The molar mass of a substance is the weight in grams of one mole of that substance. Said differently, it is how much 6.022x1023 units of a substance weighs. Thusly it has units of g/mol and is used to convert grams into moles and moles into grams. Molar mass is generally used in reference to atoms or molecules.

To calculate the molar mass of a formula (also known as relative formula mass, or Mr), simply add together the Ar (relative atomic mass) of all the elements in that formula.

One mole of atoms or molecules of any substance will have a mass in grams equal to the relative formula mass of that substance.

So, the molar mass of MgCl2 is -
Ar of Mg = 24
Ar of Cl = 35.5. * 2 because there are two Cl.
24 + (35.5 * 2 ) = 95. So the molar mass of MgCl2 is simply 95.

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