   Chapter 3, Problem 185CP

Chapter
Section
Textbook Problem

When aluminum metal is heated with an element from Group 6A of the periodic table, an ionic compound forms. When the experiment is performed with an unknown Group 6A element, the product is 18.56% AI by mass. What is the formula of the compound?

Interpretation Introduction

Interpretation: The mass percent of aluminum (Al) metal for its reaction with group 6A elements is given. The formula of ionic compound formed after this reaction is to be calculated.

Concept introduction: The ionic compound is formed by the complete transfer of electrons between atoms.

The oxidation state of 6A element is -2 and the oxidation state of aluminum is +3.

The number of moles is the ratio of mass of compound to its molar mass.

To determine: The formula of ionic compound that is formed due to reaction of aluminum with group 6A elements.

Explanation

Given

The mass percent of aluminum (Al)  metal is 18.56 %.

When aluminum metal is heated with an element from group 6A elements, an ionic compound is formed having the chemical formula,

Al2X3

Where,

• X is group 6A element.

The ratio of aluminum and element X  in Al2X3 is,

AlX=23 (1)

If the mass of ionic compound is taken 100 g., then mass of aluminum is 18.56 g. The mass of X is calculated by subtracting the total mass of ionic compound with the mass of aluminum metal, that is,

Mass of X = 100 g - 18.56 g      = 81.44 g

It is assumed that atomic mass of element X  is A.

The atomic mass of aluminum is 26.981 g/mol

Formula

The number of moles is calculated by using the formula,

Number of moles = Given massAtomic mass (2)

Substitute the values of mass and atomic mass of aluminum in equation (2).

Number of moles of Al = Given mass of AlAtomic mass =18.56 g26

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