Chapter 7

A chemist prepared an aqueous solution by mixing \(2.50 \mathrm{~g}\) of ammonium phosphate trihydrate, \(\left(\mathrm{NH}_{4}\right)_{3} \mathrm{PO}_{4} \cdot 3 \mathrm{H}_{2} \mathrm{O}\) and \(1.50 \mathrm{~g}\) of potassium phosphate, \(\mathrm{K}_{3} \mathrm{PO}_{4}\), with \(500 \mathrm{~g}\) of water. (a) Determine the number of moles of formula units of each compound that was measured. (b) How many moles of \(\mathrm{PO}_{4}{ }^{3-}\) are present in solution? (c) Calculate the mass of phosphate ions present in the solution. (d) What is the total mass of the water present in the solution?

To prepare a very dilute solution, it is advisable to perform successive dilutions of a single prepared reagent solution, rather than to wcigh a very small mass or to measure a very small volume of stock chemical. A solution was prepared by transferring \(0.661 \mathrm{~g}\) of \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) to a \(250.0\)-mL volumetric flask and adding water to the mark. A \(1.000-\mathrm{mL}\). sample of this solution. was transferred to a 500 -mL. volumetric flask and diluted to the mark with water. Then \(10.0 \mathrm{~mL}\) of the diluted solution was transferred to a 250 -mL. flask and diluted to the mark with water. (a) What is the final concentration of \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{\text {, in solution? (b) What }}\) mass of \(\mathrm{K}_{2} \mathrm{Cr}_{2} \mathrm{O}_{7}\) is in this final solution? (The answer to the last question gives the amount that would have had to have been weighed out if the solution had been prepared directly.)

What mass (in grams) of anhydrous sohute is needed to prepare each of the following solutions? (a) \(1.0 \mathrm{~L}\) of \(0.10 \mathrm{M} \mathrm{NaCl}(\mathrm{aq}) ;\) (b) \(250 \mathrm{~mL}\) of \(0.10 \mathrm{M}\) \(\mathrm{CaCl}_{2}(\mathrm{aq}) ;\) (c) \(500 \mathrm{~mL}\) of \(0.63 \mathrm{M} \mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(\mathrm{aq})\).

Describe the preparation of cach solution, starting with the anhydrous solute and warer and using the corrcsponding volumetric flask: (a) \(25.0 \mathrm{~mL}\) of \(6.0 \mathrm{M}\) \(\mathrm{NaOH}(\mathrm{aq}) ;\) (b) \(1.0 \mathrm{~L}\), of \(0.10 \mathrm{M} \mathrm{BaCl}_{2}\) (aq); (c) \(500 \mathrm{~mL}\) of \(0.0010 \mathrm{M} \mathrm{AgNO}_{3}(\mathrm{aq})\).

Calculate the mole fraction of each component in the following solutions: (a) \(2.5 .0 \mathrm{~g}\) of water and \(50.0 \mathrm{~g}\) of ethanol, \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}\); (b) \(25.0 \mathrm{~g}\) of water and \(50.0 \mathrm{~g}\) of methanol, \(\mathrm{CH}_{3} \mathrm{OH}\); (c) a glucowe solution that is \(0.10 \mathrm{~m}\) \(\mathrm{C}_{6} \mathrm{H}_{12} \mathrm{O}_{6}(a q)\).

Calculate the molality of the solute in each of the following solutions: (a) \(10.0 \mathrm{~g}\) of \(\mathrm{NaCl}\) dissolvcd in \(250 \mathrm{~g}\) water; (b) \(0.48 \mathrm{~mol}\) of \(\mathrm{KOH}\) dissolved in \(50.0 \mathrm{~g}\) water; (c) \(1.94 \mathrm{~g}\) of urea, \(\mathrm{CO}\left(\mathrm{NH}_{2}\right)_{2}\), dissolved in \(200 \mathrm{~g}\) water.

(a) What mass (in grams) of \(\mathrm{NaOH}\) must be mixed with \(250 \mathrm{~g}\) of water to prepare a \(0.22 \mathrm{~m} \mathrm{NaOH}\) solution? (b) Calculate the amount (in moles) of cthylene glycol, \(\mathrm{HOC}_{2} \mathrm{H}_{4} \mathrm{OH}\), that should be added to \(2.0 \mathrm{~kg}\) of water to prepare a \(0.44 \mathrm{~m} \mathrm{HOC}_{2} \mathrm{H}_{4} \mathrm{OH}\) (aq) solution. (c) Determine the amount (in moles) of HCl that must be dissolved in \(500 \mathrm{~g}\) water to prepare a \(0,0010 m\) HClaq) solution.

In a laboratory exercise, a student mixes \(2.5 .0 \mathrm{~g}\) of cthanol, \(\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{OH}\), with \(150 \mathrm{~g}\) of water. (a) What is the mole fraction of ethanol in the solution? (b) What is the molality of cthanol in the solution?

Nitric acid is purchased from chemical suppliers as a solution that is \(70 \% \mathrm{HNO}_{3}\) by mass. What mass (in grams) of a \(70 \% \mathrm{HNO}_{3}(\mathrm{aq})\) solution is required to prepare \(250 \mathrm{~g}\) of a \(2.0 \mathrm{~m} \mathrm{HNO}_{3}\) (aq) solution? The density of \(70 \% \mathrm{HNO}_{3}(\mathrm{aq})\) is \(1.42 \mathrm{~g} \cdot \mathrm{cm}^{-1}\).