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Topic: Chemistry problems. Not sure about how to solve 5 and 6.  (Read 164 times)

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MrLoonyMoon

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Chemistry problems. Not sure about how to solve 5 and 6.
« on: March 15, 2020, 03:53:01 PM »
Table:

Mass of 100ml beaker: 61.911g

Mass of beaker + 60mL Di H2O: 117.653g

Mass of Beaker + H2O + 12g NaCl: 130.639g

Mass of empty 50.0mL Volumetric flask: 33.810g

Mass of volumetric flask + 50.0mL NaCl(aq) solution: 90.848g

Boiling Point temperature of NaCl(aq) solution: 106.0*C

Boiling point temperature of Di H2O: 100.0*C

Q: Calculate the percent by mass NaCl in the solution.

My A: 130.639g - 61.911g = 68.728g 12g NaCl/68.728g x 100 =17.46%

2. Q: Calculate the Molality of NaCL in the solution.

My A: 12NaCl/68.728g = 0.1746 molality

3. Q:Calculate the density of the solution.

My A: 68.728g/60mL = 1.145g/mL

4. Q: Calculate the molarity of NaCl in the solution.

My A: 12g/60mL = (0.2g/mL/58.55g/mol) x 1000 = 3.42M

5. Q: Calculate the mole fraction of NaCl in solution.

?

6. Q: Given that the boiling point elevation for water is K(b) = 0.512*C/m, calculate how much the bp temperature of the water should have increased by Delta T. C is Celsius.

?

7. Q: What did you record for Delta T.

My A:Delta T = (2)(4.5*C)(3.42M) = 30.78 I used the formula Delta T = (A)(k(f))(M).  M is Molality and A is Van't Hoff Factor.

mjc123

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Re: Chemistry problems. Not sure about how to solve 5 and 6.
« Reply #1 on: March 16, 2020, 05:47:34 AM »
1. Don't use 12g NaCl. You have a measurement for the mass of NaCl you used (line 3 - line 2 of table). Use that.
2. What is the definition of molality?
3. If you took 60 mL water, that doesn't mean you have 60 mL solution after adding NaCl. Did you measure the solution volume?
4. Correct if you assume the volume is 60 mL, but see above.
5. What is the definition of mole fraction?
6. Do you know how the boiling point is affected by the presence of solute? (You quote a formula below that suggests you should know.)
7. Why use a formula (especially one that relates to freezing)? You are asked what value you recorded.