
Concept explainers
How many atoms (or ions) of each element are in 140.0 g of the following substances?
(a) H2(b) Ca(NO3)2 (c) N2O2(d) K2SO4
(a)

Interpretation:
The number of atoms (or ions) present in 140 g H2 is to be determined.
Explanation of Solution
For the number of formula units:
m(g)MM→nNA→Number of formula units
Here, m is the given mass, MM is the molar mass and n is the number of moles.
For the number of atoms or ions:
Number of formula unitsformula ratio→Atoms or ions
The number of formula units present is determined as follows:
Number of formula units=mMM×NA ...(1)
Here, NA is Avogadro’s number with a value of 6.022×1023 mol−1 .
The number of atoms is determined as follows:
Number of atoms=mMM×NA×Present ions ...(2)
By combining equations (1) and (2), the number of atoms can be determined as follows:
Number of atoms=Number of formula units×Present ions ...(3)
The molar mass of one mole of H2 is 2.016 g . The weight of H2 is 140 g . In the H2 molecule, there are two hydrogen atoms. Substitute these values in equation (2) as follows.
Number of H atoms=140 g2.016 g/mol×6.022×1023 mol−1×2 H atoms=8.364×1025 H atoms
Therefore, there are 8.364×1025 H atoms present in 140 g of H2 .
(b)

Interpretation:
The number of atoms (or ions) of each element present in 140 g Ca(NO3)2 is to be determined.
Explanation of Solution
The molar mass of Ca(NO3)2 is 164.10 g/mol . The weight of Ca(NO3)2 is 140 g . Substitute these values in equation (1) to determine the number of formula units present in 140 g of Ca(NO3)2 as follows.
Formula units Ca(NO3)2=140 g164.1 g/mol g/mol×6.022×1023 mol−1=5.14×1023 formula units Ca(NO3)2
There is one Ca2+ in Ca(NO3)2 . By using equation (3), the number of Ca2+ ions in 140 g Ca(NO3)2 is determined as follows.
Number of atoms=5.14×1023 × 1 Ca2+=5.14×1023 Ca2+
There are two NO−3 ions in Ca(NO3)2 . By using equation (3), the number of NO−3 in 140 g Ca(NO3)2 is determined as follows.
Number of atoms=5.14×1023 × 2 NO−3=1.028×1024 NO−3
Therefore, there are 5.14×1023 Ca2+ and 1.028×1024 NO−3 present in 140 g of Ca(NO3)2 .
(c)

Interpretation:
The number of atoms (or ions) of each element present in 140 g N2O2 is to be determined.
Explanation of Solution
The molar mass of N2O2 is 60.02 g/mol . The weight of N2O2 is 140 g . Substitute these values in equation (1) to determine the number of formula units present in 140 g of N2O2 as follows.
Formula units N2O2=140 g60.02 g/mol g/mol×6.022×1023 mol−1=1.405×1024 formula units N2O2
There are two N in N2O2 . By using equation (3), the number of N atoms in 140 g N2O2 is determined as follows.
Number of atoms=1.405×1024 × 1 N=2.8×1024 N atoms
There are two O ions in N2O2 . By using equation (3), the number of O in 140 g N2O2 is determined as follows.
Number of atoms=1.405×1024 × 2 O=2.8×1024 O atoms
Therefore, there are 2.8×1024 N and 2.8×1024 O atoms present in 140 g of N2O2 .
(d)

Interpretation:
The number of atoms (or ions) of each element present in 140 g K2SO4 is to be determined.
Explanation of Solution
The molar mass of K2SO4 is 174.26 g/mol . The weight of K2SO4 is 140 g . Substitute these values in equation (1) to determine the number of formula units present in 140 g of K2SO4 as follows.
Formula units K2SO4=140 g174.26 g/mol g/mol×6.022×1023 mol−1=4.838×1023 formula units K2SO4
There are 2 K+ in K2SO4 . By using equation (3), the number of K+ ions in 140 g K2SO4 is determined as follows.
Number of atoms=4.838×1023 × 2 K+=9.676×1023 K+ ions
There is one SO24− in K2SO4 . By using equation (3), the number of SO24− in 140 g K2SO4 is determined as follows.
Number of atoms=4.838×1023 × 1 SO24−=4.838×1023 SO24−
Therefore, there are 9.676×1023 K+ and 4.838×1023 SO24− present in 140 g of K2SO4 .
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