Unit 4, Lessons 15 and 16

Lesson 15: 

One can create a solution with a specific molarity by using the molar mass of the solid. If the amount of solid you have is given, you can find out how many moles of that substance are needed with a chart (which we've already gone over) and the molar mass. If the concentration or the volume is given, you can substitute it into the molarity concentration to find the other value you need. For example, if you need .01 L of a 0.5 M solution, you would need 0.5 grams of your solute. How did I get this? Plug the known values into the molarity equation to get 0.5 = n / 0.01. Multiply each side by 0.01 to get n = 0.005 moles of solute. Convert moles to grams of solute by using the molar mass of, in this case, NaBr, to get 0.515 g. That rounds to 0.5!

Problems:

1.) Explain how you would create a solution of sucrose with a molarity of 0.25. I think I would first need to know the volume of solution I am making. Then, I would multiply that volume by the molarity, 0.25 M, to get my # of moles. I would convert the moles to grams and that would tell me how much sucrose I need to make this solution.

6.) How many grams of fructose, C6H12O6, are in 1 L of soft drink if the molarity of fructose in the drink is 0.75 M? First off, the molar mass of fructose is about 180 g. In the equation 0.75 M =  n / 1, I find that this solution has 0.75 moles of fructose. Converting moles to grams, I multiply 0.75 by 180 and get 135 grams of fructose in this soft drink.



Lesson 16:

We really didn't have many notes for this lesson, since we really just perfected our skills from 15. The gist of lesson 16 is that the type of substance being dissolved in a solution affects the whole solution's mass. The mass of one mole of one substance could be very different than the mass of one mole of another substance.

Problems:

1.) Explain how you might use mass to determine if a sample of water is contaminated. You might be able to figure out if the water is contaminated by finding its density/mass and then comparing the sample to a sample of "normal" water. If the results are different, one sample of water probably has some extra substances in it, like minerals or toxins.

3.) Explain why 0.10 M CuCl2 has a greater density than 0.10 M KCl. The densities are different because the solutions contain two different compounds, and CuCl2 has a greater molar mass than KCl.