Chapter 10, Problem 204A

Interpretation Introduction

(a)

Interpretation:

Atomic masses of element X and Y must be found out when mass of 0.25 mol of XY and that of 0.25 mol of X₂Y₃ is 17.96 g and 39.92 g respectively.

Concept introduction:

Atomic mass of an element is the average weight of all the isotopes for the element.

Answer to Problem 204A

Atomic masses of X and Y are 55.84 and 16 respectively.

Explanation of Solution

0.25 mol XY has mass $\text{=17}\text{.96 g}\text{.}$

1 mol XY has mass $\text{=}\frac{\text{17}\text{.96}}{0.25}\text{ g= 71}\text{.84 g}\text{.}$

0.25 mol X₂Y₃ has mass $\text{=39}\text{.92 g}\text{.}$

1 mol X₂Y₃ has mass $\text{=}\frac{\text{39}\text{.92}}{0.25}\text{ g= 159}\text{.68 g}\text{.}$

So,

$\begin{array}{l}\text{X+Y=71}\text{.84}\mathrm{........}\text{(1)×2}\\ \text{2X+2Y=143}\text{.68}\mathrm{...}\text{(2)}\\ \text{2X+3Y=159}\text{.68}\mathrm{..}\text{(3)}\\ \text{------------------}\\ \text{(3)-(2), Y=(159}\text{.68-143}\text{.68)=16}\\ \text{X+16=71}\text{.84}\\ \text{or, X=(71}\text{.84-16)=55}\text{.84}\text{.}\end{array}$

b)

Interpretation Introduction

Interpretation:

Formulas of the two compounds must be found out for which 0.25 moles have mass of 17.96 g and 39.92 g respectively.

Concept introduction:

Elements can be identified from the atomic mass and valency.

Answer to Problem 204A

Formulas of the two compounds are FeO and Fe₂O₃.

Explanation of Solution

Atomic mass of X is 55.84 and that of Y is 16. This suggests that X is Fe and Y is O. Oxygen has valency 2 whereas Fe has two valencies which are 2 (in ferrous compound) and 3 (in ferric compound). So the expected formulas of the two compounds are FeO and Fe₂O₃.