Formula Mass.

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Presentation transcript:

Formula Mass

Formula mass Formula mass – the formula mass of any molecule, formula unit, or ion is the sum of the average atomic masses of all the atoms represented in its formula. Molar mass- the molar mass of a substance is equal to the mass in grams of one mole, or 6.022 x 1023 particles of the substance. NOTE- These are 2 different terms but both are calculated the same way. Know these terms, but usually we will refer to the molar mass as just that, or we may use “gfm” (the gram formula mass)

How to Find the Formula Mass 1. Write a CORRECT formula for the compound 2. Look up the atomic mass of each element from the periodic table in the compound and round it to a whole number (EXCEPTIONS: round Cl to 35.5 and Cu to 63.5). 3. Multiply the rounded atomic mass by the subscripts, if any. 4. Add all masses of elements together and use the unit, g/mol

Example Ex: Find the formula mass of calcium phosphate 1. Ca3(PO4)2 1. Ca3(PO4)2 2. Ca = 40 x 3 = 120 3. P = 31 x 2 = 62 O = 16 x 8 = 128 310 g/mol

Examples l. Find the formula mass of ammonium sulfate 2. Find the molecular mass of dichlorine heptoxide

Hydrates Some compounds trap water inside their crystal structure and are known as hydrates. You will not be able to predict which compounds will form hydrates. All you have to do is to be able to name them and find their molar masses (including the water) CuSO4 • 5H20 is an example of a hydrate. This says that one formula unit of copper(II) sulfate will trap 5 molecules of water inside its crystal.

Hydrates Hydrates are named by naming the ionic compound by the regular rules and then adding (as a second word) a prefix indicating the number of water molecules. You will use the word “hydrate” to indicate water. The above compound would be called copper(II) sulfate pentahydrate. To find the formula mass of a hydrate, simply find the mass of the ionic compound by itself and then ADD the mass of water molecule(s) to that mass.

Practice 1. What is the formula mass of barium chloride dihydrate? 2. What is the formula mass of aluminum sulfate octahydrate?