Name: The Mole Concept
Uploaded: 2019-07-19
Duration: 4 min 28 s
Description: The mole is the SI unit of measurement for the amount of substance. 1 mole of a substance means the Avogadro's number of atoms or molecules of that substance...

Question

Use the criss-cross method to write down the formulae of the following ionic compounds.

Ammonium chloride, calcium hydroxide, magnesium sulphate, ammonium sulphate, calcium sulphate.

Accepted Answer

a) Ammonium chloride

1. Write the symbol and the charge of the cation(metal)first and anion (nonmetal) second.

${NH}_{4}^{+} {Cl}^{-}$

Total charge of cations = Total charge of anions

2. Use a multiplier to make the total charge of the cations and anions equal to each other.

$1 (1 +) = 1 (1 -) + 1 = - 1$

3. Use the multipliers as subscript for each ion.

${NH}_{1} {Cl}_{1}$

4. Symbol/Formulae

Valencies

So, the formula is $N H_{4} C l$

b) Calcium hydroxide

1. Write the symbol and the charge of the cation(metal)first and anion (nonmetal) second.

${Ca}^{2 +} {OH}^{-}$

Total charge of cations = Total charge of anions

2. Use a multiplier to make the total charge of the cations and anions equal to each other. $1 (2 +) = 2 (1 -) 2 + = 2 -$

3. Use the multipliers as subscripts for each ion.

Symbol/FormulaeValencies

So, the formula is $Ca (OH)_{2}$

c) Magnesium sulphate

1. Write the symbol and the charge of the cation(metal)first and anion (nonmetal) second.

${Mg}^{2 +} {SO}_{4}^{2 -}$

Total charge of cations = Total charge of anions

2. Use a multiplier to make the total charge of the cations and anions equal to each other.

$1 (2 +) = 1 (2 -) 2 + = 2 -$

3. 3. Use the multipliers as subscripts for each ion. Symbol/Formulae

Valencies

So, the formula is ${MgSO}_{4}$

d) Ammonium sulphate

1. Write the symbol and the charge of the cation(metal)first and anion (nonmetal) second.

${NH}_{4}^{+} {SO}_{4}^{2 -}$

Total charge of cations = Total charge of anions

2. Use a multiplier to make the total charge of the cations and anions equal to each other.

$2 (1 +) = 1 (2 -) 2 + = 2 -$

3. Use the multipliers as subscripts for each ion.

Symbol/FormulaeValencies

So, the formula is $({NH}_{4})_{2} {SO}_{4}$

e) Calcium phosphate

1. Write the symbol and the charge of the cation(metal)first and anion (nonmetal) second.

${Ca}^{2 +} {PO}_{4}^{3 -}$

Total charge of cations = Total charge of anions

2. Use a multiplier to make the total charge of the cations and anions equal to each other.

$3 (2 +) = 2 (3 -) 6 + = 6 -$

3. Use the multipliers as subscripts for each ion.

Symbol/Formulae

Valencies

So, the formula is ${Ca}_{3} ({PO}_{4})_{2}$

Use the criss-cross method to write down the formulae of the following ionic compounds.

Ammonium chloride, calcium hydroxide, magnesium sulphate, ammonium sulphate, calcium sulphate.

Important Questions on Chemical Formula and Mole Concept

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