1. General Chemistry, Chapters 4-6 Practice Problems Dr. Badger

2. 1. A solution contains 2.0 moles of solute in 200.0 mL of solution. What is its molarity? 2.0 mol 200 mL 1000 mL 1 L =10.0 mol/L

3. 2. How many grams of Ca(OH) 2 are required to make 1.00 L of a 0.750M Ca(OH) 2 solution? 74.10 g Ca(OH) 2 1 mol Ca(OH) 2 1 L =55.6 g Ca(OH) 2 mm Ca(OH) 2 = 74.10 g Ca(OH) 2 / mol 0.750 mol Ca(OH) 2 That is, 55.6 g Ca(OH) 2 per liter of solution is 0.750M Ca(OH) 2

4. 3. How many grams of CO 2 are in 15.0 L of CO 2 at STP? 15.0 L of CO 2 44.01 g CO 2 mol CO 2 22.4 L =29.5 g CO 2 1.00 mol Could you use PV = nRT to solve this? Why didn’t we use it?

5. 4. 50 mL of an H 3 PO 4 solution was titrated with 0.50 M KOH. 235 mL of the base were required to neutralize the sample. What is the molarity of this acid solution? H 3 PO 4 + KOH  K 3 PO 4 + H 2 O Is this equation balanced? Why do we need a balanced chemical equation?

6. 4. 50 mL of an H 3 PO 4 solution was titrated with 0.50 M KOH. 235 mL of the base were required to neutralize the sample. What is the molarity of this acid solution? H 3 PO 4 + 3 KOH  K 3 PO 4 + 3 H 2 O 0.50 mol KOH 0.235 L 3 mol KOH 1 mol H 3 PO 4 0.050 L = 3.92  10 -2 mol H 3 PO 4 3.92  10 -2 mol H 3 PO 4 = 0.783 mol/L 1 L

7. 5. What volume of conc hydrochloric acid (12.0 M) is required to make 350 mL of 1.5 M hydrochloric acid? M i  V i = M f  V f 12.0 M  V i = 1.5 M  350 mL V i = 12.0 M 1.5 M  350 mL = 43.75 = 44 mL

8. 6. 0.750 mol NH 3 occupies 2.25 L at 175ºC. Calculate the pressure of NH 3 in the flask. PV = nRT P = 175ºC = 448 K V nRT = 0.750 mol  0.0821 L atm mol -1 K -1  448 K 2.25 L P = 12.3 atm

9. 7. A sample of a gas occupies 2.50  10 3 mL at 25ºC and 560 mmHg. What volume will it occupy at the same temperature and 690 mmHg? P 1 V 1 = P 2 V 2 560 mmHg  2.50  10 3 mL V 2 = 690 mmHg 0.737 atm  2.50  10 3 mL V 2 = 0.908 atm = 2.03  10 3 mL

10. 8. Calculate the mass of 3.54 L of CO gas measured at 29ºC and 800 mmHg. 29ºC = 302K PV = nRT n = RT PV = 0.0821 L atm mol -1 K -1  302 K 1.05 atm  3.54 L n = 0.150 mol CO 28.01 g CO 1 mol CO = 4.20 g CO

11. 9. Calculate the amount of heat necessary to raise the temperature of 26.0 g of water from 13.4ºC to 81.0ºC. The specific heat of water = 4.18 J/g·ºC. q = msΔt Δt = t f - t i q = 26.0 g  4.18 J/g ºC  (67.6 ºC) q = 7.35  10 3 kJ

12. 10. A sample of oxygen gas was collected over water at 25ºC and 725 mmHg. The volume of the container was 4.80 L. Calculate the mass of O 2 (g) collected. P T = P ox + P w P ox = P T - P w P ox = 725 mmHg – 23.76 mmHg n = RT PV = 0.0821 L atm mol -1 K -1  298 K 0.923 atm  4.80 L = 18.1 mol n = 18.1 mol O 2 32.00 g O 2 1 mol O 2 = 579 g O 2 P ox = 701.24mmHg = 0.923 atm

13. 11. Calculate the amount of heat required to raise the temperature of 41.7 g of Al from 20.0ºC to 145ºC. (Al has a specific heat of 0.920 J/g·ºC.) q = msΔt Δt = t f - t i q = 41.7 g  0.920 J/g ºC  (125 ºC) q = 4.80  10 3 kJ

14. 12. Determine the molar mass of an unknown gas if a sample weighing 0.211 g is collected in a flask with a volume of 0.117 L at 90ºC. The pressure of the gas is 682 mmHg. n = RT PV = 0.0821 L atm mol -1 K -1  363 K 0.897 atm  0.117 L = 3.52  10 -3 mol 3.52  10 -3 mol 0.211 g = 59.9 g / mol PV = nRT

15. 13. Given: 2 SO 2 (g) + O 2 (g)  2 SO 3 (g) Calculate the heat evolved when 88.2 g of SO 2 (g) is oxidized to SO 3 (g).  H = –198.2 kJ/mol 88.2 g SO 2 = -273 kJ 1 mol SO 2 64.07 g SO 2 1 mol -198.2 kJ Is this -273 kJ/mol? Explain.