molar mass

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