Chem Solutions Homework Key

What is the solvent and what is the solute in MgF2 (aq)? Solute: MgF2 (s) Solvent: Water DRAW what is happening at the molecular level when KBr is dissolved in water. KBr dissolving in water, showing K+ and Br ions surrounded by water molecules What is the general solubility rule? Using that rule, what do you know is true about the polarity of oil? Hint: think of an oil/water mixture. 1   General Solubility Rule: Like dissolves like. Since oil does not dissolve in water, it is nonpolar. Draw molecules in the boxes below to help understand concentration. two beakers, one with fewer solute particles and labeled Less concentrated solution, and the other with more solute particles and labeled More concentrated solution. What is the difference between a concentrated and dilute solution? Concentrated solution: More moles of solute per liter of solution. Dilute solution: Less moles of solute per liter of solution. Describe how to make a solution. Mass the solid. Dissolve the solid into some water. Fill the volumetric flask to the line with water and invert. Describe how to dilute a solution. Determine the volume of stock solution needed. Fill the volumetric flask to the line with water. Invert. Define the following terms: Saturated solution: Maximum amount of solute that can be dissolved in solution. Supersaturated solution: More solute than possible dissolved in solution. Unsaturated solution: Less solute than possible dissolved in solution. 9. How does temperature generally affect solubility? **In general, ↑ temperature means ↑ solubility** 10. Explain why salt is thrown on icy roads in the winter. Be specific about what is happening at the molecular level. The solute/solvent interactions make it harder for the water to turn to a solid phase, thus lowering the freezing point of the water. 11. What happens to the boiling point of a substance when the amount of solute increases? The boiling point increases as the amount of solute increases. **Use the diagram below to answer questions 12-15** [Image of a solubility graph] 12. List the following solutions as saturated, unsaturated, or supersaturated: a. 70 g of CaCl2 at 30°C: **saturated** b. 50 g of KCl at 60°C: **unsaturated** c. 20 g of KNO3 at 30°C: **saturated** d. 30 g of K2Cr2O7 at 50°C: **supersaturated** 13. The temperature must be at least **50°C** to dissolve 90 g of KNO3 into 100 g of water. 14. Why is Ce2(SO4)3 different than all of the other compounds on the solubility chart? Ce2(SO4)3 becomes less soluble when temperature increases. 15. I have 20 grams of KCl dissolved into 100 g of water at 70°C. About how many more grams of KCl can be dissolved in the water to make it a saturated solution? **30 more grams** **Molarity/M Problems** 16. Fill in the equation circle to the right. What are the units for concentration? **M = mol/L** Volume must be measured in the units L or mL. 17. What is the concentration of a solution when 0.77 moles of NaCl is dissolved into water to make 1.2 L of solution? M = 0.77 mol / 1.2 L = **0.64 M** 18. A 250 mL solution is made with 0.50 moles of KBr. What is the molarity of the solution? M = 0.50 mol / 0.25 L = **2 M** 19. What is the volume of a 0.25M solution that contains 2.2 moles of solute? 0.25 = 2.2 / X X = 8.8 L 20. How many moles KCI03 are in 30 mL of a 0.100 M solution? How many grams? 0.1 M = X / 0.03 L X = 0.003 mol 0.003 mol | 122.55 g ----------- = 0.37 g 1 mol 21. Calculate the molarity of the following solutions a. 1.5 g of NaCl is dissolved in water to make 250 mL of solution 1.5 g NaCl | 1 mol -------------- = 0.026 mol 58.59 g 0.026 mol / 0.25 L = 0.104 M b. 0.4 g of NaOH is dissolved in water to make 150 mL of solution 0.4 g NaOH | 1 mol -------------- = 0.01 mol 40 g 0.01 mol / 0.15 L = 0.067 M 22. How many moles of AlCl3 are in 100 mL of a 0.50M solution? 0.5 M = X / 0.1 L X = 0.05 mol 23. How many moles of Ca(OH)2 are in 25 mL of 1.5M solution? 1.5 = X / 0.025 L X = 0.038 mol 24. A reaction calls for the use of 5.00 mol of NH4OH. You have a bottle of 1.5M NH4OH. How many milliliters of the solution should you add? 1.5 M = 5 mol / X X = 3.33 L or 3,330 mL 25. A reaction calls for the use of 0.33 mol H2SO4. You have a bottle of 0.1M H2SO4. How many mL should you add? 0.1 M = 0.33 / X X = 3.33 L or 3,330 mL 26. How many grams of CaCl2 would be required to produce a 3.5 M (molar) solution with a volume of 2.0 L? 3.5 M = X / 2 L X = 7 mol | 111 g CaCl2 --------- = 777 g CaCl2 1 mol 27. How many moles of Sr(NO3)2 would be used in the preparation of 2.50 L of a 3.5 M solution? 3.5 = X / 2.5 L X = 8.75 mol