High-temperature superconducting oxides hold great promise in the utility, transportation, and computer industries

(a) One superconductor is $\mathrm{La}_{2-x} \mathrm{Sr}_{x} \mathrm{CuO}_{4}$ . Calculate the molar

masses of this oxide when $x=0, x=1,$ and $x=0.163$

(b) Another common superconducting oxide is made by heating a mixture of barium carbonate, copper(II) oxide, and ytrium(III) oxide, followed by further heating in $\mathrm{O}_{2}$ :

$$

4 \mathrm{BaCO}_{3}(s)+6 \mathrm{CuO}(s)+\mathrm{Y}_{2} \mathrm{O}_{3}(s) \longrightarrow

$$

$$

2 \mathrm{YBa}_{2} \mathrm{Cu}_{3} \mathrm{O}_{65}(s)+4 \mathrm{CO}_{2}(g)

$$

$$

2 \mathrm{YBa}_{2} \mathrm{Cu}_{3} \mathrm{O}_{6.5}(s)+\frac{1}{2} \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{YBa}_{2} \mathrm{Cu}_{3} \mathrm{O}_{7}(s)

$$

When equal masses of the three reactants are heated, which reactant is limiting?

(c) After the product in part (b) is removed, what is the mass $\%$ of each reactant in the remaining solid mixture?

## Discussion

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Chrome The most important commercial ore of chromium is chromite $\left(\mathrm{FeCr}_{2} \mathrm{O}_{4}\right) .$ One of the steps in the

process used to extract chromium from the ore is the

reaction of chromite with coke (carbon) to produce

ferrochrome (FeCr_ ).

\begin{equation}

2 \mathrm{C}(\mathrm{s})+\mathrm{FeCr}_{2} \mathrm{O}_{4}(\mathrm{s}) \rightarrow \mathrm{FeCr}_{2}(\mathrm{s})+2 \mathrm{CO}_{2}(\mathrm{g})

\end{equation}

What mole ratio would you use to convert from moles

of chromite to moles of ferrochrome?

Classify the following as acid-base reactions or oxidation-reduction reactions:

(a) $\mathrm{Na}_{2} \mathrm{S}(a q)+2 \mathrm{HCl}(a q) \longrightarrow 2 \mathrm{NaCl}(a q)+\mathrm{H}_{2} \mathrm{S}(g)$

(b) $\mathrm{PCl}_{3}(l)+\mathrm{Cl}_{2}(g) \longrightarrow \mathrm{PCl}_{5}(s)$

(c) $\mathrm{C}_{2} \mathrm{H}_{4}(g)+3 \mathrm{O}_{2}(g) \longrightarrow 2 \mathrm{CO}_{2}(g)+2 \mathrm{H}_{2} \mathrm{O}(g)$

(d) $\mathrm{Zn}(s)+\mathrm{H}_{2} \mathrm{SO}_{4}(a q) \longrightarrow \mathrm{ZnSO}_{4}(a q)+\mathrm{H}_{2}(g)$

(e) $2 \mathrm{K}_{2} \mathrm{S}_{2} \mathrm{O}_{3}(s)+\mathrm{I}_{2}(s) \longrightarrow \mathrm{K}_{2} \mathrm{S}_{4} \mathrm{O}_{6}(s)+2 \mathrm{KI}(s)$

(f) $3 \mathrm{Cu}(s)+8 \mathrm{HNO}_{3}(a q) \longrightarrow 3 \mathrm{Cu}\left(\mathrm{NO}_{3}\right)_{2}(a q)+2 \mathrm{NO}(g)+4 \mathrm{H}_{2} \mathrm{O}(l)$