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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## Recommended Questions

Cocaine is a weak organic base whose molecular formula is $\mathrm{C}_{17} \mathrm{H}_{21} \mathrm{NO}_{4}$ . An aqueous solution of cocaine was found to have a pH of 8.53 and an osmotic pressure of 52.7 torr at $15^{\circ} \mathrm{C} .$ Calculate $K_{b}$ for cocaine.

A forensic chemist is given a white solid that is suspected of being pure cocaine $\left(\mathrm{C}_{17} \mathrm{H}_{21} \mathrm{NO}_{4}, \text { molar mass }=303.35 \mathrm{g} / \mathrm{mol}\right)$ She dissolves $1.22 \pm 0.01 \mathrm{g}$ of the solid in $15.60 \pm 0.01 \mathrm{g}$ benzene. The freezing point is lowered by $1.32 \pm 0.04^{\circ} \mathrm{C}$

a. What is the molar mass of the substance? Assuming that the percent uncertainty in the calculated molar mass is the same as the percent uncertainty in the temperature change, calculate the uncertainty in the molar mass.

b. Could the chemist unequivocally state that the substance is cocaine? For example, is the uncertainty small enough to distinguish cocaine from codeine $\left(\mathrm{C}_{18} \mathrm{H}_{21} \mathrm{NO}_{3}, \text { molar }\right.$ mass $=299.36 \mathrm{g} / \mathrm{mol}$ )?

c. Assuming that the absolute uncertainties in the measurements of temperature and mass remain unchanged, how could the chemist improve the precision of her results?

Cocaine is a widely abused, addicting drug. Cocaine is usually obtained as its hydrochloride salt (cocaine hydrochloride) but can be converted to crack (the neutral molecule) by treatment with base. Which of the two compounds here has a higher boiling point? Which is more soluble in water? How does the relative solubility explain why crack is usually smoked but cocaine hydrochloride is injected directly into the bloodstream?

At $0^{\circ} \mathrm{C}$ and 1.00 atm, as much as 0.70 $\mathrm{g}$ of $\mathrm{O}_{2}$ can dissolve in 1 $\mathrm{L}$ of water. At $0^{\circ} \mathrm{C}$ and $4.00 \mathrm{atm},$ how many grams of $\mathrm{O}_{2}$ dissolve in 1 $\mathrm{L}$ of water?

Is octanoic acid more soluble in $1 M \mathrm{HCl}, 1 M \mathrm{NaOH}$ , or pure water? Explain. Drugs such as morphine $\left(\mathrm{C}_{17} \mathrm{H}_{19} \mathrm{NO}_{3}\right)$ are often treated with strong acids. The most commonly used form of morphine is morphine hydrochloride $\left(\mathrm{C}_{17} \mathrm{H}_{20} \mathrm{ClNO}_{3}\right)$ Why is morphine treated in this way? (Hint: Morphine is an amine.)