Thursday, March 24

We started class today by talking about how we should memorize the equations on page 5 for molality, molarity, mass % and volume % for the test tomorrow. We then reviewed number 9 on page 17. We wwere given some information and had to find the change in freezing point. You have to recognize that hexane is a non-electrolyte so i=1. We are given the constant, 30.0 C/m, so all we need to know to find the change in freezing point is the molarity. We are told that 0.421 moles are dissolved into 500 g CCl4, so we can use that to find the molarity. The molarity would be .421 mol/.500 kg. If you plug that into the m for the equation Tf=i m Kf, you get Tf= 25.3 C. We did number 10 on page 18 which is basically the same problem but wiht different numbers. The answer is 0.430 C.

Next we went to page 13 to do 21 and 22. For 21, the relationship between gas solubility and temperature is inverse becuase as temperature increases, the solubility for a gas decreases. This is because if there were oxygen bubbles in a solution, they would be heading upwards towards the surface of the solution. As you increase the teperature, the oxygen molecules will move faster so they will reach the surface of the solution and get out of the solid more quickly. Once they're out, there's less gas in the solution so the solubility decreases.

For 22, the relationship between gas solubility and pressure is direct. This is because if there are gas molcules right above a solution, as the temperature increases the molecules wil start moving around really quickly. Some of the gas molecules will be forced into the liquid by this so there will be more gas in the liquid, therefore increasing the pressure.

Next we did the colligative properties lab. The goal of this lab was to find the molar mass of an unknown solid by using boiling point elevation. We started by pouring 50 mL of water into 3 seperate beakers. We put them all on the hot plate and when they reached 80 C, we took 2 off. In each of the ones we took off, we put a certain mass of the unknown solid and then put them back on the hot plate. The third beaker was used to find the boiling point of pure water. When the other 2 beakers started to boil, we measured their boiling points. Then you subtracted the boiling point you reocrded of pure water from the boiling points of the solutions and recorded the difference. By using the boiling point elevation formula, we could use the change in temperatures to find the molality. To do this you had to divide the difference by 2x0.52 since the constant was 0.52 C/m and we were told i=3. Once the molality was found, that could be used to find the moles of the solute since molality=moles solute/kg solvent. The final step was to find the molar mass which is mass of solute/moles solute. The mass was the number of grams you were supposed to measure of the solid. Each group had a different mass. The theoretical value was 58 g/mol. Using this, you were supposed to do a percent error calculation.

The homework was to study for the test tomorrow and finish our lab notebooks.