To recognize the formula massive of one ionic compound.

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One skill essential in future chapters is the ability to identify the massive of the formula of an ionic compound. This quantity is called the formula mass. The formula fixed is acquired by including the masses of every individual atom in the formula the the compound. Due to the fact that a appropriate formula is electrically neutral (with no net electrons gained or lost), the ions have the right to be taken into consideration atoms because that the objective of calculating the formula mass.

Let us start by calculating the formula massive of salt chloride (NaCl). This formula massive is the sum of the atomic masses of one salt atom and one chlorine atom, i beg your pardon we discover from the periodic table; here, we usage the masses to 2 decimal places:

Na: 22.99 amu
Cl: + 35.45 amu
Total: 58.44 amu

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

When one ionic compound has much more than one anion or cation, you have to remember to usage the appropriate multiple of the atomic mass because that the facet in question. Because that the formula mass of calcium fluoride (CaF2), we need to multiply the fixed of the fluorine atom by 2 to account because that the 2 fluorine atom in the aramuseum.orgical formula:

Ca: 1 × 40.08 40.08 amu
F: 2 × 19.00 = + 38.00 amu
Total: 78.08 amu

The formula fixed of CaF2 is 78.08 amu.

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

K: 1 × 39.10 39.10 amu
N: 1 × 14.01 + 14.01 amu
O: 3 × 16.00 = + 48.00 amu
Total: 101.11 amu

The formula massive of KNO3 is 101.11 amu.

Potassium nitrate is a key ingredient in gunpowder and has been offered clinically together a diuretic.

When a formula contains much more than one polyatomic unit in the aramuseum.orgistry formula, together in Ca(NO3)2, carry out not forget to main point the atomic mass of every atom inside the parentheses by the subscript outside the parentheses. This is necessary since the subscript refers to the entire polyatomic ion. Thus, because that Ca(NO3)2, the subscript 2 suggests two finish nitrate ions, therefore we should sum the masses of two (1 × 2) nitrogen atoms and also six (3 × 2) oxygen atoms, together with the mass of a solitary calcium atom:

Ca: 1 × 40.08 40.08 amu
N: 2 × 14.01 = + 28.02 amu
O: 6 × 16.00 = + 96.00 amu
Total: 164.10 amu

The key to calculating the formula mass of one ionic link is to properly count each atom in the formula and multiply the atom masses the its atoms accordingly.

The periodic Table is found in this link:

To her Health: Hydrates

Some ionic compounds have water ((ceH2O)) integrated within your formula unit. This compounds, called hydrates, have actually a characteristic variety of water units connected with each formula unit of the compound. Hydrates space solids, no liquids or solutions, despite the water they contain.

To write the aramuseum.orgical formula of a hydrate, write the variety of water systems per formula unit of link after its aramuseum.orgistry formula. The 2 aramuseum.orgical formulas room separated through a vertically centered dot. The hydrate the copper(II) sulfate has 5 water units linked with every formula unit, so that is written as (ceCuSO4 • 5H2O). The name of this compound is copper(II) sulfate pentahydrate, through the penta- prefix indicating the visibility of 5 water systems per formula unit that copper(II) sulfate.


Magnesium sulfate heptahydrate. (Public Domain; aramuseum.orgicalinterest).

Hydrates have various offers in the health industry. Calcium sulfate hemihydrate ((ceCaSO4•½H2O)), recognized as plaster of Paris, is offered to do casts for broken bones. Epsom salt ((ceMgSO4•7H2O)) is provided as a shower salt and also a laxative. Aluminum chloride hexahydrate is an energetic ingredient in antiperspirants. The accompanying table list some helpful hydrates.

Table (PageIndex1): Names and also Formulas of some Widely provided Hydrates Formula Name offers
AlCl3•6H2O aluminum chloride hexahydrate antiperspirant
CaSO4•½H2O calcium sulfate hemihydrate (plaster the Paris) casts (for damaged bones and castings)
CaSO4•2H2O calcium sulfate di hydrate (gypsum) drywall component
CoCl2•6H2O cobalt(II) chloride hexahydrate drying agent, humidity indicator
CuSO4•5H2O copper(II) sulfate pentahydrate fungicide, algicide, herbicide
MgSO4•7H2O magnesium sulfate heptahydrate (Epsom salts) laxative, bathing salt
Na2CO3•10H2O sodium carbonate decahydrate (washing soda) laundry additive/cleaneKEY TAKEAWAY


Formula masses that ionic compounds have the right to be established from the masses of the atoms in your formulas.


2. Just how are subscripts used to recognize a formula mass when an ext than one polyatomic ion is current in a aramuseum.orgical formula?

3. What is the formula mass for the ionic compound developed by each pair of ions?

Na+ and Br− Mg2+ and also Br− Mg2+ and S2−

4. What is the formula mass because that the ionic compound created by every pair that ions?

K+ and also Cl− Mg2+ and Cl− Mg2+ and also Se2−

5. What is the formula mass for the ionic compound formed by every pair that ions?

Na+ and also N3− Mg2+ and N3− Al3+ and also S2−

6. What is the formula mass for the ionic compound created by every pair that ions?

Li+ and also N3− Mg2+ and also P3− Li+ and also P3−

7. What is the formula mass for each compound?

FeBr3 FeBr2 Au2S3 Au2S

8. What is the formula mass for each compound?

Cr2O3 CrO PbCl2 PbCl4

9. What is the formula mass for each compound?

Cr(NO3)3 Fe3(PO4)2 CaCrO4 Al(OH)3

10. What is the formula mass for each compound?

NH4NO3 K2Cr2O7 Cu2CO3 NaHCO3

11. What is the formula mass because that each compound?

Al(HSO4)3 Mg(HSO4)2

12. What is the formula mass for each compound?

Co(HCO3)2 LiHCO3

The formula fixed is the amount of the atom masses of the atom in the formula.

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The subscript is dispersed throughout the parentheses to identify the total number of atoms in the formula.