Chapter 22, Problem 78A

(a)

Interpretation Introduction

Interpretation:

The number of atoms in 56.1 g of Al needs to be determined.

Concept introduction:

In 1 mol of a substance there are $6.023\times {10}^{23}$ units. Also, number of moles is related to mass and molar mass as follows:

$n=\frac{m}{M}$

Here, m is mass and M is molar mass.

Answer to Problem 78A

$1.252\times {10}^{24}$

Explanation of Solution

The molar mass of Al is 26.98 g/mol

In 1 mol there are $6.023\times {10}^{23}$ atoms of Al. Thus, $6.023\times {10}^{23}$ molecules of Al has 26.98 g of mass.

Therefore, number of atoms in 56.1 g can be calculated s follows:

$N=\frac{6.023\times {10}^{23}}{26.98\text{ g}}\times 56.1\text{ g}=1.252\times {10}^{24}$

Thus, the number of atoms in 56.1 g of Al is $1.252\times {10}^{24}$.

(b)

Interpretation Introduction

Interpretation:

The number of atoms in 2 mol of C needs to be determined.

Concept introduction:

In 1 mol of a substance there are $6.023\times {10}^{23}$ units. Also, number of moles is related to mass and molar mass as follows:

$n=\frac{m}{M}$

Here, m is mass and M is molar mass.

Answer to Problem 78A

$1.2046\times {10}^{24}$

Explanation of Solution

In 1 mol there are $6.023\times {10}^{23}$ atoms of C thus, number of atoms in 2 mol will be twice of the given value.

$N=2\times 6.023\times {10}^{23}=1.2046\times {10}^{24}$

Therefore, number of atoms in 2 mol of C is $1.2046\times {10}^{24}$.