Homework 2-solutions PDF

Preview

Summary

Homework ...

Description

ortega (jdo868) – Homework 2 – fakhreddine – (49215) This print-out should have 11 questions. Multiple-choice questions may continue on the next column or page – find all choices before answering. 001 1.0 points A solution is 40.0% silver nitrate (AgNO3) by mass. The density of this solution is 1.48 grams/mL. The formula weight of AgNO3 is 170 grams/mol. Calculate the molality of AgNO3 in this solution. Correct answer: 3.92157 m . Explanation: density = 1.48 g/mL FWAgNO3 = 170 g/mol % AgNO3 = 40% by mass molality =

nAgNO3 kg water

In 100 g of 40.0% AgNO3 (aq) there are 40.0 g AgNO3 and 60.0 g water. nAgNO3 = 40.0 g AgNO3  1.0 mol AgNO  3 × 170 g AgNO3 = 0.235294 mol AgNO3 m=

0.235 mol AgNO3 = 3.92157 m 0.060 kg water

002 1.0 points The mole fraction of a certain nonelectrolyte compound in a solution containing only that substance and water is 0.100. The molecular weight of water is 18.0 g/mol. What additional information is needed to determine the molality of the solution? 1. The mole fraction of water in the solution. 2. The density of the solution. 3. No additional information; the molality can be calculated from the information given. correct 4. The density of the solute.

1

5. The molecular weight of the compound. Explanation: Here we can assume that we have 1 mol total. (In fact, we can choose any number of moles, but the math is easier if you choose 1 mol.) If the mole fraction of the substance is 0.100, you can then assume that you have 0.100 mol of the substance, and the remaining 0.900 mol is H2O. The molality of a solution is determined by the following formula: mol solute m= kg solvent We’ve already assumed that we have 0.100 mol of solute, and we can determine the kg of H2O in the usual   way:   1 kg 18.0 g 0.900 mol H 2O 1 mol 1000 g = 0.0162 kg, and we can calculate the molality of this solution: 0.100 mol m= = 6.17 m 0.0162 kg H2 O So, it is possible to determine the molality of this solution without any additional information. 003 1.0 points Water causes many electrolytes to dissociate 1. because it undergoes hydrogen bonding to large halide ions. 2. because of repulsive interactions between ions in the crystalline state. 3. because the dispersion forces between ion and solvent are strong. 4. because water molecules are dipoles and the dipoles orient in an energetically favorable manner to solvate the ions. correct Explanation: The more negative end of the water molecule orients toward the cation and the more positive end orients toward the anion; the ions remain apart.

ortega (jdo868) – Homework 2 – fakhreddine – (49215) 004 1.0 points For which of the following ions would the hydration process give off the least heat? 1. K+ correct 2. Al3+ 3. Li+

(decreases, increases, may increase or decrease) in magnitude. 1. increases; decreases; decreases 2. decreases; decreases; increases 3. increases; does not change; may increase or decrease

4. Ba+2

4. decreases; increases; increases

5. Mg+2

5. increases; increases; decreases

Explanation: A small charge-to-radius ratio means that the hydration process will give off a small amount of heat. This term is smallest for K+ . 005 1.0 points Consider two solutes with the same lattice energy in kJ/mol; however, the particles of solute A are strongly solvated by water, while those of solute B are only weakly solvated. Which of the two dissolution processes is more exothermic, on a kJ/mol basis? 1. dissolving B in water 2. It depends on the molecular weights of the solutes.

2

6. increases; decreases; increases 7. increases; increases; increases correct Explanation: As charge-to-size ratio increases, hydration energy and crystal latice energy also increase. 007 1.0 points Which of the following is the best explanation of why sugar is not very soluble in gasoline? Note that all of the statements below are true. 1. Sugar does not ionize in gasoline. 2. There are dipole-dipole interactions between sugar molecules but not between gasoline molecules.

3. dissolving A in water correct 4. Cannot be answered without knowing the identities of the solutes. 5. Both processes are equally exothermic. Explanation: Since the solvation process is exothermic, dissolving A (the more strongly solvated solute) will be the more exothermic process. 006 1.0 points Consider a series of chloride salts (MCl2). As the charge-to-size ratio of M2+ (decreases, increases) the hydration energy of M2+ (decreases, increases, does not change) in magnitude and the crystal lattice energy of MCl2

3. The disorder of the system would increase if the sugar dissolved in gasoline. 4. The attractive forces between gasoline molecules are weaker than the forces between gasoline and sugar molecules. 5. There is hydrogen bonding in the sugar crystal, but not between sugar and gasoline. correct Explanation: The sugar molecule has several polar OH groups which are strongly attracted to polar water molecules but poorly attracted to nonpolar gasoline (hydrocarbon) molecules.

ortega (jdo868) – Homework 2 – fakhreddine – (49215) 008 1.0 points Which of the following alcohols would be the least miscible with water?

3

2. HI, SO3 3. C2H4 , SO3

1. ethanol (CH3CH2 OH) 4. SO3 , HI correct 2. methanol (CH3OH) 5. SF6 , HI 3. pentanol (CH3CH2CH2CH2CH2 OH) 4. propanol (CH3CH2 CH2OH) 5. hexanol (CH3CH2 CH2CH2CH2CH 2 OH) correct Explanation: The polar OH group is miscible with water but as the nonpolar hydrocarbon chain lengthens, solubility decreases. 009 1.0 points Based simply on the molecular formula provided, which of the following compounds is least likely to be miscible in cyclohexane (C6H12)? 1. naphthalene (C10 H8)

Explanation: Sulfur trioxide can react with with water to create sulfuric acid, much as carbon dioxide can react with water to form carbonic acid, like so: H2O + SO3 → H2 SO4 H2O + CO2 → H2 CO3 Hydriodic acid, unlike the other gases which are non-polar, is highly polar, and thus more likely to dissolve in significant quantities in water, a polar solvent. 011 1.0 points Which of the following is a FALSE statement? 1. The solubility of a gas increases with increasing pressure of the gas above the solution.

2. benzene (C6H6 ) 3. methanol (CH3OH) correct 4. Each contains a hydrocarbon unit and should be equally miscible. 5. phenol (C6H5 OH) Explanation: 010 1.0 points Consider the following list of gases: H2 , SF6, SO3 , HI, C2H4 The gas most likely to dissolve in water as a result of chemical reaction is and the gas most likely to dissolve in water because of strong intermolecular interactions is . 1. SF6 , H2

2. The solubility of a gas decreases with increasing temperature. 3. The solubility of a gas depends both on temperature and pressure. 4. The solubility of a gas decreases with increasing pressure of a gas above the solution. correct Explanation: Gases become more soluble as pressures increases and temperatures decrease....