9.E: Exercises
- Page ID
- 83112
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Calcium nitrate reacts with sodium carbonate to precipitate solid calcium carbonate:
Ca(NO3)2(aq) + Na2CO3(aq) → CaCO3(s) + 2NaNO3(aq)
A) Balance the chemical equation. B) How many grams of Na2CO3 are needed to react with 50.0 mL of 0.450 M Ca(NO3)2? C) What was the osmolarity of the 50.0 mL of 0.450 M Ca(NO3)2 solution before the reaction? -
Estimate the freezing point of concentrated aqueous HCl, which is usually sold as a 12 M solution. Assume complete ionization into H+ and Cl− ions. (The normal freezing point of water is zero celsius. For every mole of particles in a liter of water, the freezing point decreases by about 1.9°C.)
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Seawater can be approximated by a 3.0% m/m solution of NaCl in water. Determine the molarity and osmolarity of seawater. Assume a density of 1.0 g/mL. (Hint: Think about the units on top and on bottom of each concentration, then how to convert the top and bottom.)
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How much water must be added to 25.0 mL of a 1.00 M NaCl solution to make a resulting solution that has a concentration of 0.250 M?
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Occasionally we hear a sensational news story about people stranded in a lifeboat on the ocean who had to drink their own urine to survive. While distasteful, this act was probably necessary for survival. Why not simply drink the ocean water?
Answers
- A) Ca(NO3)2(aq) + Na2CO3(aq) → CaCO3(s) + 2NaNO3(aq) ; B) 2.39 g; C) 1.35 osmol
- −45.6°C
- 3.0% (m/m) is 3.0 g NaCl per 100 g solution. The 3.0 g NaCl can be converted to moles using molar mass. The 100 g solution can be converted to mL using density, and then to liters. Moles NaCl, divided by liters of solution gives 0.51 M. Since each NaCl breaks up into two ions (i = 2), the molarity is doubled to get the molarity of particles (a.k.a. osmolarity), 1.0 osmol.
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The final volume needs to be 100. mL total. This means that 75.0 mL of water must be added to the initial 25.0 mL of solution.
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The osmotic pressure of seawater is too high. Drinking seawater would cause water to go from inside our cells into the more concentrated seawater, ultimately collapsing and killing the cells (crenation).