Question

A student has samples of two pure compounds, XClO3 and ZClO3, which contain unknown alkali metals X and Z. The student measures the mass of each sample and then strongly heats the samples to drive off all the oxygen, leaving solid residues of XCl and ZCl. The student measures the mass of the solid residue from each sample. Which of the following questions can be answered from the results of the experiment?
A. Which has the greater molar mass, X or Z?

B. Which has the higher boiling point, X or Z?

C. Which has the higher melting point, XCl or ZCl?

D. Which has the greater density, XCl or ZCl?

Answer 1

Answer:

• A. Which has the greater molar mass, X or Z?

Explanation:

By strongly heating up the starting substances to drive off all the oxygen, the student performs the following chemical decomposition reactions:

• 2XClO₃ (s)  → 2XCl + 3O₂ (g)

• 2ZClO₃ (s) → 2ZCl + 3O₂ (g)

The student knows the mass of each sample and the mass of the solid residue from each sample. That permits her to determine the mass of oxygen gas released from each sample:

• mass of oxygen = mass of sample - mass of solid residue

With the mass of oxygen released, she can calculate the number of moles of oxygen in the original sample:

• number of moles of oxygen = mass in grams of oxygen / molar mass of oxygen

Then she can calculate the number of moles of each sample:

• There are 3 moles of O in every mole of  XClO₃ and in every ZClO₃.

So, the number of moles of each sample is the number of moles of oxygen released by each one divided by 3.

Now, the formula molar mass = mass in grams / number of moles will permit to determine the molar mass of each sample.

Hence, the student can answer the question A. Which has the greater molar mass, X or Z?