A vital skill for future chaptersis the capability to identify the mass of the formula of an ionic compound. This amount is dubbed the formula mass. The formula fixed is acquired by including the masses of each individual atom in the formula that the compound. Because a ideal formula is electrically neutral (with no net electrons acquired or lost), the ions deserve to be considered atoms because that the function of calculating the formula mass.

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Let us begin by calculating the formula fixed of salt chloride (NaCl). This formula mass is the sum of the atomic masses that one salt atom and also one chlorine atom, which we find from the routine table; here, we use the masses to two decimal places:

Na: 22.99 amu

Cl: +35.34 amu

Total: 58.44 amu

To 2 decimal places, the formula fixed of NaCl is 58.44 amu.

When an ionic compound has an ext than one anion or cation, you must remember to usage the appropriate multiple of the atom mass for the aspect in question. For the formula mass of calcium fluoride (CaF2), we need to multiply the mass of the fluorine atom by 2 to account for the 2 fluorine atom in the tasiilaq.netical formula:

Ca: 1 x 40.08 = 40.08 amu

F: 2 x 19.00 = +38.00 amu

Total = 78.08 amu

The formula mass of CaF2 is 78.08 amu.

For ionic compounds through polyatomic ions, the amount must include the number and also mass of each atom in the formula because that the polyatomic ion. For example, potassium nitrate (KNO3) has actually one potassium atom, one nitrogen atom, and three oxygen atoms:

K: 1 x 39.10 = 39.10 amu

N: 1 x 14.00 = +14.00 amu

O: 3 x 16.00 = +48.00 amu

Total = 101.10 amu

The formula fixed of KNO3 is 101.10 amu.

Potassium nitrate is a crucial ingredient in gunpowder and also has been used clinically together a diuretic.

When a formula contains an ext than one polyatomic unit in the tasiilaq.netistry formula, together in Ca(NO3)2, do not forget to multiply the atomic mass of every atom within of the bracket by the subscript external of the parentheses. This is necessary since the subscript refers to the entire polyatomic ion. Thus, because that Ca(NO3)2, the subscript 2 means two finish nitrate ions, for this reason we must sum the masses of two (1 × 2) nitrogen atoms and also six (3 × 2) oxygen atoms, along with the mass of a single calcium atom:

Ca: 1 x 40.08 = 40.08 amu

N: 2 x 14.00 = +28.00 amu

O: 6 x 16.00 = +96.00 amu

Total = 164.08 amu

The vital to calculating the formula mass of an ionic link is to properly count each atom in the formula and multiply the atomic masses the its atom accordingly.

Example (PageIndex1)

Use the atom masses (rounded to two decimal places) to identify the formula mass because that each ionic compound.

FeCl3 (NH4)3PO4



Fe: 1 x 55.85 = 55.85 amu

Cl: 1 x 35.45 = +106.35 amu


Total = 162.20 amu

The formula mass of FeCl3 is 162.2 amu.

b. When we distribute the subscript 3 v the parentheses comprise the formula for the ammonium ion, we view that we have actually 3 nitrogen atoms and 12 hydrogen atoms. Thus, we collection up the sum as follows:

N: 3 x 14.00 = 42.00 amu

H: 12 x 1.00 = +12.00 amu

P: 1 x 30.97 = +30.97 amu

O: 4 x 16.00 = +64.00 amu

Total = 148.97 amu

The formula mass for (NH4)3PO4 is 149.0 amu.

Exercise (PageIndex1)

Use the atomic masses (rounded to two decimal places) to determine the formula mass for each ionic compound.

TiO2 AgBr Au(NO3)3 Fe3(PO4)2


a. 79.87 amu

b. 187.77 amu

c. 383.0 amu

Some ionic compounds have water (H2O) integrated within their formula unit. These compounds, dubbed hydrates, have a characteristic variety of water units linked with every formula unit the the compound. Hydrates space solids, not liquids or solutions, regardless of the water they contain.

To compose the tasiilaq.netistry formula the a hydrate, create the number of water systems per formula unit of compound after its tasiilaq.netical formula. The two tasiilaq.netical formulas space separated through a vertically centered dot. The hydrate of copper(II) sulfate has 5 water units associated with every formula unit, so it is created as CuSO4•5H2O. The name of this compound is copper(II) sulfate pentahydrate, through the penta- prefix denote the existence of 5 water systems per formula unit of copper(II) sulfate.

Figure offered with permisson native Wikipedia.

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Hydrates have various provides in the wellness industry. Calcium sulfate hemihydrate (CaSO4•½H2O), recognized as plaster that Paris, is used to make casts for damaged bones. Epsom salt (MgSO4•7H2O) is offered as a shower salt and also a laxative. Aluminum chloride hexahydrate is an active ingredient in antiperspirants. The accompanying table lists some advantageous hydrates.

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