
Concept explainers
(a)
Interpretation:
The number of formula units in 29.6 g of lithium nitrate is to be calculated.
Concept introduction:
A mole is a basic unit that is used in the International system of units (SI). It is abbreviated as mol. It can be defined as 6.022×1023 units of some quantity which can be atoms, molecules or ions. The quantity 6.022×1023 is called the

Answer to Problem 31E
The number of formula units in 29.6 g of lithium nitrate is 2.58×1023 formula units.
Explanation of Solution
The molecular formula for lithium nitrate is LiNO3.
The molar mass of nitrogen is 14.01 g mol−1.
The molar mass of oxygen is 16.00 g mol−1.
The molar mass of lithium is 6.94 g mol−1.
The molar mass of LiNO3 is calculated below.
Total molar mass =6.94+14.01+(3×16.00)=68.95 g mol−1
Therefore, the molar mass of LiNO3 is 68.95 g mol−1.
Thus, one mole of LiNO3 is 68.95 g.
The number of moles in 29.6 g of LiNO3 is given below.
Moles of LiNO3=1 mole of LiNO368.95 g of LiNO3×weight of LiNO3 in g=1 mole of LiNO368.95 g of LiNO3×29.6 g of LiNO3 =0.429 moles of LiNO3
The number of formula units in LiNO3 is calculated as,
Formula units of LiNO3=(6.022×1023 formula units of LiNO3×moles of LiNO31 mole of LiNO3)
Substitute the number of moles of LiNO3 in above equation,
Formula units of LiNO3=(6.022×1023 formula units of LiNO3×0.429 moles of LiNO31 mole of LiNO3)=2.58×1023 formula units of LiNO3
The number of formula units in 29.6 g of lithium nitrate is 2.58×1023 formula units.
(b)
Interpretation:
The number of formula units in 0.151 g of lithium sulfide is to be calculated.
Concept introduction:
A mole is a basic unit that is used in the International system of units (SI). It is abbreviated as mol It can be defined as 6.022×1023 units of some quantity which can be atoms, molecules or ions. The quantity 6.022×1023 is called the Avogadro’s number.

Answer to Problem 31E
The number of formula units in 0.151 g of lithium sulfide is 1.98×1021 formula units.
Explanation of Solution
The molecular formula for lithium sulfide is Li2S.
The molar mass of lithium is 6.94 g mol−1.
The molar mass of sulfur is 32.06 g mol−1
The molar mass of Li2S is calculated below.
Total molar mass =(2×6.94)+32.06=45.94 g mol−1
Therefore, the molar mass of Li2S is 45.94 g mol−1.
Thus one mole of Li2S is 45.94 g.
The number of moles in 0.151 g of Li2S is given below.
Moles of Li2S=1 mole of Li2S45.94 g of Li2S×weight of Li2S in g=1 mole of Li2S45.94 g of Li2S×0.151 g of Li2S =0.00328 moles of Li2S
The number of formula units in Li2S is calculated as,
Formula units of Li2S=(6.022×1023 formula units of Li2S×moles of Li2S1 mole of Li2S)
Substitute the number of moles of Li2S in above equation,
Formula units of Li2S=(6.022×1023 formula units of Li2S×0.00328 moles of LiNO31 mole of Li2S)=1.98×1021 formula units of Li2S
The number of formula units in 0.151 g of lithium sulfide is 1.98×1021 formula units.
(c)
Interpretation:
The number of formula units in 457 g of iron (III) sulfate is to be calculated.
Concept introduction:
A mole is a basic unit that is used in the International system of units (SI). It is abbreviated as mol. It can be defined as 6.022×1023 units of some quantity which can be atoms, molecules or ions. The quantity 6.022×1023 is called the Avogadro’s number.

Answer to Problem 31E
The number of formula units in 457 g of iron (III) sulphate is 6.88×1023 formula units.
Explanation of Solution
The molecular formula for iron (III) sulfate is Fe2(SO4)3.
The molar mass of iron is 55.85 g mol−1.
The molar mass of sulfur is 32.06 g mol−1
The molar mass of oxygen is 16.00 g mol−1.
The molar mass of Fe2(SO4)3 is calculated below.
Total molar mass =(2×55.85) +(3×32.06)+(12×16)=399.88 g mol−1
Therefore, the molar mass of Fe2(SO4)3 is 399.88 g mol−1.
Thus one mole of Fe2(SO4)3 is 399.88 g mol−1.
The number of moles in 457 g of Fe2(SO4)3 is given below.
Moles of Fe2(SO4)3=(1 mole of Fe2(SO4)3×weight of Fe2(SO4)3 in g399.88g of Fe2(SO4)3)=(1 mole of Fe2(SO4)3×457 g of Fe2(SO4)3399.88 g of Fe2(SO4)3)=1.1428 moles of Fe2(SO4)3
The number of formula units in Fe2(SO4)3 is calculated as,
Formula units of Fe2(SO4)3=(6.022×1023 formula units of Fe2(SO4)3×moles of Fe2(SO4)31 mole of Fe2(SO4)3)
Substitute the number of moles of Fe2(SO4)3 in above equation,
Formula units of Fe2(SO4)3=(6.022×1023 formula units of Fe2(SO4)3×1.1428 moles of Fe2(SO4)31 mole of Fe2(SO4)3)=6.88×1023 formula units of Fe2(SO4)3
The number of formula units in 457 g of iron (III) sulphate is 6.88×1023 formula units.
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