Chapter 13

In chemical research we often send newly synthesized compounds to commercial laboratories for analysis. These laboratories determine the weight percent of \(\mathrm{C}\) and \(\mathrm{H}\) by burning the compound and collecting the evolved \(\mathrm{CO}_{2}\) and \(\mathrm{H}_{2} \mathrm{O}\) They determine the molar mass by measuring the osmotic pressure of a solution of the compound. Calculate the empirical and molecular formulas of a compound, \(\mathrm{C}_{x} \mathrm{H}_{y} \mathrm{Cr},\) given the following information: (a) The compound contains \(73.94 \%\) C and \(8.27 \%\) \(\mathrm{H} ;\) the remainder is chromium. (b) At \(25^{\circ} \mathrm{C},\) the osmotic pressure of a solution containing \(5.00 \mathrm{mg}\) of the unknown dissolved in exactly \(100 \mathrm{mL}\) of chloroform solution is \(3.17 \mathrm{mm} \mathrm{Hg}\)

When salts of \(\mathrm{Mg}^{2+}, \mathrm{Ca}^{2+},\) and \(\mathrm{Be}^{2+}\) are placed in water, the positive ion is hydrated (as is the negative ion). Which of these three cations is most strongly hydrated? Which one is least strongly hydrated?

Explain why a cucumber shrivels up when it is placed in a concentrated solution of salt.

If you dissolve equal molar amounts of \(\mathrm{NaCl}\) and \(\mathrm{CaCl}_{2}\) in water, the \(\mathrm{CaCl}_{2}\) lowers the freezing point of the water almost 1.5 times as much as the NaCl. Why?

A \(100 .\) -gram sample of sodium chloride \((\mathrm{NaCl})\) is added to \(100 .\) mL of water at \(0^{\circ} \mathrm{C}\). After equilibrium is reached, about 64 g of solid remains undissolved. Describe the equilibrium that exists in this system at the particulate level.

Account for the fact that alcohols such as methanol \(\left(\mathrm{CH}_{3} \mathrm{OH}\right)\) and ethanol \(\left(\mathrm{C}_{2} \mathrm{H}_{5} \mathrm{OH}\right)\) are quite miscible with water, whereas an alcohol with a long-carbon chain, such as octanol \(\left(\mathrm{C}_{8} \mathrm{H}_{17} \mathrm{OH}\right),\) is poorly soluble in water.

Starch contains \(\mathrm{C}-\mathrm{C}, \mathrm{C}-\mathrm{H}, \mathrm{C}-\mathrm{O},\) and \(\mathrm{O}-\mathrm{H}\) bonds. Hydrocarbons have only \(\mathrm{C}-\mathrm{C}\) and \(\mathrm{C}-\mathrm{H}\) bonds. Both starch and hydrocarbons can form colloidal dispersions in water. Which dispersion is classified as hydrophobic? Which is hydrophilic? Explain briefly.

You have two aqueous solutions separated by a semipermeable membrane. One contains \(5.85 \mathrm{g}\) of NaCl dissolved in \(100 .\) mL of solution, and the other contains 8.88 g of \(\mathrm{KNO}_{3}\) dissolved in \(100 .\) mL of solution. In which direction will solvent flow: from the NaCl solution to the KNO \(_{3}\) solution, or from \(\mathrm{KNO}_{3}\) to \(\mathrm{NaCl}\) ? Explain briefly.

A protozoan (single-celled animal) that normally lives in the ocean is placed in freshwater. Will it shrivel or burst? Explain briefly.

It is interesting how the Fahrenheit temperature scale was established. One report, given by Fahrenheit in a paper in \(1724,\) stated that the value of \(0^{\circ} \mathrm{F}\) was established as the freezing temperature of saturated solutions of sea salt. From the literature we find that the freezing point of a \(20 \%\) by mass solution of NaCl is \(-16.46^{\circ} \mathrm{C} .\) (This is the lowest freezing temperature reported for solutions of NaCl.) Does this value lend credence to this story of the establishment of the Fahrenheit scale?