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?

Explanation:

1. For option A: When heating \( \text{XClO}_3 \) and \( \text{ZClO}_3 \), the reactions are \( 2\text{XClO}_3 \stackrel{\Delta}{=\!=\!=} 2\text{XCl} + 3\text{O}_2\uparrow \) and \( 2\text{ZClO}_3 \stackrel{\Delta}{=\!=\!=} 2\text{ZCl} + 3\text{O}_2\uparrow \). Let the mass of \( \text{XClO}_3 \) be \( m_1 \), mass of \( \text{XCl} \) be \( m_{1r} \); mass of \( \text{ZClO}_3 \) be \( m_2 \), mass of \( \text{ZCl} \) be \( m_{2r} \). The moles of \( \text{XClO}_3 \) equal moles of \( \text{XCl} \), and same for \( \text{ZClO}_3 \) and \( \text{ZCl} \). Molar mass of \( \text{X} \) is \( M_X \), \( \text{Z} \) is \( M_Z \). Molar mass of \( \text{XClO}_3 = M_X + 35.45 + 3\times16 = M_X + 83.45 \), molar mass of \( \text{XCl}= M_X + 35.45 \). The mass of \( \text{O}_2 \) lost from \( \text{XClO}_3 \) is \( m_1 - m_{1r} \), moles of \( \text{O}_2 \) is \( \frac{m_1 - m_{1r}}{32} \), and moles of \( \text{XClO}_3 \) is \( \frac{2}{3} \times \frac{m_1 - m_{1r}}{32} \). Also, moles of \( \text{XCl} \) is \( \frac{m_{1r}}{M_X + 35.45} \). Since moles of \( \text{XClO}_3 \) = moles of \( \text{XCl} \), we can solve for \( M_X \) (similarly for \( M_Z \)) and compare \( M_X \) and \( M_Z \).
2. For option B: Boiling point depends on intermolecular forces and is not directly determinable from the mass - loss (oxygen loss) data in this experiment. The experiment only deals with mass changes during a chemical reaction, not with the physical property of boiling point.
3. For option C: Melting point of ionic compounds like \( \text{XCl} \) and \( \text{ZCl} \) depends on lattice energy, which is related to ion size and charge. The experiment provides mass - related data from a chemical reaction, not information about ion sizes or lattice energy, so melting point cannot be determined.
4. For option D: Density is \( \frac{\text{mass}}{\text{volume}} \). The experiment gives mass data but no information about the volume of \( \text{XCl} \) or \( \text{ZCl} \), so density cannot be determined.

Answer:

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