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:

Step1: Write the decomposition reactions

$2XClO_3\xrightarrow{\Delta}2XCl + 3O_2\uparrow$ and $2ZClO_3\xrightarrow{\Delta}2ZCl + 3O_2\uparrow$. Let the molar - mass of $X$ be $M_X$ and of $Z$ be $M_Z$. The molar - mass of $XClO_3$ is $M_X + 35.5+3\times16 = M_X + 83.5$, and the molar - mass of $XCl$ is $M_X + 35.5$. Similarly for $Z$.

Step2: Use mass - change relationship

The mass of oxygen lost from $XClO_3$ sample is related to the mass of $XClO_3$ and $XCl$. If the initial mass of $XClO_3$ is $m_1$ and the mass of $XCl$ (residue) is $m_{1r}$, the mass of oxygen lost, $m_{O_1}=m_1 - m_{1r}$. From the stoichiometry, if we assume we have $n_1$ moles of $XClO_3$, the mass of oxygen lost per mole of $XClO_3$ decomposition is based on the ratio of molar - masses. The same is true for $ZClO_3$. We can set up equations based on the mass - loss data: For $XClO_3$, if $m_1$ grams of $XClO_3$ decompose to $m_{1r}$ grams of $XCl$, and for $ZClO_3$, if $m_2$ grams of $ZClO_3$ decompose to $m_{2r}$ grams of $ZCl$. Using the molar - mass relationships and the mass - loss data from the decomposition reactions ($2XClO_3
ightarrow2XCl + 3O_2$ and $2ZClO_3
ightarrow2ZCl + 3O_2$), we can calculate and compare the molar - masses of $X$ and $Z$.

Step3: Analyze other properties

Boiling point, melting point, and density are physical properties that are not directly related to the mass - change data from the decomposition reaction. Boiling and melting points depend on intermolecular forces, lattice energy etc., and density depends on mass and volume (volume is not measured in this experiment).

Answer:

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