mol. [5]

Now, to calculate the molar mass of Alum, you just have to add the molar mass of all the individual atoms that are present in Alum.

So, Molar mass of Alum [KAl(SO4)2 · 12H2O] = [Molar mass of Potassium atom + Molar mass of Aluminum + {Molar mass of Sulfur atom + (Molar mass of Oxygen atom × 4)} × 2] + [12 × {(Molar mass of Hydrogen × 2) + Molar mass of Oxygen}]
= [39.098 + 26.982 + {32.06 + (15.999 × 4)} × 2] + [12 × {(1.008 × 2) + 15.999}]
= [39.098 + 26.982 + {32.06 + 63.996} × 2] + [12 × {2.016 + 15.999}]
= [39.098 + 26.982 + {96.056} × 2] + [12 × {18.015}]
= [39.098 + 26.982 + 192.112] + [216.18]
= [258.192] + [216.18]
= [258.192] + [216.18]
= 474.372 g/mol

Hence the Molar mass of Alum is 474.372 g/mol.

I hope you have understood the short and simple calculation for finding the molar mass of Alum.


  • In some books, you may see the unit of molar mass as grams/mole or g/mole. But all these units (i.e g/mol, grams/mole and g/mole) are the same. 
  • Always follow the calculation order to avoid any mistakes in calculation. First solve the brackets, then multiplications and at last do the final addition.
  • And don’t forget to put the unit g/mol to your final calculated molar mass.

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Jay is an educator and has helped more than 100,000 students in their studies by providing simple and easy explanations on different science-related topics. With a desire to make learning accessible for everyone, he founded Knords Learning, an online learning platform that provides students with easily understandable explanations.

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